This group covers semiconductors: diodes, LEDs and the two main transistor families, plus the idea of gain. Semiconductor devices are the active parts that rectify, switch and amplify signals in modern radios.
Diodes
A diode allows current to flow in only one direction. Its two electrodes are the anode and the cathode. Current flows easily from anode to cathode when forward biased and is blocked in the reverse direction.
On a physical diode, the cathode end is often marked with a stripe or band. In the schematic symbol, the bar on the symbol is the cathode side and the triangle points from anode toward cathode.
Forward voltage drop
When a diode conducts, a small voltage is lost across it. This forward voltage drop depends on the type of diode. Silicon diodes drop roughly 0.6 to 0.7 V, while Schottky and germanium diodes drop less. A drop is normal and does not mean the diode is defective. It does reduce the voltage delivered to the load, which matters in low-voltage supplies. It is not proportional to the peak inverse voltage rating.
LEDs
A light-emitting diode (LED) is a diode that emits light when forward current passes through it. Reverse current does not light it. LEDs need a series resistor to limit current, and they are widely used as indicators.
Transistors
A transistor is a semiconductor device that can act as an electronic switch or as an amplifier. Because a small control signal governs a larger current, it can provide power gain. Passive parts like transformers, inductors (reactors) and resistors cannot produce power gain.
Bipolar junction transistors (BJT)
A bipolar transistor is built from three regions of semiconductor material, arranged NPN or PNP. Its electrodes are the emitter, base and collector. A small current into the base controls a larger current between collector and emitter. Vacuum tubes such as triodes and pentodes also have several elements, but they are not semiconductors.
Field-effect transistors (FET)
FET stands for Field Effect Transistor. Its electrodes are the gate, drain and source. The voltage on the gate sets up an electric field that controls current flowing between source and drain. If a question mentions gate, drain and source, the answer is field-effect; if it mentions base, collector and emitter, the answer is bipolar junction.
Gain
Gain describes how much an amplifier increases a signal. It is the ratio of output to input, and it can be expressed for voltage, current or power. An amplifier’s power gain is the output signal power relative to the input signal power. Voltage gain and current gain compare output and input voltage or current in the same way.
| Device | Electrodes |
|---|---|
| Diode | Anode, cathode |
| Bipolar junction transistor | Emitter, base, collector |
| Field-effect transistor | Source, gate, drain |
Key facts to memorize
- Diode: passes current one way only; electrodes are anode and cathode.
- Cathode is usually marked with a stripe.
- Forward voltage drop varies between diode types (silicon about 0.6 to 0.7 V; Schottky lower) and is normal.
- LED emits light when forward current flows.
- BJT has three semiconductor regions: emitter, base, collector.
- FET (Field Effect Transistor): gate, drain, source.
- A transistor can be used as an electronic switch and can provide power gain.
- Gain: output relative to input, for voltage, current or power.
Memory hooks
- Cathode = stripe = the bar on the symbol.
- “Gate” on a field-effect device acts like a gate that lets current pass; BJT uses “base.”