The invention of transistors revolutionized or changed the human civilization like no other technology. At the heart of a smartphone lies a processor and this processor holds about 2 billion transistors. What do these incredibly tiny devices do? How do they work?
Transistors can act like a switch with no moving parts. Amplification is the basic function of a transistor. They can amplify a weak signal. First, let’s understand the basis of transistors.
Transistors are made of semiconductors such as silicon. Each silicon atom is bonded with four neighbouring silicon atoms. Silicon has 4 valence electrons. Let’s replace the silicon atom with a four-handed smiley. Each hand holds one electron. Each one of these electrons goes for sharing with a neighbouring 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. A technique called doping is used to improve the conductivity of semiconductors. For example say you inject phosphorus with five valence electrons. Here one electron will be free to move in the system. This is known as N-type doping. On the other-hand, if you inject boron with three valence electrons. There will be a vacant position for an electron this vacant position is known as a hole and a neighbouring electron can fill this hole at any time. This electron movement is visualized as holes moving in the opposite direction. We call this P-type doping.
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. A diode is formed when you dope one part of silicon as a P-type and the other part as an N-type. Something very interesting happens at the boundary of the N and P joint
The abundant electrons on the inside will have a natural tendency to migrate to the holes that are available on the P side. 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.
If you apply an external power source across the diode, 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.
Just keep this simple principle of a diode in mind, you will understand the operation of a transistor very easily
Now back to the transistor. Note that the P-layer is really thin and lightly doped You can easily see that a transistor is essentially two diodes sandwiched back to back. So whichever way you connect the power source,
one diode will always be reverse biased and block the electricity flow. This means the transistor is in the off state.
Now let’s connect a second power supply. The power supply should have enough voltage to overcome the potential barrier. So this is just a forward biased diode. Thus a high number of electrons will be emitted from the N region. Just like in a diode a few electrons will combine with the holes and jump across the neighbouring holes and flow to the base. But there are a lot more electrons that have crossed to the P side, what will these remaining electrons do?
Think for a moment, The remaining electrons will get attracted by the positive terminal of the first power source and will flow straight. 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. 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. 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 two BJT’s you can build the basic dynamic memory element of computer: