Semiconductors: Diodes, Transistors, and What Gain Means

Concept

Every component the last lesson covered — resistors, capacitors, inductors — is passive: each one only dissipates, stores, or releases the energy a circuit already has, and none of them can make a signal bigger than it started. A semiconductor device changes that. Semiconductors are built from a material, most often silicon, that sits electrically between a conductor and an insulator: on its own it barely conducts, but adding tiny controlled amounts of other elements — a process called doping — creates two flavors of material, one with a surplus of loosely-held electrons and one with a shortage. Neither flavor by itself does much, but joining them together at a boundary inside a single crystal creates a junction that behaves in ways plain resistance never could: it can let current flow one direction and block the other, or let a small signal control a much larger one. That junction behavior is the basic principle every diode and transistor in this lesson builds on, and it's also what makes these parts active rather than passive — some of them can add power to a signal instead of just shaping or absorbing it, which is exactly what the last block in this lesson explores under the name gain.

Concept

The simplest semiconductor device is the diode: a single junction between the two doped materials, packaged with a lead coming off each side. Its defining behavior is one-way current flow — apply voltage in the direction the junction favors and current passes through easily; reverse it and almost none gets through. The two leads have names of their own instead of being interchangeable like a resistor's: the anode is the lead current enters, and the cathode is the lead it exits. Because getting the direction backward matters, diode packages mark the cathode lead clearly, most often with a stripe printed near that end of the body, though some packages print a lone K by that lead instead, borrowed from the German word for cathode. Diodes aren't perfectly lossless in the direction they do conduct; crossing the junction costs a small forward voltage drop, and that drop isn't the same figure for every diode type, since different semiconductor materials and junction designs give up different amounts of voltage to get current moving. A drop that's a little higher or lower than another diode's isn't a sign anything is defective — it's simply a property of that diode's construction.

Concept

A light-emitting diode, or LED, is an ordinary diode junction built from a semiconductor material chosen specifically so that forward current flowing across it releases some of its energy as light instead of only heat. Send current through in the forward direction — the direction the diode already favors — and the LED lights; there's no reverse-current version of this effect, and no separate circuit is needed to make it glow beyond getting the polarity right and keeping the current within what the device can handle. The same anode-and-cathode vocabulary that describes any diode applies here without changes.

Concept

A transistor takes the semiconductor junction a step further by combining three regions of doped material into one device instead of two. That third region is what turns a diode's simple one-way gate into something far more useful: a device where a small signal applied to one part controls a much larger current flowing through the other two, which is what lets a transistor work as an electronic switch — fully on or fully off, controlled by a signal far weaker than the current it's switching. Two transistor families cover nearly everything you'll encounter in amateur equipment, and they're built and described differently even though both rest on the same three-region idea. A bipolar junction transistor, or BJT, has three electrodes named emitter, base, and collector, and a small current fed into the base controls a much larger current flowing from collector to emitter. A field-effect transistor, or FET, has three electrodes named gate, drain, and source instead, and rather than a control current, it's a voltage applied to the gate that controls current flowing from drain to source. Different names, different control mechanism, same underlying job: a small input governs a large output.

Concept

Gain is the word for exactly that relationship — how much bigger the output of a device is than its input — and it's worth being precise about what's being compared, because gain can describe three different pairs of numbers. Voltage gain compares output signal voltage to input signal voltage; current gain compares output signal current to input signal current; and power gain compares output signal power to input signal power. All three are legitimate uses of the term, and which one a spec sheet or a question means depends on context, not on gain having one single definition. What matters most for understanding why transistors matter is this: a transistor can provide power gain, taking a weak input signal and delivering a stronger version of it at the output, with the extra power coming from the transistor's own power supply rather than from the input signal itself. A plain resistor can never do that — it only ever dissipates power, never adds it — and even a transformer, which can step voltage or current up while stepping the other down, can't increase the total power passing through it either. That capacity to add power, drawn from an external supply rather than squeezed out of the input, is the line between an active device and everything passive in the previous lesson, and it's the reason a handful of transistors, not a pile of resistors, is what turns a weak received signal into one you can actually hear, or a low-power oscillator into a signal strong enough to put on the air.