Explain how diodes pass current one way, how transistors switch and amplify, and what gain means.
Syllabus: Semiconductors: basic principles and applications of solid-state devices, diodes and transistors; Gain
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
A diode lets current flow in one direction only. Its two electrodes are the anode (current goes in) and the cathode (current goes out). On the real part, a stripe marks the cathode.
Memory hookThe symbol's triangle is an arrow: current goes with the arrow, from anode to the bar (cathode).
An LED is a diode that glows. Current has to flow forward through it: reverse it and nothing lights.
A conducting diode always uses up a little voltage, its forward voltage drop. It is normal, it varies by diode type, and it is not a fault.
| Type | Typical forward drop |
|---|---|
| Schottky diode | about 0.3 V |
| Silicon diode | about 0.7 V |
| LED | about 2 V or more, by colour |
RememberLED light comes from forward current. Forward drop is normal and differs between diode types.
A transistor has three leads, and the common bipolar kind is built from three regions of semiconductor. One lead is the control; the other two carry the main current.
| Family | Leads |
|---|---|
| Bipolar junction (BJT) | emitter, base, collector |
| Field-effect (FET) | source, gate, drain |
Memory hookGate, drain, source: a FET has a gate. A BJT has a base.
A transistor uses a small base (or gate) signal to control a much larger current. Use it fully on or off and it is an electronic switch. Keep it in between and it amplifies.
Gain is output ÷ input. It can be measured in voltage, current or power, so all three are gain.
RememberOnly active parts such as transistors give power gain. Transformers, resistors and reactors cannot.