When one poses the question what is a transistor or how does a transistor work one can literally find tons of material that answers those questions and gives more detail than perhaps one would care for.
Indeed, many books exist on the subject of transistors, semiconductor physics, transistor characteristics, and their operation. They range from the beginner level to the engineering level and even on to cutting edge research and new breakthroughs in the design of transistors and semiconductors.
Because of this, the answers one will find to the questions above (and similar ones) range from the simplistic to the extremely complicated.
Even while ignoring engineering-level concepts concerning transistors it’s absurd to think that all the things a hobbyist should know about them can appear in one article.
Toward that end, this will be the first post on the ubiquitous transistor and the first of many others that will show up in the future.
If you’re new to electronics or need to brush up on semiconductor basics I strongly suggest you read How Diodes Work – An Introduction. This post will cover some basic semiconductor principles if you’re unfamiliar with the topic.
I’m going to take three things as a given before we dive into the meat of the post.
- People of all levels of experience — from complete novice to electronics guru — will read this.
- There is a crap-load of things to say and learn about transistors.
- I’m going to assume you know nothing about transistors and start with very simple, basic information in this post.
In that vein, this introductory post on how transistors work will start with some history of the transistor (and what was in use before it), the main types of transistors a hobbyist will see, and a mile-high overview of what transistors are used for and how they work. In the future we’ll go into a lot more detail on the individual types of transistors and their uses.
What is a Transistor?
The Land Before Transistors
Before the transistor the device of choice (perhaps because it was the only choice) was the vacuum tube.
These days vacuum tubes are pretty much obsolete, though they still do find their way into some niche applications. In fact, certain audiophiles swear vacuum tubes give sound a warmer, richer quality than those omnipresent transistors.
The structure of a basic tube is as follows…
A glass container encloses the vacuum tube, sealing it. As you may guess the tube contains no air, hence the name vacuum tube. The parts of a basic tube are the heater, cathode and anode. Electrical connections to these parts are made through the base of the container.
Figure 1 depicts a variety of vacuum tubes.
Figure 1: an assortment of vacuum tubes.
Unfortunately, vacuum tubes have a few drawbacks.
First, they are fragile and break easily. Shock or vibration can kill a tube, so you need to handle them with care.
Second, they slurp up watts like a ravenous man coming off a hunger strike. The heater requires a great deal of power which makes them unsuitable for applications requiring battery power.
Their size is another disadvantage. Early computers, composed of vacuum tubes, would fill a large room (in addition to consuming more than a hundred kilowatts of power). Today we can cram all that computing power on a single, inexpensive chip. Transistors are the basic building blocks of such chips. This is possible because transistors can be made much smaller than tubes.
And finally, the tube heater has a finite life span. This means the tubes needed to be replaced regularly. Modern transistors are much more robust.
History of the Transistor
In 1948 William Shockley, Walter Brattain, and John Bardeen were able to produce the first working point-contact transistor at Bell Telephone Laboratories.
Unfortunately, the original point-contact transistors made by Shockley and company were extremely unreliable. It would take another twelve years — until 1960 — to make transistors more rugged and viable for use in commercial products. A transistor many of us are still familiar with today – the bipolar junction transistor or BJT – had arrived.
In the early 1960s, many electronics manufactures started replacing vacuum tubes with transistors, especially in low-power and low-frequency applications.
Around the same time Robert Noyce found that more than one transistor could be constructed on a single piece of semiconductor material. The integrated circuit, or IC, was born.
Research and development into semiconductors later yielded another type of transistor many of us are familiar with – the field effect transistor or FET. These (especially the MOSFET) became viable in the mid-1960s and started showing up in products late in that decade.
Since then, electronics has never been the same. Many of the electronic devices we take for granted today are only possible because of the transistor. Everything from your smartphone, laptop, TV, kitchen appliances, IoT devices, audio equipment and more exist in the form they do today because of transistors.
What is a Transistor? Two Main Transistor Types
The Bipolar Junction Transistor (or BJT)
For our purposes, we’re going to talk about the two most common types of transistors that you’ll see.
The first type is the bipolar junction transistor, which we’ll refer to as the BJT from here on.
BJTs were the original large-scale production transistor and are robust.
They sport three terminals: the base, collector, and emitter.
Refer to figures 2 and 3 for the following discussion. Current flowing into the base controls the amount of current flowing from the collector to the emitter. If the base current is below a certain threshold, the transistor is in cutoff mode and no current flows in the collector. Think of this as an open switch.
Figure 2: an NPN BJT along with its schematic symbol. When no current flows into the base, the transistor acts like an open switch. With the proper amount of current flowing into the base, current flows from the collector to emitter, essentially shorting them out, like a switch with closed contacts.
When the base current increases, the BJT enters the active region where the collector current is proportional to the base current by a gain factor (a.k.a. beta). For example, if the base current is 10 mA the collector current may be 10 times that (or some other gain factor). If the base current continues to increase, the collector current reaches a maximum value and stops increasing. The BJT is now in saturation mode. Think of saturation mode like a closed switch.
Figure 3: the current through the BJT is proportional by some gain factor to the current flowing into the base. So, a little current flowing into base yields a greater current flowing from the collector, through the transistor to the emitter.
For now, that’s all we’re going to say about the operation of the BJT.
There are two types of BJTs: NPN and PNP. The names have to do with their semiconductor junctions (this is a good time to stop and read the post on diodes if you need to).
An NPN BJT sandwiches P-type semiconductor material between two layers of N-type semiconductor material. Take a gander at figure 4 if you need help visualizing this.
Figure 4: an NPN BJT has P-type material between two layers of N-type material.
The schematic symbol for a PNP transistor is slightly different. Figure 5 depicts the schematic symbol for both NPN and PNP BJTs. You may be guessing that a PNP BJT sandwiches N-type material between two layers of P-type material, sort of the opposite of an NPN transistor. If so, you’re right. Think of figure 4 and swap the N-regions with the P-regions to visualize a PNP BJT.
You’ll use NPN transistors more often than the PNP version, but it’s important to be aware of both types.
Figure 5: schematic symbols for the 2 types of BJTs.
The second common transistor type likely to cross your bench is the MOSFET.
Like the BJT, the MOSFET is a three terminal device. However, the terminals on a MOSFET have different names than those of the BJT.
Before we give the names of the MOSFET’s three terminals it may be useful to know what MOSFET stands for.
MOSFET stands for metal oxide semiconductor field effect transistor. A MOSFET is a type of FET or field effect transistor. There are other less common types of field effect transistors out there such as the JFET (junction field effect transistor), but for now we’re just going to talk about MOSFETs.
The terminals on a MOSFET are the gate, drain, and source.
Figure 6: a typical MOSFET.
The gate usually appears on the left pin if the transistor is facing you with the drain in the middle and the source on the right. The gate of the MOSFET turns it on and off just the like base of a BJT turns it on and off.
And, like the BJT, MOSFETs come in two flavors: n-channel (the schematic symbol in figure 7 depicts an n-channel device) and p-channel. However, there is a big difference between the base of a BJT and gate of a MOSFET. While current entering the base controls the BJT, voltage on the gate controls the MOSFET. This gives the MOSFET an advantage: it consumes very little current (like nano or pico amps which is just leakage current), and thus power while BJTs consume more.
There are other advantages, too. MOSFETs can be made smaller and they are relatively easy to manufacture. Plus, they have a high input impedance.
A disadvantage is that MOSFETs can be sensitive to static electricity, so you need to take care when handling them. An antistatic wrist strap helps with this.
Figure 7: a typical n-channel MOSFET and its schematic symbol. If this were a p-channel MOSFET, the arrow would be pointing away from the gate instead of towards it. The schematic symbols for MOSFETs may appear slightly different depending on what the source is, but it’s easy to tell them apart either way.
Figure 8: for further clarification, we can see 2 different schematic symbols for the same n-channel enhancement type device. The substrate is just a fancy term for the main body of the transistor.
There is yet another important consideration when choosing MOSFETs. In addition to being n-channel or p-channel type, they can also be either depletion-mode or enhancement-mode MOSFETs. This gives us a total of four possible MOSFET types: n-channel depletion-mode, p-channel depletion-mode, n-channel enhancement-mode, and p-channel enhancement mode.
The fact that there are four choices sounds bewildering, but the enhancement-mode MOSFETs are more common than the depletion-mode version. And, as is a similar case with BJTs, the n-channel MOSFETs are more common than the p-channel variety. But remember, things exist for a reason, and since all MOSFET varieties have their special little place in the world of electronics it’s important to be aware of all four MOSFET types.
Since this is an introductory treatment of transistors, we won’t go into any more detail on enhancement vs depletion mode MOSFETs here.
How MOSFETs are Made: Their Structure
While N-type and P-type semiconductor material compose MOSFETs, their structure is a bit different than that of a BJT. Refer to figures 9 and 10 for the following discussion.
Figure 9: basic structure of n-channel and p-channel enhancement MOSFETs with their schematic symbols. These symbols include the substrate and a 4th terminal, but don’t let that confuse you.
Like the BJT, we can see that the different types of semiconductor material form a sandwich – sort of. A closer look reveals that the substrate (or body), whether N-type of P-type, spans the whole length of the device unlike the material in a BJT.
Also, unlike the BJT, there is no base. Instead, the gate is a very thin layer of silicon dioxide. This is what makes MOSFETs vulnerable to static – high voltage can punch a hole right through the thin gate destroying the transistor. But this design has some pros; it offers a high input impedance (it’s essentially a capacitor) and enables voltage on the gate to control the device.
On top of the gate, source, and drain lie metal contacts for the electrical connections.
Let’s take a look at a cross section of one of these babies.
Figure 10: cross section of the structure of a MOSFET.
Figure 10 shows us a nifty 3D view of the cross section of an n-channel enhancement type MOSFET on top and a flat view on the bottom. The n+ indicates heavily doped N-type material. The device is fabricated on a P-type substrate which is a single crystal silicon wafer. The n+ regions are created in the substrate. Then a thin layer of silicon dioxide (a great insulator) is deposited on the substrate’s surface. Metal is then placed on top of that to form the electrical connection. Metal also goes on the drain and source regions for electrical connections.
For the purposes of this introductory article, that’s all we need to know about the structure of the MOSFET for now.
Transistor Operation: Two Modes
We’ve already seen how the BJT finds use in two different modes. They can be used as an amplifier or as a switch. This depends on whether it’s cutoff, in active mode, or saturation.
We already know that to use a BJT as a switch we need to run it in cutoff (turns the “switch” off) or saturation (turns the “switch” on).
If we want to use a BJT as an amplifier, we need to run it in active mode. Amplifier design is a whole other huge subject on its own, so that’s all were going to say about it here.
Similarly, the MOSFET has the same uses. They can be used as either a switch or an amplifier.
MOSFETs operate in a cutoff and a saturation region, but, unlike the BJT, the third region is the triode (or ohmic, as some people call it) region. To use it as a switch, we need to operate the MOSFET in the cutoff and triode regions. If we want to use it as an amplifier, we need to operate it in saturation mode. This can be confusing because while the three modes or regions of operation between a BJT and a MOSFET are similar, they don’t map to each other perfectly.
In digital circuits, MOSFETs are used as switches. In fact, modern digital ICs including microcontrollers and microprocessors employ MOSFETs (along with capacitors, resistors, and other parts) as their most basic building block.
BJTs aren’t used in most modern digital circuits because of their power requirements and other reasons. However, TTL or transistor to transistor logic chips, which use BJTs, are still around and make great tools for learning, experimentation and prototyping. And, while MOSFETs dominate the digital world, BJTs dominate the analog world finding a home in op amps, amplifiers, audio, motor control, radio, and much more.
How Transistors Work – Wrapping Up
We’ve now answered some basic questions.
We know that vacuum tubes were in use before transistors and we have a little transistor history.
We also answered the questions what is a transistor? and how does a transistor work?
The basic structure and operation of the two most common transistors you’re going to see – BJTs and MOSFETs – were discussed.
But there is much more to say about transistors, as was hinted in the beginning of the post.
Future articles will dive a bit deeper into a particular transistor type, use, or mode of operation.
Until then, drop me a comment and let me know what types of transistors you find yourself using most often. Or just let me know if you have any questions about transistors. Maybe I’ll answer it in the next transistor related article.
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