What Is a Transistor?

Two Basic Functions

• Amplification: boosting a weak signal like a microphone's enough to drive a speaker.
• Switching: turning a larger load (a motor, an LED strip) on and off with a small control signal — the basis of digital circuits.

The BJT vs. MOSFET Difference

The two most common transistor families are the BJT and the MOSFET; they're controlled differently:

Terminals and Operation

In a BJT, a small current into the base controls the large current between collector and emitter. In a MOSFET, the voltage applied to the gate governs the current between drain and source; because the gate draws almost no current, MOSFETs switch with low loss.

Beyond these there are special types: the high-input-impedance JFET, the high-gain Darlington, the high-current power transistor and the high-frequency RF transistor.

Why Is It So Important?

The billions of transistors inside a microchip work as switches representing 0s and 1s. In that sense, the transistor is the fundamental building block of all digital technology. When choosing a transistor for your project, watch the current, voltage and power limits.

Common Mistakes

• Mixing up the terminals: the same package can have different pinouts; always check the datasheet (how to read a datasheet).
• Forgetting the base/gate resistor: in a BJT, a series resistor is essential to limit the base current.
• Exceeding the power limit: the load you switch must not exceed the transistor's current/power rating; use a heat sink if needed.