Providing amazing functionality in almost all aspects of electronics, transistors are powerful yet fairly simple to use.
The Classroom this month continues delving into the fundamental knowledge of common components that many makers will benefit from understanding. In this issue, we are looking at the humble transistor, which is one of the most fundamental components in modern electronics.
A transistor is a device which takes a small current as an input, and uses it to control a much larger current's behaviour - acting as an amplifier, or even a switch.
We will concentrate purely on bipolar transistors in this issue. The family broadly referred to as Field Effect Transistors, or FETs, with an extended range of sub-types, will be explored separately in a future edition.
WHAT IS A TRANSISTOR?
The transistor is the basis not only of many electronic circuits, but also the basis of most integrated circuits as well. Most use anywhere from a dozen through to many thousands of transistors. Put simply, the transistor is a combination of three pieces of semiconductor material, arranged like two diodes pushed together, with a common semiconductor material between them. When a small current is fed into the middle of the two diodes, one becomes forward biased. Because of the close proximity of the second diode it also conducts, allowing a larger current to flow across the three semiconductor layers. The end effect is that the transistor can either amplify or switch; more on this later.
It would be nice to assume everyone has read the article on diodes, where PN junctions were explained, but of course that isn't always the case. Here is a brief recap:
Most semiconductors are based on elemental silicon, with impurities of selected materials added to change the properties of the silicon. This results in either an “N” or a “P” type material. There is considerable science behind what this means at an atomic level but essentially “N” type materials have an excess of electrons, while “P” type materials have a deficiency of electrons. When they are joined, the electrons move to balance the ionisation, and an insulator is formed, which is called the “barrier region”. This is microscopic in size, and the materials must be in the closest contact, so a slice of “P” type material cannot simply be glued to a slice of “N” type material.
NOT JUST A PAIR OF DIODES?
Joining two diodes together does not form a transistor. To make a transistor, the materials must be arranged with their PN junctions opposed so, in fact, we have either PN facing NP, (PN>NP) or NP facing PN, (NP>PN). The region in the middle must be very small indeed, with the common region being microscopically thin. Transistors, therefore, have three layers known as PNP or NPN. In reality, it is not two separate junctions joined, but three pieces of material joined to form two junctions. It may help to remember it is the forward biased junction that turns the transistor on, so in an NPN, one PN junction is designed to turn on like a forward biased diode, and this is given an arrow symbol to show whether it is (N)PN  or (P)NP .
With three layers, each transistor requires three connections, which are called the Collector, Base, and Emitter. The emitter always has an arrow head indicating whether it is an NPN or PNP type. Some have learned to identify NPN symbols from PNP by remembering the maxim, “NPN = Never Points North”.
THE NUTS AND BOLTS
Okay, we’ve covered how transistors are made and arranged, let’s now examine how they’re used. All PN junctions have a “barrier voltage”. When a forward biased potential difference (the technically correct, but not-often-heard term for what we usually call “voltage”) that exceeds that barrier voltage is applied, the PN junction conducts.
In most general purpose bipolar transistors, the barrier voltage is ~0.6V. When the base of an NPN transistor is raised to about 0.6V to 0.65V, forward biased relative to the emitter, the PN junction begins to conduct, just as it would as a diode. If the collector is also raised to a more positive voltage (i.e., a positive potential difference across the collector-emitter terminals), current will conduct through the collector to the emitter. That current can be much larger than the one flowing across the base-emitter circuit, and so an amplifier has been formed.
The NPN transistor  shows a practical circuit that is an ideal first circuit, recommended for any maker to experiment with. It uses a common BC337 transistor, although any NPN transistor can be used to test this circuit and theory.
|4 x AA Battery Pack With Switch||PH9282||S5043|
|1 x 1k0 Potentiometer||RP3504||R2202|
|1 x BC337 Transistor||ZT2112||Z1035|
|1 x BC327 Transistor||ZT2110||Z1030|
|1 x 6.3V MES Globe||SL2654||S4045|
|1 x MES Base||SL2659||S4057|
You’ll also need standard prototyping equipment like a breadboard and jumper leads.
The circuit in these Classroom series of experiments require a small lamp be connected between the positive of a 6V (e.g., 4 x AA batteries) power supply and the collector. The emitter is connected to the negative of the power supply. A 1k0 variable resistor is connected with its outside pins to the positive and negative rails of the power supply, and its variable pin to the transistor base. Before connecting the batteries, or with the power turned off, turn the pot so the wiper turns toward the negative-connected terminal of the pot.
Turn on the power and measure the voltage across the collector and emitter. It should be almost the same as the battery voltage. Now turn the pot until the voltage across the collector and emitter begins to fall. This indicates that the base emitter junction has begun to turn on. Continue increasing the “bias” (i.e., the voltage on the base), until the voltage across the collector emitter is very low and no longer changing – or at least, not changing much. Now the transistor is fully on. Measure the voltage across the base and emitter; it should be around 0.62V to 0.67V.
Congratulations! You have made a transistor switch, and amplifier.
WHAT ABOUT PNP?
Why do we even need PNP types? Some jobs require the opposite polarity. A diode can be easily reversed, but a transistor cannot be reversed. It needs to swap P and N layers, so they become PNP instead on NPN. Some circuits require both, but that’s a story for another day.
For PNP transistors, everything is mirrored; it’s like looking at the circuit upside down, but with the same power supply. The diagram in  shows the PNP version of . You can see that the base-emitter junction still has a forward bias.
The experiment given for NPN can also be used to test the operation of a PNP. Just remember to follow the arrows as current flows in the direction of the arrows; electrons go against the current.
So, what can you actually do with a transistor? Amplifying a signal is not just the preserve of audio circuits, although they are a classic use of transistors, and one of the earliest uses. Radio frequency amplifiers, as well as instrument amplifiers are equally, if not more common.
There are many cases where you may wish to use a small circuit to drive a larger current. Many makers drive motors with their circuits, and transistors are integral to this. The ubiquitous H-bridge is an arrangement of transistors, although how it works is beyond the scope of this article, for now.
In this month’s Fundamentals, we have presented a series of sensor circuits, which take a small signal and make it usable. Many of these circuits use transistors, and many more use integrated circuits, which happen to be full of transistors!
The Supersize Me article had an LED bar graph, which used PNP transistors to simultaneously invert the signal from a chip that gave a low signal when we wanted a high, and to drive the LED arrays with that signal. Transistors are, in effect, a current amplifier that can be switched hard on.
So far, we have presented the transistor as an abstract concept, but of course in the real world they are a physical thing, which requires a package for mechanical protection. A tiny piece of silicon crystal, doped with often-toxic elements would not be a nice thing to handle. Transistors are encased in a plastic, metal or ceramic case to protect it mechanically, chemically and electrically. A standard case may have alternate pin connections however, which can cause confusion. Always check the pinout data.
There are several common packages available for retail sale. Many others exist but are not commonly found in maker applications, and if you are already encountering them then you probably already know all you need to know about this!
Generally, the only things printed on a transistor are part number, a manufacturer’s logo or code, and a batch number. The part numbers, again sadly, are not always written the same way. For example, a BC548, which is a common transistor in a TO92 package, may be labelled as 548, BC548, C548, ST548, or something else. This is because manufacturers sometimes have their own way of writing things on cases. When shopping recently, three different codes were present in one draw of BC548s at an electronics retailer, and anyone who works in the retailing of these goods knows there are many more. Unfortunately, there is no rule to figure it out, but the information usually becomes clear with a little practise and some consideration of what is in your hands. Having said this, and again as anyone who has retailed these will tell you, the full part number is the most common thing to find, even if it is split across two lines.
As always, the manufacturers’ data sheets are the best source of information for the exact parameters of your transistor. VCEO-max is used to give the maximum voltage rating a transistor can stand, between collector and emitter, and with the base open-circuit (not connected). Be careful as the reverse voltage is often not included, and is also often quite low, below 10V for many. It is not normal use to operate a transistor with a reversed voltage applied. However, the maximum voltage that can be applied to the base-emitter junction is often in the data, and is important, particularly if you are making an inverter circuit.
Maximum current is also included, and refers to the current that can flow between the collector and emitter. You will need to know the base current to drive a transistor; however, this is also often not stated. What is given is the “current gain”, which is labelled as hFE. This is actually not a number in amperes or milliamperes, but a ratio of collector current and base current. It is given as a minimum and maximum range, because of manufacturing considerations, which is not actually as unhelpful as it sounds. Other data present will be connections, temperature limits, measurements, and a bunch of stuff that most makers, and indeed some engineers, will never need to use.
A NOTE ABOUT CASES: The size of the case is generally a good indication of the ruggedness of a device. A TO92 package, for example, will likely be a low current device, whereas something in a TO3P package will likely handle quite a large current. All but the smallest packages can mount to a heat sink, and this will be needed when running these products to their design limits. In fact, it is even possible to heat sink a TO92, although a way of clamping it to the heat sink must be employed, as no bolt-hole is present on this package.
So, you have read about transistors and their basic operation, and you’re now probably thinking, “Great, but what can I make with them?” You could start with some of the sensors in this month’s Fundamentals article. You could also breadboard the basic demonstrator circuits we used earlier to explain the concept – adjusting the variable resistor to change the bias in the base, therefore causing a change in the current in the load, and see what effect you can get – you may even have a basic light dimmer! It’s a good idea to use cheap transistors, such as the BC5XX series for this, as they do the job but do not cost the earth when you let the smoke out of them (where BC5XX designates a lot of part numbers such as BC548, BC549 or BC559).