Semiconductor Electronics

Materials are classified by how easily they conduct. Conductors (metals) conduct freely; insulators hardly conduct; and semiconductors like silicon and germanium lie in between. In the energy-band picture, a semiconductor has a small energy gap (about $1\ \text{eV}$) between its filled valence band and empty conduction band — small enough that some electrons can cross it as temperature rises.

A pure semiconductor is called intrinsic. At absolute zero it behaves as an insulator, but at room temperature thermal energy frees a few electrons into the conduction band, each leaving behind a vacancy called a hole that acts like a positive charge carrier. In an intrinsic semiconductor the number of electrons equals the number of holes ($n_e = n_h$). A counterintuitive but key NEET fact: unlike metals, the conductivity of a semiconductor increases with temperature, because heating creates more carriers.

Pure semiconductors conduct too weakly to be useful, so their conductivity is boosted by doping — adding tiny amounts of impurity. Doping silicon with a pentavalent impurity (such as phosphorus or arsenic, group 15) donates extra electrons, giving an n-type semiconductor in which electrons are the majority carriers and holes the minority.

Doping instead with a trivalent impurity (such as boron or aluminium, group 13) creates extra holes, giving a p-type semiconductor in which holes are the majority carriers and electrons the minority. Crucially, both n-type and p-type materials remain electrically neutral overall — doping changes the type of carrier, not the net charge. These doped (extrinsic) semiconductors are the building blocks of every diode and transistor.

Joining a p-type and an n-type region forms a p-n junction, the basic semiconductor device. Near the junction, electrons and holes diffuse across and recombine, leaving a thin region with no free carriers called the depletion layer. The exposed fixed charges set up an internal potential barrier (about $0.7\ \text{V}$ for silicon, $0.3\ \text{V}$ for germanium) that opposes further crossing.

The junction's behaviour depends on how it is connected to a battery — its bias. In forward bias, the p-side is connected to the positive terminal; this lowers the barrier and narrows the depletion layer, so once the applied voltage exceeds the barrier, a large current flows easily. The diode then behaves almost like a closed switch.

In reverse bias, the p-side is connected to the negative terminal; this raises the barrier and widens the depletion layer, so only a tiny leakage current (from minority carriers) flows. The diode then behaves almost like an open switch. This asymmetry — easy current one way, almost none the other — gives the diode its defining property: it allows current in essentially one direction only.

This one-way behaviour shows up as the diode's V–I characteristic: negligible current in reverse bias, then a sharp rise once the forward voltage passes the knee. A heavily reverse-biased diode can also undergo breakdown at a high enough voltage. The rectifying action of the p-n junction is the single most important idea in this module, underlying rectifiers, logic and most analog circuits.

The diode's one-way conduction is used to convert AC into DC — a process called rectification. A half-wave rectifier uses a single diode that conducts only during the half of each AC cycle when it is forward biased, blocking the other half. The output is a pulsing DC present for only half the time, so it is inefficient and has a low average value.

A full-wave rectifier uses two diodes (with a centre-tapped transformer) or four diodes in a bridge, arranged so that current flows through the load in the same direction during both halves of the cycle. This gives an output for the whole cycle, with double the frequency and a higher average value — far more efficient, which is why it is used in practice. The remaining ripple is smoothed by a filter (typically a capacitor) to produce steady DC.

Several diodes are designed for special jobs. A Zener diode is built to operate safely in reverse breakdown at a fixed voltage, so it is used as a voltage regulator that holds the output steady. A light-emitting diode (LED) emits light when forward biased, converting electrical energy directly to light — used in displays and lighting.

Two more are commonly tested. A photodiode is operated in reverse bias and its current increases when light falls on it, so it works as a light detector. A solar cell is a large-area junction that generates a voltage when illuminated, converting light directly into electrical energy. Recognising which diode does which job — Zener regulates, LED emits, photodiode detects, solar cell generates — covers most NEET special-diode questions.

Logic gates are the building blocks of digital electronics. They operate on binary inputs — represented as $0$ (low) and $1$ (high) — and produce a binary output according to a fixed rule. The behaviour of a gate is summarised in a truth table listing the output for every combination of inputs, which NEET expects you to read and reproduce.

There are three basic gates. The OR gate outputs $1$ if any input is $1$ (output is $0$ only when all inputs are $0$). The AND gate outputs $1$ only if all inputs are $1$. The NOT gate (inverter) has a single input and simply reverses it, turning $0$ into $1$ and $1$ into $0$.

Combining a NOT with the basic gates gives two very important compound gates. The NAND gate is an AND followed by NOT, so its output is $0$ only when all inputs are $1$. The NOR gate is an OR followed by NOT, so its output is $1$ only when all inputs are $0$. These are simply the inverted versions of AND and OR.

NAND and NOR are called universal gates because any logic circuit whatsoever can be built using only NAND gates, or only NOR gates — a fact NEET likes to test directly. For example, a NOT gate can be made from a single NAND gate by tying its two inputs together. Mastery here is mostly about memorising the simple rule for each gate and being able to fill in its truth table quickly, which makes this final topic one of the most reliable and predictable sources of marks in the whole paper.