Transistors, How do they work?

May 4, 2019

The invention of transistors revolutionized human civilization like no other technology. At the heart of a smartphone lies a processor, and this processor holds around 2 billion transistors (Fig:1). What do these incredibly tiny devices do? How do they work?

Fig:1 Billion transistors inside the processor

Introduction of Transistor

Transistors can act like a switch with no moving parts. They can amplify a weak signal. In fact, amplification is the basic function of a transistor. First, let’s understand the basis of transistors. We will come back to the application part later

Transistors are made of semiconductors such as silicon. Each silicon atom is bonded with 4 neighboring silicon atoms (Fig:2). Silicon has 4 electrons in its valence shell. Each hand holds one electron. Each one of these electrons goes for sharing with a neighboring silicon atom. This is known as a covalent bond. Currently, the electrons are in their valence band. If the pure silicon has to conduct electricity the electrons have to absorb some energy and become free electrons. Thus, the pure silicon will have a low electrical conductivity.

Fig:2 Silicon atom

A technique called doping is used to improve the conductivity of semiconductors. For example, say you inject phosphorus with 5 valence electrons. Here 1 electron will be free to move in the system. This is known as N-type doping. (Fig:3)

Fig:3 N-type Doping

On the other hand, if you inject boron with 3 valence electrons, there will be a vacant position for an electron. This vacant position is known as a hole, and a neighboring electron can fill this hole at any time. This electron movement is visualised as holes moving in opposite direction. We will call this P-type doping (Fig:4).

Fig:4 P-type Doping

Working of Diode

If you dope a silicon wafer in the following manner, a transistor is born. But if you really want to understand how a transistor works, we have to get a clear idea of what happens at the electron level of a more basic component, a diode.

Something very interesting happens at the boundary of the N and P joint. The abundant electrons on the N side will have a natural tendency to migrate to the holes that are available on the P side (Fig:5). This will make the P side boundary slightly negatively charged and the N side slightly positively charged. The resulting electric field will oppose any further natural migration of the electrons.

Fig:5 Depletion layer at P-N Junction

If you apply an external power source across the diode as shown, the power source will attract the electrons and holes. Electricity flow is impossible in this case.

However, if you reverse the power connection, the situation is quite different. Assume that the power source has enough voltage to overcome the potential barrier. You can immediately see that the electrons will be pushed away by the negative terminal. When the electrons cross the potential barrier, they will be drained of energy and will easily occupy the holes in the P region. But due to the attraction of the positive terminal, these electrons can now jump to the holes nearby in the P region and flow through the external circuit. This is known as the forward biasing of a diode (Fig:6B).

Fig:6A Reverse biasing
Fig:6B Forward biasing

Just keep this simple principle of a diode in mind, you will understand the operation of a transistor very easily.

Working of Transistor

Note that the P layer is really thin and lightly doped. You can easily see that the transistor is essentially 2 diodes sandwiched back-to-back. So, whichever way you connect the power source, 1 diode will always be reverse-biased and block the electricity flow. This means the transistor is in the off state.

The remaining electrons will get attracted by the positive terminal of the first power source and will flow straight as shown (Fig:7A). Note that the P region is very narrow, which ensures that no remaining electrons flow to the positive terminal of the second power source. In short, a small base current is amplified to a high collector current. You can easily correlate the naming of the transistor terminal with the nature of electron flow. If you can increase the base current, the collector current will increase proportionally. This is a clear case of current amplification.

The kind of transistor we have discussed is called a bipolar junction transistor. Let's replace this representative transistor with a realistic one. (Fig:7B)

Fig:7A Representative transistor
Fig:7B Realistic transistor

You can further improve the amplification by introducing one more transistor. The base of this transistor is connected with the emitter of the first transistor. If you introduce a weak fluctuating signal at the input, like what you would find in a microphone, you will get an amplified signal at the loudspeaker (Fig:8A).

Fig:8A The amplified signal at the loudspeaker
Fig:8B Symbol of BJT

The other interesting thing you can note about this basic circuit is that depending on the value of the applied voltage, the transistor can be either on or off. Here the transistor acts as a switch. This property of the transistor opens the doors to the world of digital electronics and digital memory. Using 2 BJTs, you can build the basic dynamic memory element of computer, a flip-flop. To know more about it, please watch the video on MOSFET.