Semiconductor Devices and Diodes
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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.