The transistor is one of the marvelous inventions of human beings and yet they are so common in our lives. This post attempts to uncover some of the physical makeup and characteristics of a transistor.
TL; DR Transistor is like a voltage-controlled switch.
A transistor is like a voltage-controlled switch. How is that functionality achieved? How do human beings make such devices? Let's start with the most fundamental component—silicon.
Silicon is a semiconductor. Each silicon element has 4 electrons on its outer shell. It needs 8 electrons to be stable. When they are bound together (e.g. in silicon crystalline), each silicon element get 8 electrons by sharing electrons, forming a stable structure. There are no extra electrons or holes. Pure silicon doesn't have extra electrons so its conductivity is low.
If we dope silicon with phosphorus, each element having 5 electrons in its outer shell, in addition to the original structure, we get excess electrons that can move around. These electrons act as charge carriers.
If we dope silicon with aluminum or boron, each element having 3 electrons at its outer shell, in addition to the original structure, we get excess holes of electrons. These holes can also act as charge carriers.
Interesting things happen when you form a P-N junction by combining the two crystals mentioned above.
By applying an external voltage, this potential can be overcome or enhanced
You know what, this behavior is exactly the functionality of a diode! Electric current can flow through one direction but not the other (not considering the breakdown scenario of course).
There are two kinds of transistors
MOSFET is more common nowadays. It's faster to switch because it relies on the field effect. It consumes less power and consumes less space. But let's look at both of them.
A BJT is like a sandwich where you wrap a P-crystal with two N-crystals or an N-crystal with two P-crystal. You get either an NPN transistor or a PNP transistor.
A BJT has three pins: collector, base, and emitter. The base is lightly doped; the collector is moderately doped; the emitter is heavily doped. The emitter is usually connected to the ground and there are voltage sources at the base and the collector.
A BJT can be used as a switch or an amplifier.
Based on different voltage conditions, an NPN transistor has different operating regions/modes
In my opinion, a MOSFET is closer to the metaphor of a voltage-controlled switch.
You have a gate in the middle and drain and source on both sides.
The gate is insulated with a metal layer; it used field-affect to control the conductivity.
Take an NMOS as an example, when a gate voltage is applied, electrons are attracted and build a channel in the p-crystal, connecting the two n-crystal. It also has several operating regions
The MOSFET can have different default modes when no voltage is applied. The default can be ON (depletion mode device) or OFF (Enhancement mode device). The described example is an enhancement mode device.
Illustrative images can be found at
https://www.halbleiter.org/en/fundamentals/construction-of-a-field-effect-transistor/
The whole process looks like planting teeth lol.
Random BJT notes
(I spent quite some time trying to figure out details of BJT, but later realized that it's not as common as MOSFET. So I didn't bother digging further. Just dumping the notes here.)
BJT saturation mode
BJT active mode
Base_collector is reversed biased but it's not a normal PN junction. The base crystal is already filled with electrons due to the forward biased in the base_emitter. So the electrons can move forward from the base. The higher collector voltage provides higher attraction.
BJT application
If you gradually increase the base voltage
https://www.halbleiter.org/en/fundamentals/
https://www.circuitbread.com/tutorials/different-regions-of-bjt-operation
https://www.circuitbread.com/tutorials/how-to-use-a-bipolar-junction-transistor-bjt-as-a-switch
[Youtube] How does a reverse biased diode work at the molecular level?