Question

How do transistors work?

Answer 1

Transistors are fundamental semiconductor devices that play a crucial role in modern electronics. They are the building blocks of integrated circuits (ICs) and are used to amplify and switch electronic signals. Transistors can be thought of as tiny electronic switches that control the flow of current in a circuit.

There are two main types of transistors: bipolar junction transistors (BJTs) and field-effect transistors (FETs). I'll provide a general explanation of how both types work:

Bipolar Junction Transistor (BJT):
A BJT consists of three layers of semiconductor material: an N-type region sandwiched between two P-type regions (NPN transistor) or a P-type region sandwiched between two N-type regions (PNP transistor). These layers are referred to as the emitter, base, and collector.

NPN Transistor:

The N-type region (emitter) has an excess of electrons.
The P-type region (base) has a deficit of electrons (holes).
The N-type region (collector) has an excess of positively charged holes.

PNP Transistor:

The P-type region (emitter) has a deficit of electrons (holes).
The N-type region (base) has an excess of electrons.
The P-type region (collector) has an excess of negatively charged electrons.

Transistors operate in two main modes: the active mode and the cutoff mode.

Active Mode (Amplification):

In the active mode, a small current flows from the emitter to the base (for NPN) or from the base to the emitter (for PNP).
This current flow from the emitter to the base causes a larger current to flow from the collector to the emitter (for NPN) or from the emitter to the collector (for PNP).
The current between the collector and emitter is amplified based on the small current flowing between the base and emitter.
The transistor acts as an amplifier, where a weak input signal at the base controls a larger output signal between the collector and emitter.

Cutoff Mode (Switching):

In the cutoff mode, there is no current flow between the base and emitter (for NPN) or between the emitter and base (for PNP).
As a result, there is no current flow between the collector and emitter (for NPN) or between the emitter and collector (for PNP).
The transistor acts as an open switch, where no current passes through it.
Field-Effect Transistor (FET):
FETs work on a different principle compared to BJTs. They have three terminals: gate, source, and drain. The FET's behavior is controlled by an electric field.

N-channel FET:

The semiconductor material is of N-type.
The gate terminal controls the flow of electrons between the source and drain terminals.
When a voltage is applied to the gate, it creates an electric field that either allows or blocks the flow of electrons between the source and drain.
When the gate-source voltage is high enough, it forms a conductive channel, allowing current to flow from the source to the drain.

P-channel FET:

The semiconductor material is of P-type.
The gate terminal controls the flow of holes between the source and drain terminals.
When a voltage is applied to the gate, it creates an electric field that either allows or blocks the flow of holes between the source and drain.
When the gate-source voltage is low enough, it forms a conductive channel, allowing current to flow from the drain to the source.

FETs are voltage-controlled devices, and they act as voltage-controlled resistors. By varying the voltage at the gate terminal, the current flowing between the source and drain can be controlled, allowing FETs to function as amplifiers or switches.

Both BJTs and FETs are crucial components in various electronic devices, including computers, smartphones, amplifiers, and many other electronic systems. They have revolutionized modern technology and enabled the development of powerful and efficient electronic devices.