Transistors

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).

Images of transistors

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.
Switches – Application circuits showing how transistors are used as electronically controlled switches.

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:

NPN and PNP symbols

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:

Transistors as two diodes

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 NPN construction

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).

Active transistor current flow

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.

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.

water analogies for current, voltage, and resistance

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.

Valve open, water flowing, short circuit

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.

valve closed, water stopped, open circuit

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.

Valve half-open, water flow rate controlled

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.

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:

Mode Quadrants

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.

Saturation mode model

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.

Saturation mode voltage relations

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.

Cutoff mode model

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.

Cutoff mode voltage relations

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.

$V_{C} > V_{B} > V_{E}$

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):

$I_{C} = \beta I_{B}$

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

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:

$I_{C}= \alpha I_{E}$

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:

$\beta=\frac{\alpha}{(1-\alpha), alpha=\frac{\beta}{\beta+1}$

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).

$V_{C} < V_{B} < V_{E}$

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
$V_E < V_B < V_C$	Active	Reverse
$V_E < V_B > V_C$	Saturation	Cutoff
$V_E > V_B < V_C$	Cutoff	Saturation
$V_E > V_B > V_C$	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.

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!

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:

NPN switch to control an 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:

PNP switch example

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.

Switching an LED with a transistor

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.

These gates (which we’ll talk more about in the next class) form the basis of operations on Boolean values, which you’ll be familiar with from both C (where they’re written as &&, ||, and !) as well as Scheme (where the operators are the more intellegible and, or, and not).

In circuits for digital logic, a (continuous) voltage is used to represent the (discrete) Boolean values you are more accustomed to seeing in programs. One possible convention might be that 0V represents “off” or false, while +5V represents “on” or true. In the following, we look at how to use transistors in circuits to code these values and combine them with the typical Boolean operators, 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:

BJT inverter circuit

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:

BJT AND circuit

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:

BJT OR circuit

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.

This derivative work is used and licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license. The original Transistors is by Jimb0.