# Semiconductor Devices and Diodes

> Semiconductor devices and diodes for JEE Physics: p-n junction biasing, knee voltages, rectifier ripple at 50 and 100 Hz, Zener and LED.

- Canonical URL: https://prepelephant.com/topics/jee/physics/semiconductor-devices-and-diodes
- Exam / course: JEE · Subject: Physics
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Semiconductor Devices and Diodes", PrepElephant, https://prepelephant.com/topics/jee/physics/semiconductor-devices-and-diodes

## Direct answer

Forward bias a p-n junction diode above its knee voltage — about 0.7 V for silicon, 0.3 V for germanium — and current climbs exponentially as I = I(0)(e^(eV/kT) − 1); reverse bias it and only a tiny saturation current trickles until breakdown. Pentavalent donors (arsenic, antimony) give n-type with electrons as majority carriers, trivalent acceptors (boron, aluminium, indium) give p-type with holes. The junction's one-way behaviour rectifies: on the Indian 230 V, 50 Hz mains, a half-wave rectifier delivers pulses at 50 Hz (maximum efficiency 40.6%), a full-wave bridge at 100 Hz (81.2%). The Zener diode, operated in reverse breakdown, holds voltage constant and regulates; the LED converts forward current to light (gallium arsenide for infrared, GaAsP for visible).

## What you must remember

- **Doping table:** group-15 donors As, Sb, P give n-type (electrons majority); group-13 acceptors B, Al, In give p-type (holes majority); both types stay electrically neutral overall.
- **Junction physics:** diffusion creates a depletion region of immobile ions; the barrier potential sits near 0.7 V (Si) and 0.3 V (Ge); forward bias narrows the depletion layer, reverse bias widens it.
- **Diode equation:** I = I(0)(e^(eV/kT) − 1); every 60-mV rise in forward bias multiplies current roughly tenfold at room temperature; dynamic resistance r(d) = dV/dI is small when conducting.
- **Rectifier numbers:** half-wave — one diode, ripple at mains frequency (50 Hz in India), maximum efficiency 40.6%; full-wave bridge — four diodes, ripple at 100 Hz, maximum efficiency 81.2%; a capacitor across the load smooths the output toward DC.
- **Zener diode:** designed reverse breakdown (avalanche or Zener mechanism); used in reverse bias as a voltage regulator since its voltage stays nearly constant over a wide current range.
- **Optoelectronic trio:** LED (forward-biased spontaneous emission; GaAs infrared, GaAsP red — efficiency far above filament lamps); photodiode (reverse-biased, light generates carriers, current proportional to illumination); solar cell (photovoltaic emf without bias, silicon cell about 0.5-0.6 V).
- **Transistor cameo:** currents obey I(E) = I(B) + I(C) with beta = I(C)/I(B) typically 50-300 and alpha = I(C)/I(E).
- **Pattern note:** Main asks bias identification, ripple frequency and Zener regulation numericals; Advanced asks diode circuits with two sources and logic-gate combinations of diodes.

## Reading a rectifier, then repairing it

Feed a bridge rectifier from a transformer secondary and trace one half-cycle: two diagonally opposite diodes conduct, the load sees the same polarity as on the next half-cycle when the other pair takes over — output pulses at 100 Hz on a 50 Hz mains, both halves harvested. Half-wave uses one diode and throws away alternate halves: 50 Hz ripple and half the average output. The filter capacitor then charges to the peak and discharges slowly through the load; a bigger capacitor or load resistance means smaller ripple — the design equation behind every DC adapter. When such an adapter fails, suspect the transformer, then a bridge diode, then the capacitor.

The Zener regulator earns its own paragraph. A 12 V Zener in reverse across a load, fed through a series resistance from an 18 V supply: the Zener absorbs current swings so the load voltage stays 12 V — the arithmetic of every Main regulation numerical.

## Where students slip

Ripple frequency is the easiest mark and the most missed: half-wave follows the mains (50 Hz in India), full-wave doubles it (100 Hz) — answers quoting 50 Hz for a bridge forget the second half-cycle. Second, the depletion layer's response is inverted by instinct: forward bias narrows it (carriers flood in), reverse widens it — the direction candidates most often reverse. Third, a Zener regulator works in reverse breakdown; drawing it forward-biased in a circuit diagram forfeits the question. Fourth, knee voltage versus barrier potential: both near 0.7 V for silicon but distinct ideas — the barrier is the junction's built-in field, the knee is where conduction visibly takes off in the I–V curve.

## Frequently asked questions

### What happens to the depletion layer under forward and reverse bias?

Forward bias narrows it as majority carriers pour across and current rises exponentially; reverse bias widens it, leaving only a tiny saturation current until breakdown.

### Why are silicon and germanium diodes quoted with different knee voltages?

Silicon's barrier potential is near 0.7 V and germanium's near 0.3 V, set by their band gaps; conduction visibly takes off only once the applied forward voltage beats these values.

### What ripple frequencies do half-wave and full-wave rectifiers produce on Indian mains?

Half-wave gives 50 Hz (one pulse per cycle of the 50 Hz supply); full-wave or bridge gives 100 Hz (one pulse per half-cycle), easing filtering.

### How does a Zener diode regulate voltage?

Operated in reverse breakdown, its voltage stays almost constant while its current varies widely; a series resistor absorbs the surplus so the load voltage is pinned at V(z).

### Why is a photodiode operated in reverse bias?

Reverse bias widens the depletion region, so light-generated carriers are swept quickly into a reverse current that varies almost linearly with illumination — detection, not emission.