What is a transistor and how does it work?

Transistors are the key component of modern electronics, they are the building block of digital electronics and amplifiers as well as many other applications. They are one of the most important electronic components of all time.

Unfortunately transistors are often taught very poorly at university with heavy emphasis on quantum mechanics, instead of simple physics principles, which ruins the sheer beauty of these devices, in this post you will see this is absolutely unnecessary and hopefully will have a greater appreciation of just how amazing transistors really are.

Transistors are a 3 terminal component and like diodes they are also semiconductors. Fundamentally transistors are used to amplify and switch electronic signals.

Let's take a look at what is known as a BJT or Bipolar Junction Transistor which looks as shown below:

In the above B, C and E refer to the Base, Collector, and Emitter terminals respectively. The NPN denotes the type of the BJT transistor, we will discuss all of these in the section below on Transistor Action - how a transistor works.

Transistor Action

To describe how transistor's work, it is best to start with a picture! Shown below is the structure of a transistor at the semiconductor level:

From above we can see that a transistor is composed of 3 semiconductor pieces, an N, a P and an N. Similar to Diodes, these refer to semiconductors being doped, and so it is an N-type followed by a P-type then an N-type connected together, hence the name NPN. There is also a PNP configuration. For now we are showing them slightly separated before showing what happens once connected.

Recall that by N-type we simply mean an impurity was added to the semiconductor that has more electrons than the semiconductor atoms.

Note also that the P-type semiconductor is much thinner than the other two, this is fundamental to transistor action which we will discuss below.

The diagram above shows a transistor with the Collector, Base and Emitter terminals shown. Note that the Collector section is the widest, followed by the emitter then the base. The Base and Collector and Base and Emitter form two PN junctions.

In terms of doping the emitter is the most heavily doped, followed by the collector and the base is lightly doped.

The two pn junctions are like two diodes. We can say that we have a collector-base diode and a emitter-base diode. The emitter-base diode is forward biased and the collector-base diode is reverse biased. The electron flow is as shown below:

So where is the magic? The magic is that the vast majority of electrons enter the emitter and pass through to the collector and only the minority recombine with holes in the thin base. Hence the names, electrons are emitted from the emitter and then collected by the collector. Why does this happen? Due to the base being so narrow and more lightly doped, the vast majority of the electrons diffuse through to the collector and do not recombine with the holes in the base and so this is a fundamental part of the design of a transistor. The wider collector width also has a role to play in this, but that's more than we need to know about how this all works, it really is this simple!

In mathematical terms we have typically 95% of the emitter current flowing through to the collector and 5% through the base. This is Transistor Action folks, a small base current and a large emitter/collector current, if we increase the base current we get a proportional gain in the collector, i.e. our base current is amplified.

Regarding the switching operation, if we do not have a base emitter voltage around 0.7V then we have no current flow through the collector and emitter, which is analogous to an open switch, likewise if we have a base emitter voltage around 0.7V then we have current flow through the collector and emitter which is analogous to a closed switch. This allows us to turn an LED on/off from a microcontroller output for example.

Before we wrap up never forget this amplification comes from somewhere and that somewhere is the voltage applied to the collector, the genius however, is that this device allows us to reproduce input signals provided at the base of a BJT with a larger amplitude at the output, the simplest example is in audio, at the output we would have a larger audio signal which would have a louder volume.

We will take an extensive look into amplifiers and even build our own high quality audio amplifier in the Analog Electronics course!