JFET Animation

How A JFET Works 

Watch it work with an interactive diagram

Using the Animation

Start by clicking and dragging up, either of the handles on the V_GS or V_DS voltage sliders. This applies appropriate voltages to the JFET. Watch the "What´s Happening" box for information about the transistor´s operation as you adjust the V_GS and V_DS sliders.
The V_DS control changes the voltage across the drain and source, which sets up a voltage gradient to attract electrons from the source to the drain, creating a drain current I_D. Notice how the initial increase in current stops rising once the pinch off point is exceeded. The increase in current normally caused by an increase in applied voltage is approximately balanced by the increase in resistance of the conducting channel, caused by the growth of depletion layers around the gate regions. This is due to a reverse bias on the gate PN junctions caused by the P type gate being at a lower voltage than the N type channel (especially nearer the positive Drain end) as the drain becomes more positive.
With V_DS in the saturation area and V_GS at zero volts, maximum current is flowing. This current can be reduced to zero just by increasing the negative gate−source voltage V_GS to narrow the conducting channel. As the Gate−channel junction is reverse biased, the JFET controls a large drain current by using a changing voltage on the gate with practically no gate current. The advantage of a JFET is that, although it´s gain (it´s FORWARD or MUTUAL TRANSCONDUCTANCE) is not very high compared with some other devices; because there is no gate current, the input impedance is extremely high. Also unlike a bipolar transistor the current does not have to pass through any PN junctions (which in effect are small capacitors), therefore the JFET high frequency performance is very good.

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Field Effect Transistors

JFETs (Junction Field Effect Transistors)

Although there are lots of confusing names for field effect transistors (FETs) there are basically two main types:
1. The reverse biased PN junction types, the JFET or Junction FET, (also called the JUGFET or Junction Unipolar Gate FET).
2. The insulated gate FET devices (IGFET).
All FETs can be called UNIPOLAR devices because the charge carriers that carry the current through the device are all of the same type i.e. either holes or electrons, but not both. This distinguishes FETs from the bipolar devices in which both holes and electrons are responsible for current flow in any one device.

The JFET

This was the earliest FET device available. It is a voltage−controlled device in which current flows from the SOURCE terminal (equivalent to the emitter in a bipolar transistor) to the DRAIN (equivalent to the collector). A voltage applied between the source terminal and a GATE terminal (equivalent to the base) is used to control the source − drain current. The main difference between a JFET and a bipolar transistor is that in a JFET no gate current flows, the current through the device is controlled by an electric field, hence "Field effect transistor". The JFET construction and circuit symbols are shown in Figures 1, 2 and 3.

JFET Construction

The construction of JFETs can be theoretically quite simple, but in reality difficult, requiring very pure materials and clean room techniques. JFETs are made in different forms, some being made as discrete (single) components and others, using planar technology as integrated circuits.

Fig.1.1 Diffusion JFET Construction

Fig. 1.1 shows the (theoretically) simplest form of construction for a Junction FET (JFET) using diffusion techniques. It uses a small slab of N type semiconductor into which are infused two P type areas to form the Gate. Current (electrons) flows through the device from source to drain along the N type silicon channel. As only one type of charge carrier (electrons) carry current in N channel JFETs, these transistors are also called "Unipolar" devices.

Fig. 1.2 JFET Planar Construction

Fig. 1.2 shows the cross section of a N channel planar Junction FET (JFET) The load current flows through the device from source to drain along a channel made of N type silicon. In the planar device the second part of the gate is formed by the P type substrate.

P channel JFETs are also available and the principle of operation is the same as the N channel type described here, but polarities of the voltages are of course reversed, and the charge carriers are holes.

Fig. 1.3 JFET Circuit symbols

How a JFET Works.

The JFET is a Voltage Operated Transistor.

Fig. 2.1 JFET Operation Below "Pinch Off".

In the N channel device, the N channel is sandwiched between two P type regions (the gate and the substrate) that are connected together electrically to form the gate. The N type channel is connected to the source and drain terminals via more heavily doped N+ type regions. The drain ic connected to a positive supply, and the source to zero volts. N+ type silicon has a lower resistivity than N type. This gives it a lower resistance, increasing conduction and reducing the effect of placing standard N type silicon next to the aluminium connector, which because aluminium is a tri−valent material, having three valence electrons whilst silicon has four, would tend to create an unwanted junction, similar in effect to a PN junction at this point.
The P type gate is at 0V and is therefore negatively biased compared to the channel, which has a potential gradient on it, as one end is connected to 0 volts (the source), and the other end to a positive voltage (the drain). Any point on the channel (apart from the extreme end near the source terminal) must therefore be more positive than the gate. Therefore the two PN junctions formed between the N type channel and the P type areas of the gate and the substrate are both reverse biased, and so have a depletion layer that extends into the channel as shown in Fig. 2.1.
The shape of the depletion layer is not symmetrical, as can be seen from Fig. 2.1. It is generally thicker towards the drain end of the channel, because the voltage on the drain is more positive than that on the source due to voltage gradient that exists along the channel. This causes a larger potential across the junctions nearer the drain, and so a thickening of the depletion layer. The effect becomes more marked when the voltage between drain and source is greater than about 1volt or so.

Fig. 2.2 JFET Operation Above "Pinch Off"

When a voltage is applied between drain and source (V_DS) current flows and the silicon channel acts rather like a conventional resistor. Now if V_DS is increased (with V_GS held at zero volts) towards what is called the pinch off value V_P, the drain current I_D also at first, increases. The transistor is working in the "ohmic region" as shown in Fig. 2.1.
However as drain source voltage V_DS increases, the depletion layers at the gate junctions are also becoming thicker and so narrowing the N type channel available for conduction. There comes a point, known as "pinch off" where the conducting channel has become narrow enough to cancel out the effect of current increasing with the applied voltage V_DS as shown in fig 2.2. Above this point there is little further increase in drain current and the transitor is said to operating in "saturation mode". With the JFET biased in this way, a small change in V_GS can be used to control the current through the source−drain channel from its maximum(saturated) value to zero current.
This type of operation is shown in the fairly flat top to the output characteristics shown in fig 2.3. Notice that each curve is drawn for a particular value of negative voltage between gate and source, and that when sufficient reverse bias is applied to the gate (e.g. more than −2.5V, the lowest value on the graph) the drain current ceases completely.

Fig. 2.3 JFET Output Characteristic

In the JFET output characteristics shown in Fig. 2.3, the Drain current I_D shows very little change, and the curves are very nearly horizontal at voltages greater than the pinch off voltage. Almost all of the expected increase in current, due to the increase in voltage between Source and Drain (V_DS), is offset by the narrowing of the conducting channel due to the growing depletion layers.

Fig. 2.4 JFET Transfer Characteristic

The transfer characteristic for a JFET, which shows the change in Drain current (I_D) for a given change in Gate−Source voltage (V_GS), is shown in Fig 2.4. Because the JFET input (the Gate) is voltage operated, the gain of the transistor cannot be called current gain, as with bipolar transistors. The drain current is controlled by the Gate−Source voltage, so the graph shows milliamperes per volt (mA / V), and as I / V is CONDUCTANCE (the inverse of resistance V / I) the slope of this graph (the gain of the device) is called the FORWARD or MUTUAL TRANSCONDUCTANCE, which has the symbol g_m. Therefore the higher the value of g_m the greater the amplification.

Notice that V_GS is always shown as being negative; in reality it may be zero or slightly above zero, but the gate is always more negative than the N type channel between source and drain. Note also that the slope of the curve in the transfer characteristic is less steep than that of the transfer characteristic for a typical bipolar transistor. This means that a JFET will have a lower gain than that of a bipolar transistor.
This disadvantage is offset by the advantage of having an extremely high input resistance. A typical input resistance for a JFET would be in the region of 1 x 10¹⁰ ohms (10,000 Megohms!) compared with 2K to 3K Ohms for a bipolar device.
This makes the JFET ideal for applications where the circuit or device driving the JFET amplifier cannot supply any appreciable current, an example being the Electret microphone, which uses a FET within the microphone to amplify the tiny voltage variations appearing across the vibrating diaphragm element.
Another feature of the JFET that makes it more suited to very high frequency use than bipolar transistors, is the absence of junctions in the JFET conducting channel. In a bipolar transistor two PN junctions forming tiny capacitances, exist between base and emitter, and base and collector, due to the PN junctions. These capacitances will limit high frequency performance, as they provide negative feedback paths at high frequencies. Because the JFET is in effect just a slab of silicon between Source and Drain, the stray capacitances that exist in bipolar devices are absent, so high frequency performance is improved, making JFETs usable even at hundreds of MHz.

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Transistors

Introduction Transistors make our electronics world go ‘round. They’re critical as a control source in just about every modern circuit. Sometimes you see them, but more-often-than-not they’re hidden deep within the die of an integrated circuit. In this tutorial we’ll introduce you to the basics of the most common transistor around: the bi-polar junction transistor (BJT).

In small, discrete quantities, transistors can be used to create simple electronic switches, digital logic, and signal amplifying circuits. In quantities of thousands, millions, and even billions, transistors are interconnected and embedded into tiny chips to create computer memories, microprocessors, and other complex ICs.

Covered In This Tutorial

After reading through this tutorial, we want you to have a broad understanding of how transistors work. We won’t dig too deeply into semiconductor physics or equivalent models, but we’ll get deep enough into the subject that you’ll understand how a transistor can be used as either a switch or amplifier.
This tutorial is split into a series of sections, covering:

• Symbols, Pins, and Construction – Explaining the differences between the transistor’s three pins.
• Extending the Water Analogy – Going back to the water analogy to explain how a transistor acts like a valve.
• Operation Modes – An overview of the four possible operating modes of a transistor.
• Applications I: Switches – Application circuits showing how transistors are used as electronically controlled switches.
• Applications II: Amplifiers – More application circuits, this time showing how transistors are used to amplify voltage or current.

There are two types of basic transistor out there: bi-polar junction (BJT) and metal-oxide field-effect (MOSFET). In this tutorial we’ll focus on the BJT, because it’s slightly easier to understand. Digging even deeper into transistor types, there are actually two versions of the BJT: NPN and PNP. We’ll turn our focus even sharper by limiting our early discussion to the NPN. By narrowing our focus down – getting a solid understanding of the NPN – it’ll be easier to understand the PNP (or MOSFETS, even) by comparing how it differs from the NPN.

Symbols, Pins, and Construction

Transistors are fundamentally three-terminal devices. On a bi-polar junction transistor (BJT), those pins are labeled collector (C), base (B), and emitter (E). The circuit symbols for both the NPN and PNP BJT are below:

The only difference between an NPN and PNP is the direction of the arrow on the emitter. The arrow on an NPN points out, and on the PNP it points in. A useful mnemonic for remembering which is which is:

NPN: Not Pointing iN

Backwards logic, but it works!

Transistor Construction

Transistors rely on semiconductors to work their magic. A semiconductor is a material that’s not quite a pure conductor (like copper wire) but also not an insulator (like air). The conductivity of a semiconductor – how easily it allows electrons to flow – depends on variables like temperature or the presence of more or less electrons. Let’s look briefly under the hood of a transistor. Don’t worry, we won’t dig too deeply into quantum physics.

A Transistor as Two Diodes

Transistors are kind of like an extension of another semiconductor component: diodes. In a way transistors are just two diodes with their cathodes (or anodes) tied together:

The diode connecting base to emitter is the important one here; it matches the direction of the arrow on the schematic symbol, and shows you which way current is intended to flow through the transistor.
The diode representation is a good place to start, but it’s far from accurate. Don’t base your understanding of a transistor’s operation on that model (and definitely don’t try to replicate it on a breadboard, it won’t work). There’s a whole lot of weird quantum physics level stuff controlling the interactions between the three terminals.
(This model is useful if you need to test a transistor. Using the diode (or resistance) test function on a multimeter, you can measure across the BE and BC terminals to check for the presence of those “diodes”.)

Transistor Structure and Operation

Transistors are built by stacking three different layers of semiconductor material together. Some of those layers have extra electrons added to them (a process called “doping”), and others have electrons removed (doped with “holes” – the absence of electrons). A semiconductor material with extra electrons is called an n-type (n for negative because electrons have a negative charge) and a material with electrons removed is called a p-type (for positive). Transistors are created by either stacking an n on top of a p on top of an n, or p over n over p.

Simplified diagram of the structure of an NPN. Notice the origin of any acronyms?

With some hand waving, we can say electrons can easily flow from n regions to p regions, as long as they have a little force (voltage) to push them. But flowing from a p region to an n region is really hard (requires a lot of voltage). But the special thing about a transistor – the part that makes our two-diode model obsolete – is the fact that electrons can easily flow from the p-type base to the n-type collector as long as the base-emitter junction is forward biased (meaning the base is at a higher voltage than the emitter).

The NPN transistor is designed to pass electrons from the emitter to the collector (so conventional current flows from collector to emitter). The emitter “emits” electrons into the base, which controls the number of electrons the emitter emits. Most of the electrons emitted are “collected” by the collector, which sends them along to the next part of the circuit.
A PNP works in a same but opposite fashion. The base still controls current flow, but that current flows in the opposite direction – from emitter to collector. Instead of electrons, the emitter emits “holes” (a conceptual absence of electrons) which are collected by the collector.

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The transistor is kind of like an electron valve. The base pin is like a handle you might adjust to allow more or less electrons to flow from emitter to collector. Let’s investigate this analogy further…

Extending the Water Analogy

If you’ve been reading a lot of electricity concept tutorials lately, you’re probably used to water analogies. We say that current is analogous to the flow rate of water, voltage is the pressure pushing that water through a pipe, and resistance is the width of the pipe.

Unsurprisingly, the water analogy can be extended to transistors as well: a transistor is like a water valve – a mechanism we can use to control the flow rate.
There are three states we can use a valve in, each of which has a different effect on the flow rate in a system.

1) On – Short Circuit

A valve can be completely opened, allowing water to flow freely – passing through as if the valve wasn’t even present.

Likewise, under the right circumstances, a transistor can look like a short circuit between the collector and emitter pins. Current is free to flow through the collector, and out the emitter.

2) Off – Open Circuit

When it’s closed, a valve can completely stop the flow of water.

In the same way, a transistor can be used to create an open circuit between the collector and emitter pins.

3) Linear Flow Control

With some precise tuning, a valve can be adjusted to finely control the flow rate to some point between fully open and closed.

A transistor can do the same thing – linearly controlling the current through a circuit at some point between fully off (an open circuit) and fully on (a short circuit).
From our water analogy, the width of a pipe is similar to the resistance in a circuit. If a valve can finely adjust the width of a pipe, then a transistor can finely adjust the resistance between collector and emitter. So, in a way, a transistor is like a variable, adjustable resistor.

Amplifying Power

There’s another analogy we can wrench into this. Imagine if, with the slight turn of a valve, you could control the flow rate of the Hoover Dam’s flow gates. The measly amount of force you might put into twisting that knob has the potential to create a force thousands of times stronger. We’re stretching the analogy to its limits, but this idea carries over to transistors too. Transistors are special because they can amplify electrical signals, turning a low-power signal into a similar signal of much higher power.

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Kind of. There’s a lot more to it, but that’s a good place to start! Check out the next section for a more detailed explanation of the operation of a transistor.

Operation Modes

Unlike resistors, which enforce a linear relationship between voltage and current, transistors are non-linear devices. They have four distinct modes of operation, which describe the current flowing through them. (When we talk about current flow through a transistor, we usually mean current flowing from collector to emitter of an NPN.)
The four transistor operation modes are:

• Saturation – The transistor acts like a short circuit. Current freely flows from collector to emitter.
• Cut-off – The transistor acts like an open circuit. No current flows from collector to emitter.
• Active – The current from collector to emitter is proportional to the current flowing into the base.
• Reverse-Active – Like active mode, the current is proportional to the base current, but it flows in reverse. Current flows from emitter to collector (not, exactly, the purpose transistors were designed for).

To determine which mode a transistor is in, we need to look at the voltages on each of the three pins, and how they relate to each other. The voltages from base to emitter (V_BE), and the from base to collector (V_BC) set the transistor’s mode:

The simplified quadrant graph above shows how positive and negative voltages at those terminals affect the mode. In reality it’s a bit more complicated than that.
Let’s look at all four transistor modes individually; we’ll investigate how to put the device into that mode, and what effect it has on current flow.
Note: The majority of this page focuses on NPN transistors. To understand how a PNP transistor works, simply flip the polarity or > and < signs.

Saturation Mode

Saturation is the on mode of a transistor. A transistor in saturation mode acts like a short circuit between collector and emitter.

In saturation mode both of the “diodes” in the transistor are forward biased. That means V_BE must be greater than 0, and so must V_BC. In other words, V_B must be higher than both V_E and V_C.

Because the junction from base to emitter looks just like a diode, in reality, V_BE must be greater than a threshold voltage to enter saturation. There are many abbreviations for this voltage drop – V_th, V_γ, and V_d are a few – and the actual value varies between transistors (and even further by temperature). For a lot of transistors (at room temperature) we can estimate this drop to be about 0.6V.
Another reality bummer: there won’t be perfect conduction between emitter and collector. A small voltage drop will form between those nodes. Transistor datasheets will define this voltage as CE saturation voltage V_CE(sat) – a voltage from collector to emitter required for saturation. This value is usually around 0.05-0.2V. This value means that V_C must be slightly greater than V_E (but both still less than V_B) to get the transistor in saturation mode.

Cutoff Mode

Cutoff mode is the opposite of saturation. A transistor in cutoff mode is off – there is no collector current, and therefore no emitter current. It almost looks like an open circuit.

To get a transistor into cutoff mode, the base voltage must be less than both the emitter and collector voltages. V_BC and V_BE must both be negative.

In reality, V_BE can be anywhere between 0V and V_th (~0.6V) to achieve cutoff mode.

Active Mode

To operate in active mode, a transistor’s V_BE must be greater than zero and V_BC must be negative. Thus, the base voltage must be less than the collector, but greater than the emitter. That also means the collector must be greater than the emitter.

In reality, we need a non-zero forward voltage drop (abbreviated either V_th, V_γ, or V_d) from base to emitter (V_BE) to “turn on” the transistor. Usually this voltage is usually around 0.6V.

Amplifying in Active Mode

Active mode is the most powerful mode of the transistor because it turns the device into an amplifier. Current going into the base pin amplifies current going into the collector and out the emitter.
Our shorthand notation for the gain (amplification factor) of a transistor is β (you may also see it as β_F, or h_FE). β linearly relates the collector current (I_C) to the base current (I_B):

The actual value of β varies by transistor. It’s usually around 100, but can range from 50 to 200…even 2000, depending on which transistor you’re using and how much current is running through it. If your transistor had a β of 100, for example, that’d mean an input current of 1mA into the base could produce 100mA current through the collector.

Active mode model. V_BE = V_th, and I_C = βI_B.

What about the emitter current, I_E? In active mode, the collector and base currents go into the device, and the I_E comes out. To relate the emitter current to collector current, we have another constant value: α. α is the common-base current gain, it relates those currents as such:

α is usually very close to, but less than, 1. That means I_C is very close to, but less than I_E in active mode.
You can use β to calculate α, or vice-versa:

If β is 100, for example, that means α is 0.99. So, if I_C is 100mA, for example, then I_E is 101mA.

Reverse Active

Just as saturation is the opposite of cutoff, reverse active mode is the opposite of active mode. A transistor in reverse active mode conducts, even amplifies, but current flows in the opposite direction, from emitter to collector. The downside to reverse active mode is the β (β_R in this case) is much smaller.
To put a transistor in reverse active mode, the emitter voltage must be greater than the base, which must be greater than the collector (V_BE<0 and V_BC>0).

Reverse active mode isn’t usually a state in which you want to drive a transistor. It’s good to know it’s there, but it’s rarely designed into an application.

Relating to the PNP

After everything we’ve talked about on this page, we’ve still only covered half of the BJT spectrum. What about PNP transistors? PNP’s work a lot like the NPN’s – they have the same four modes – but everything is turned around. To find out which mode a PNP transistor is in, reverse all of the < and > signs.
For example, to put a PNP into saturation V_C and V_E must be higher than V_B. You pull the base low to turn the PNP on, and make it higher than the collector and emitter to turn it off. And, to put a PNP into active mode, V_E must be at a higher voltage than V_B, which must be higher than V_C.
In summary:

Voltage relations	NPN Mode	PNP Mode
VE < VB < VC	Active	Reverse
VE < VB > VC	Saturation	Cutoff
VE > VB < VC	Cutoff	Saturation
VE > VB > VC	Reverse	Active

Another opposing characteristic of the NPNs and PNPs is the direction of current flow. In active and saturation modes, current in a PNP flows from emitter to collector. This means the emitter must generally be at a higher voltage than the collector.

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If you’re burnt out on conceptual stuff, take a trip to the next section. The best way to learn how a transistor works is to examine it in real-life circuits. Let’s look at some applications!

Applications I: Switches

One of the most fundamental applications of a transistor is using it to control the flow of power to another part of the circuit – using it as an electric switch. Driving it in either cutoff or saturation mode, the transistor can create the binary on/off effect of a switch.
Transistor switches are critical circuit-building blocks; they’re used to make logic gates, which go on to create microcontrollers, microprocessors, and other integrated circuits. Below are a few example circuits.

Transistor Switch

Let’s look at the most fundamental transistor-switch circuit: an NPN switch. Here we use an NPN to control a high-power LED:

Our control input flows into the base, the output is tied to the collector, and the emitter is kept at a fixed voltage.
While a normal switch would require an actuator to be physically flipped, this switch is controlled by the voltage at the base pin. A microcontroller I/O pin, like those on an Arduino, can be programmed to go high or low to turn the LED on or off.
When the voltage at the base is greater than 0.6V (or whatever your transistor’s V_th might be), the transistor starts saturating and looks like a short circuit between collector and emitter. When the voltage at the base is less than 0.6V the transistor is in cutoff mode – no current flows because it looks like an open circuit between C and E.
The circuit above is called a low-side switch, because the switch – our transistor – is on the low (ground) side of the circuit. Alternatively, we can use a PNP transistor to create a high-side switch:

Similar to the NPN circuit, the base is our input, and the emitter is tied to a constant voltage. This time however, the emitter is tied high, and the load is connected to the transistor on the ground side.
This circuit works just as well as the NPN-based switch, but there’s one huge difference: to turn the load “on” the base must be low. This can cause complications, especially if the load’s high voltage (V_CC in this picture) is higher than our control input’s high voltage. For example, this circuit wouldn’t work if you were trying to use a 5V-operating Arduino to switch on a 12V motor. In that case it’d be impossible to turn the switch off because V_B would always be less than V_E.

Base Resistors!

You’ll notice that each of those circuits uses a series resistor between the control input and the base of the transistor. Don’t forget to add this resistor! A transistor without a resistor on the base is like an LED with no current-limiting resistor.
Recall that, in a way, a transistor is just a pair of interconnected diodes. We’re forward-biasing the base-emitter diode to turn the load on. The diode only needs 0.6V to turn on, more voltage than that means more current. Some transistors may only be rated for a maximum of 10-100mA of current to flow through them. If you supply a current over the maximum rating, the transistor might blow up.
The series resistor between our control source and the base limits current into the base. The base-emitter node can get its happy voltage drop of 0.6V, and the resistor can drop the remaining voltage. The value of the resistor, and voltage across it, will set the current.

The resistor needs to be large enough to effectively limit the current, but small enough to feed the base enough current. 1mA to 10mA will usually be enough, but check your transistor’s datasheet to make sure.

Digital Logic

Transistors can be combined to create all our fundamental logic gates: AND, OR, and NOT.
(Note: These days MOSFETS are more likely to be used to create logic gates than BJTs. MOSFETs are more power-efficient, which makes them the better choice.)

Inverter

Here’s a transistor circuit that implements an inverter, or NOT gate:

An inverter built out of transistors.

Here a high voltage into the base will turn the transistor on, which will effectively connect the collector to the emitter. Since the emitter is connected directly to ground, the collector will be as well (though it will be slightly higher, somewhere around V_CE(sat) ~ 0.05-0.2V). If the input is low, on the other hand, the transistor looks like an open circuit, and the output is pulled up to VCC
(This is actually a fundamental transistor configuration called common emitter. More on that later.)

AND Gate

Here are a pair of transistors used to create a 2-input AND gate:

2-input AND gate built out of transistors.

If either transistor is turned off, then the output at the second transistor’s collector will be pulled low. If both transistors are “on” (bases both high), then the output of the circuit is also high.

OR Gate

And, finally, here’s a 2-input OR gate:

2-input OR gate built out of transistors.

In this circuit, if either (or both) A or B are high, that respective transistor will turn on, and pull the output high. If both transistors are off, then the output is pulled low through the resistor.

H-Bridge

An H-bridge is a transistor-based circuit capable of driving motors both clockwise and counter-clockwise. It’s an incredibly popular circuit – the driving force behind countless robots that must be able to move both forward and backward.
Fundamentally, an H-bridge is a combination of four transistors with two inputs lines and two outputs:

Can you guess why it’s called an H bridge?

(Note: there’s usually quite a bit more to a well-designed H-bridge including flyback diodes, base resistors and Schmidt triggers.)
If both inputs are the same voltage, the outputs to the motor will be the same voltage, and the motor won’t be able to spin. But if the two inputs are opposite, the motor will spin in one direction or the other.
The H-bridge has a truth table that looks a little like this:

Input A	Input B	Output A	Output B	Motor Direction
0	0	1	1	Stopped (braking)
0	1	1	0	Clockwise
1	0	0	1	Counter-clockwise
1	1	0	0	Stopped (braking)

Oscillators

An oscillator is a circuit that produces a periodic signal that swings between a high and low voltage. Oscillators are used in all sorts of circuits: from simply blinking an LED to the producing a clock signal to drive a microcontroller. There are lots of ways to create an oscillator circuit including quartz crystals, op amps, and, of course, transistors.
Here’s an example oscillating circuit, which we call an astable multivibrator. By using feedback we can use a pair of transistors to create two complementing, oscillating signals.

Aside from the two transistors, the capacitors are the real key to this circuit. The caps alternatively charge and discharge, which causes the two transistors to alternatively turn on and off.
Analyzing this circuit’s operation is an excellent study in the operation of both caps and transistors. To begin, assume C1 is fully charged (storing a voltage of about V_CC), C2 is discharged, Q1 is on, and Q2 is off. Here’s what happens after that:

• If Q1 is on, then C1’s left plate (on the schematic) is connected to about 0V. This will allow C1 to discharge through Q1’s collector.
• While C1 is discharging, C2 quickly charges through the lower value resistor – R4.
• Once C1 fully discharges, its right plate will be pulled up to about 0.6V, which will turn on Q2.
• At this point we’ve swapped states: C1 is discharged, C2 is charged, Q1 is off, and Q2 is on. Now we do the same dance the other way.
• Q2 being on allows C2 to discharge through Q2’s collector.
• While Q1 is off, C1 can charge, relatively quickly through R1.
• Once C2 fully discharges, Q1 will be turn back on and we’re back in the state we started in.

It can be hard to wrap your head around. You can find another excellent demo of this circuit here.
By picking specific values for C1, C2, R2, and R3 (and keeping R1 and R4 relatively low), we can set the speed of our multivibrator circuit:

So, with the values for caps and resistors set to 10µF and 47kΩ respectively, our oscillator frequency is about 1.5 Hz. That means each LED will blink about 1.5 times per second.

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As you can probably already see, there are tons of circuits out there that make use of transistors. But we’ve barely scratched the surface. These examples mostly show how the transistor can be used in saturation and cut-off modes as a switch, but what about amplification? Time for more examples!

Applications II: Amplifiers

Some of the most powerful transistor applications involve amplification: turning a low power signal into one of higher power. Amplifiers can increase the voltage of a signal, taking something from the µV range and converting it to a more useful mV or V level. Or they can amplify current, useful for turning the µA of current produced by a photodiode into a current of much higher magnitude. There are even amplifiers that take a current in, and produce a higher voltage, or vice-versa (called transresistance and transconductance respectively).
Transistors are a key component to many amplifying circuits. There are a seemingly infinite variety of transistor amplifiers out there, but fortunately a lot of them are based on some of these more primitive circuits. Remember these circuits, and, hopefully, with a bit of pattern-matching, you can make sense of more complex amplifiers.

Common Configurations

Three of the most fundamental transistor amplifiers are: common emitter, common collector and common base. In each of the three configurations one of the three nodes is permanently tied to a common voltage (usually ground), and the other two nodes are either an input or output of the amplifier.

Common Emitter

Common emitter is one of the more popular transistor arrangements. In this circuit the emitter is tied to a voltage common to both the base and emitter (usually ground). The base becomes the signal input, and the collector becomes the output.

The common emitter circuit is popular because it’s well-suited for voltage amplification, especially at low frequencies. They’re great for amplifying audio signals, for example. If you have a small 1.5V peak-to-peak input signal, you could amplify that to a much higher voltage using a slightly more complicated circuit, like:

One quirk of the common emitter, though, is that it inverts the input signal (compare it to the inverter from the last page!).

Common Collector (Emitter Follower)

If we tie the collector pin to a common voltage, use the base as an input, and the emitter as an output, we have a common collector. This configuration is also known as an emitter follower.

The common collector doesn’t do any voltage amplification (in fact, the voltage out will be 0.6V lower than the voltage in). For that reason, this circuit is sometimes called a voltage follower.
This circuit does have great potential as a current amplifier. In addition to that, the high current gain combined with near unity voltage gain makes this circuit a great voltage buffer. A voltage buffer prevents a load circuit from undesirably interfering with the circuit driving it.
For example, if you wanted to deliver 1V to a load, you could go the easy way and use a voltage divider, or you could use an emitter follower.

As the load gets larger (which, conversely, means the resistance is lower) the output of the voltage divider circuit drops. But the voltage output of the emitter follower remains steady, regardless of what the load is. Bigger loads can’t “load down” an emitter follower, like they can circuits with larger output impedances.

Common Base

We’ll talk about common base to provide some closure to this section, but this is the least popular of the three fundamental configurations. In a common base amplifier, the emitter is an input and the collector an output. The base is common to both.

Common base is like the anti-emitter-follower. It’s a decent voltage amplifier, and current in is about equal to current out (actually current in is slightly greater than current out).
The common base circuit works best as a current buffer. It can take an input current at a low input impedance, and deliver nearly that same current to a higher impedance output.

In Summary

These three amplifier configurations are at the heart of many more complicated transistor amplifiers. They each have applications where they shine, whether they’re amplifying current, voltage, or buffering.

	Common Emitter	Common Collector	Common Base
Voltage Gain	Medium	Low	High
Current Gain	Medium	High	Low
Input Impedance	Medium	High	Low
Output Impedance	Medium	Low	High

Multistage Amplifiers

We could go on and on about the great variety of transistor amplifiers out there. Here are a few quick examples to show off what happens when you combine the single-stage amplifiers above:

Darlington

The Darlington amplifier runs one common collector into another to create a high current gain amplifier.

Voltage out is about the same as voltage in (minus about 1.2V-1.4V), but the current gain is the product of two transistor gains. That’s 2β, upwards of 1000!
The Darlington pair is a great tool if you need to drive a large load with a very small input current.

Differential Amplifier

A differential amplifier subtracts two input signals and amplifies that difference. It’s a critical part of feedback circuits, where the input is compared against the output, to produce a future output.
Here’s the foundation of the differential amp:

This circuit is also called a long tailed pair. It’s a pair of common-emitter circuits that are compared against each other to produce a differential output. Two inputs are applied to the bases of the transistors; the output is a differential voltage across the two collectors.

Push-Pull Amplifier

A push-pull amplifier is a useful “final stage” in many multi-stage amplifiers. It’s an energy efficient power amplifier, often used to drive loudspeakers.
The fundamental push-pull amp uses an NPN and PNP transistor, both configured as common collectors:

The push-pull amp doesn’t really amplify voltage (voltage out will be slightly less than that in), but it does amplify current. It’s especially useful in bi-polar circuits (those with positive and negative supplies), because it can both “push” current into the load from the positive supply, and “pull” current out and sink it into the negative supply.
If you have a bi-polar supply (or even if you don’t), the push-pull is a great final stage to an amplifier, acting as a buffer for the load.

Putting Them Together (An Operational Amplifier)

Let’s look at a classic example of a multi-stage transistor circuit: an Op Amp. Being able to recognize common transistor circuits, and understanding their purpose can get you a long way! Here is the circuit inside an LM3558, a really simple op amp:

The internals of an LM358 operational amplifier. Recognize some amplifiers?

There’s certainly more complexity here than you may be prepared to digest, however you might see some familiar topologies:

• Q1, Q2, Q3, and Q4 form the input stage. Looks a lot like an common collector (Q1 and Q4) into a differential amplifier, right? It just looks upside down, because it’s using PNP’s. These transistors help to form the input differential stage of the amplifier.
• Q11 and Q12 are part of the second stage. Q11 is a common collector and Q12 is a common emitter. This pair of transistors will buffer the signal from Q3’s collector, and provide a high gain as the signal goes to the final stage.
• Q6 and Q13 are part of the final stage, and they should look familiar as well (especially if you ignore R_SC) – it’s a push-pull! This stage buffers the output, allowing it to drive larger loads.
• There are a variety of other common configurations in there that we haven’t talked about. Q8 and Q9 are configured as a current mirror, which simply copies the amount of current through one transistor into the other.

After this crash course in transistors, we wouldn’t expect you to understand what’s going on in this circuit, but if you can begin to identify common transistor circuits you’re on the right track!

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Transistor history

Who Invented the Transistor?

On reading my recent @CHM blog “Who invented the diode?” CHM senior curator Dag Spicer pointed me to a fascinating scholarly treatise, “Singletons and Multiples in Scientific Discovery: A Chapter in the Sociology of Science,” that describes how multiple independent discoveries of scientific phenomena are the norm rather than the exception. The author, Robert K. Merton, traces this understanding back to Elizabethan philosopher, statesman, and scientist Sir Francis Bacon.

Merton also paraphrases Bacon’s observation that “once the right path is followed, discoveries in limitless number will arise from the growing stock of knowledge.” This pattern was readily apparent in the history of the diode. And as this blog describes, it was repeated in the development of the next great leap forward in semiconductor devices, the transistor.

Early in the last century scientists knew how to make a two terminal diode by placing a sharp metal probe in contact with a semiconductor crystal. These point-contact diodes could change an oscillating signal to a steady signal and found wide use as detectors in crystal radio receivers. By the 1920’s inventors began to investigate the use of semiconductors for amplifying and switching signals.

Early semiconductor amplifiers

Some of the earliest work on semiconductor amplifiers emerged from Eastern Europe. In 1922-23 Russian engineer Oleg Losev of the Nizhegorod Radio Laboratory, Leningrad, found that a special mode of operation in a point-contact zincite (ZnO) crystal diode supported signal amplification up to 5 MHz. Although Losev experimented with the material in radio circuits for years, he died in the 1942 Siege of Leningrad and was unable to advocate for his place in history. His work is largely unknown.

Austro-Hungarian physicist, Julius E. Lilienfeld, moved to the US and in 1926 filed a patent for a “Method and Apparatus for Controlling Electric Currents” in which he described a three-electrode amplifying device using copper-sulfide semiconductor material. Lilienfeld is credited with inventing the electrolytic capacitor but there is no evidence that he built a working amplifier. His patent, however, had sufficient resemblance to the later field effect transistor to deny future patent applications for that structure.

Oleg V. Losev (1903 – 1942)

Julius E. Lilienfeld (1882 –1963), Courtesy of AIP Emilio Segre Visual Archives

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German scientists also contributed to this early research. While working at Cambridge University, England in 1934, German electrical engineer and inventor Oskar Heil filed a patent on controlling current flow in a semiconductor via capacitive coupling at an electrode – essentially a field-effect transistor. And in 1938, Robert Pohl and Rudolf Hilsch experimented on potassium-bromide crystals with three electrodes at Gottingen University. They reported amplification of low-frequency (about 1 Hz) signals. None of this research led to any applications but Heil is remembered in audiophile circles today for his air motion transformer used in high fidelity speakers.

The first transistors

Because of their poor reliability and large power consumption, by the late 1930’s engineers at American Telephone and Telegraph knew that vacuum-tube circuits would not meet the company’s rapidly growing demand for increased phone call capacity. Bell Laboratories’ director of research Mervin J. Kelly assigned William Shockley to investigate the possibility of using semiconductor technology to replace tubes.

Using improved semiconductor materials developed for radar detectors during the war, in early 1945 Shockley experimented with a field-effect amplifier, similar in concept to those patented by Heil and Lilienfeld, but it failed to work as he intended. Physicist John Bardeen suggested that electrons on the semiconductor surface might be blocking penetration of electric fields into the material. Under Shockley’s direction, together with physicist Walter Brattain, Bardeen began researching the behavior of these “surface states.”

John Bardeen, William Shockley and Walter Brattain in 1948, Courtesy of Bell Telephone Laboratories

On December 16, 1947, their research culminated in a successful semiconductor amplifier. Bardeen and Brattain applied two closely-spaced gold contacts held in place by a plastic wedge to the surface of a small slab of high-purity germanium. On December 23 they demonstrated their device to lab officials and in June 1948, Bell Labs publicly announced the revolutionary solid-state device they called a “transistor.”

Early that same year, while examining a phenomenon he called “interference,” German physicist Herbert Mataré and his colleague Heinrich Welker independently fabricated an amplifier based on germanium with two point-contacts touching its surface at a Westinghouse laboratory in Paris, France. When they learned of the Bell Labs’ announcement, Mataré and Welker applied for patents on their own device, which they called a “transistron.”

Improving the transistor

Realizing that the point-contact structure had serious limitations, and spurred by professional jealousy as he resented not being involved with its discovery, Shockley worked alone to conceive a more reliable and reproducible device. Introduced in 1952, Shockley’s bipolar junction transistor, which was made from a solid piece of semiconductor material and no point contacts, dominated the industry for the next 30 years. All three Bell Labs scientists received the 1956 Nobel Prize in Physics for their contributions.

Over the next decade many different manufacturing methods were developed to produce faster, cheaper and ever more reliable transistors. An important advance in 1954 was the silicon transistor, first from Morris Tanenbaum at Bell Labs and shortly after by a team led by chemist Willis Adcock at upstart Texas Instruments. By the end of the 1950s, silicon had become the industry’s preferred material and TI the dominant semiconductor vendor.

The 1954 Texas Instruments’ silicon-transistor team: W. Adcock, M. Jones, E. Jackson, and J. Thornhill, Courtesy of Texas Instruments, Inc.

The founders of Fairchild Semiconductor, a start-up in California’s Silicon Valley, started their company on the premise of making an even better silicon transistor. Their day-to-day challenges in developing a new technology are described in detail in several of the Fairchild Patent Notebooks in the Museum collection, notably those written by Gordon Moore and Sheldon Roberts. Coinciding with the beginning of the “space race,” their 1958 introduction of a double-diffused silicon mesa transistor was a great commercial success. Reliability problems that threatened the future of the company were resolved with Swiss physicist Jean Hoerni’s revolutionary planar process. Hoerni’s planar technique not only transformed transistor manufacturing from a semi-hand-crafted operation to high volume automated production. It also enabled the development of the modern integrated circuit (IC).

The MOS Transistor

The ideas of Lilienfeld and Heil and Shockley’s failed early experiments finally bore fruit in 1959 when, working for Egyptian engineer Martin M. (John) Atalla on the study of semiconductor surfaces at Bell Labs, Korean electrical engineer Dawon Kahng built the first successful field-effect transistor (FET) comprising a sandwich of layers of metal (M – gate), oxide (O – insulation), and silicon (S – semiconductor). The MOSFET, popularly shortened to MOS, promised a significantly smaller, cheaper, and lower power transistor.

Martin M. Atalla (1924 – 2009), Courtesy of the Atalla Family

Dawon Kahng (1931 – 1992), Courtesy NEC Corporation

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Fairchild and RCA introduced commercial MOS transistors in 1964. But in the decade that it took to resolve early manufacturing problems with the MOS process, individual transistors had largely been replaced by ICs in computer systems.  In the long term, MOS transistors proved the most practical approach to building high-density ICs, such as microprocessors and memories. Close to 100% of the billions of transistors manufactured every day are MOS devices.

As with most technological developments, the creation of the modern transistor followed the Baconian pattern of gradually emerging from “the growing stock of knowledge” built by a truly international cast of engineers and scientists rather than from the lone efforts of a single heroic “inventor.”

 

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