T6B: Semiconductors – Ham Radio Technician License Study Guide

Semiconductors are the foundation of modern electronics. Unlike conductors (which always carry current) or insulators (which block it), semiconductors can be engineered to control current flow in precise ways. The two most important semiconductor devices for this exam are the diode and the transistor — components that appear in every piece of amateur radio equipment from handheld radios to high-power amplifiers.

T6B focuses on what these devices are, how they work, what their electrodes are called, and how they are identified. Understanding diodes and transistors at this level gives you the foundation to understand rectifiers, amplifiers, switches, and oscillators in later subelements.

Diodes

A diode is a two-terminal semiconductor device that allows current to flow in only one direction. This one-way property makes diodes essential for rectification (converting AC to DC) and for protecting circuits from reverse voltage. Current flows from the anode to the cathode — in the forward direction. If voltage is applied in reverse, a normal diode blocks current flow entirely.

The two electrodes of a diode are called the anode and the cathode. On a physical diode package, the cathode lead is typically marked with a stripe — a band around one end of the component body. This stripe is the key identifier you need to know. Different diode types have different forward voltage drops — the voltage consumed by the diode when it is conducting. Some types (like Schottky diodes) have a lower forward voltage drop than standard silicon diodes; this is a designed characteristic, not a defect.

Light-Emitting Diodes (LEDs)

A light-emitting diode (LED) is a special type of diode that emits light when forward current flows through it. The physics behind this involves electrons releasing energy as photons when they recombine across the semiconductor junction. Like all diodes, an LED only conducts in the forward direction — reverse current does not cause it to emit light. LEDs are used as visual indicators in amateur radio equipment: power-on lights, transmit indicators, and status displays.

Transistors

A transistor is a three-terminal semiconductor device that can function as an electronic switch or as an amplifier. As a switch, a small control signal at one terminal can turn on or off a much larger current through the other two terminals. As an amplifier, a small input signal produces a larger output signal — this is called gain. Transistors can provide power gain, meaning the output signal carries more power than the input signal.

Structurally, a transistor consists of three regions of semiconductor material joined together. The arrangement and doping of these regions determines the transistor's type and behavior.

Bipolar Junction Transistors and Field-Effect Transistors

There are two main transistor families on the exam: bipolar junction transistors (BJTs) and field-effect transistors (FETs).

A bipolar junction transistor (BJT) has three electrodes: the emitter, the base, and the collector. Current into the base controls the much larger current flowing between the emitter and collector. BJTs are called "bipolar" because both electrons and holes carry current inside them.

A field-effect transistor (FET) — FET stands for Field Effect Transistor — has three electrodes: the gate, the drain, and the source. The voltage at the gate controls current flowing between the source and drain. FETs control current using an electric field rather than a base current, which makes them useful in many radio frequency applications.

Gain and Amplification

Gain is the term that describes a device's ability to amplify a signal. When a transistor (or any amplifying device) produces a larger output signal than its input, it is said to have gain. Gain can refer to voltage gain, current gain, or power gain. Transistors are the components that can provide power gain — a transformer can change voltage and current ratios but does not increase power; only an active device like a transistor can amplify power.

T6B Practice Questions