Basics
A transistor is an active device: energy from a power supply can be controlled by a smaller input signal. This is why a transistor can amplify. The input does not create the output energy; it controls energy supplied by the circuit's power source.
BJT terminals
- Emitter: supplies charge carriers.
- Base: thin control region.
- Collector: receives most of the carriers crossing the base.
An NPN transistor and a PNP transistor contain opposite arrangements of semiconductor regions. Their voltage polarities and conventional current directions are correspondingly reversed.
FET terminals
- Source: one end of the conductive channel.
- Gate: controls the channel.
- Drain: the other end of the channel.
In a MOSFET the gate is insulated from the semiconductor channel by a thin dielectric, so almost no steady DC gate current is required. The gate still has capacitance and must be charged and discharged during switching.
Switching idea
When used as a switch, a transistor is controlled so that the load current is either very small or strongly conducted.
Amplification idea
When biased in an amplifier region, a small changing input signal causes a larger changing output signal. The transistor is arranged with resistors, capacitors, and a power supply so the output can move both upward and downward without immediately reaching cutoff or saturation.
BJT current gain
A simple first model of a BJT uses:
where β is DC current gain. This is useful for learning, but β is not a precision constant. It varies substantially from device to device and with collector current and temperature.
Base-emitter voltage
The BJT base-emitter junction behaves much like a silicon diode. During normal forward operation it may have roughly 0.6 to 0.8 V across it, depending on current and temperature.
MOSFET gate voltage
A MOSFET begins forming a channel near its threshold voltage, but threshold is not the same as fully on. A power MOSFET may need a much larger gate-to-source voltage than its threshold specification to reach the low on-resistance promised by the datasheet.
Testing with a meter
A BJT can often be partly identified using a multimeter's diode-test range. The base forms diode-like junctions to emitter and collector. An NPN typically shows forward junction readings when the positive meter lead is on the base; a PNP shows them with the negative lead on the base.
This test can identify gross junction failures but does not measure transistor gain accurately or prove operation under load.
Heat
When a transistor carries current while voltage exists across it, power becomes heat:
Small transistors may dissipate only a fraction of a watt. Power transistors may need copper planes, heat sinks, airflow, thermal pads, or forced cooling.