Semiconductor Diode Applications
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Direct answer
A p-n junction conducts freely once forward bias exceeds a small threshold — about 0.7 V for silicon, 0.2 V for germanium — and blocks reverse bias, and that one-way traffic is the rectifier: half-wave rectification uses a single diode in series with the load to pass only one half of each AC cycle; full-wave circuits (two diodes on a centre-tapped transformer, or four in a bridge) turn both halves the same way so the output never falls to zero, doubling the ripple frequency to 100 Hz on 50 Hz mains. Around the rectifier NCERT arrays the whole junction family: the LED (forward-biased light emitter), the photodiode (reverse-biased light detector), the Zener diode (breakdown voltage regulator) and the solar cell (light straight to EMF, without any bias).
What you must remember
- Conduction threshold: forward bias collapses the depletion layer above about 0.7 V (Si) or 0.2 V (Ge); reverse bias widens it — the diode's entire identity in one sentence.
- Half-wave rectifier: one diode with the load across the secondary; output during alternate half cycles only; ripple frequency equals the mains frequency (50 Hz).
- Full-wave, centre-tap: two diodes fed by a centre-tapped secondary, each conducting on alternate halves; output pulses at twice the mains frequency (100 Hz).
- Bridge rectifier: four diodes needing no centre tap; one diagonal pair conducts each half cycle; better transformer utilisation — NCERT's most quoted circuit.
- Filter logic: a capacitor across the load charges to peaks and discharges slowly, smoothing the pulses toward steady DC — the ripple idea.
- LED: heavily doped forward-biased junction emitting light on electron-hole recombination; silicon and germanium junctions emit only heat, so GaAs and GaAsP-type materials are used; low power draw, long life.
- Photodiode: reverse-biased junction whose reverse current grows with illumination — light detection, as in optical receivers and counters.
- Solar cell: a photodiode operated with no external bias, converting incident light directly into electrical power; the photovoltaic effect.
Tracing one cycle through a rectifier
Picture a bridge fed by 50 Hz mains through a step-down transformer. In the positive half cycle, one diagonal pair of diodes conducts and pushes current down through the load; in the negative half cycle the other pair conducts, and because of how the bridge is wired, the load current again flows the same way. The load therefore sees 100 humps per second — ripple at 100 Hz — which a shunt capacitor then flattens into near-DC. Count the diodes and you have the discriminator: one for half-wave, two for centre-tap full-wave, four for bridge. The counting is examinable on its own: "the output frequency of a full-wave rectifier connected to 50 Hz supply is" is a perennial one-liner with 50 Hz sitting in the options to catch the inattentive.
Where NEET sets the trap
Electronics questions here are identification and counting, rarely computation. The polarity swap is the classic: an LED is forward biased, a photodiode reverse biased — an assertion-reason item flips them and expects you to notice that detection needs the reverse current's light sensitivity, not conduction. The threshold 0.7 V appears both ways: "below this the diode effectively does not conduct" and, inverted, "the minimum forward voltage for conduction". Doping questions ride along in the same chapter: trivalent impurities (aluminium, boron, indium) make p-type, pentavalent (arsenic, antimony, phosphorus) make n-type, and intrinsic-versus-extrinsic conductivity is asserted constantly. The Zener diode, operated in reverse breakdown as a voltage regulator, closes the set — its detailed working forms its own page.
Frequently asked questions
What is the output frequency of a full-wave rectifier on 50 Hz mains?
100 Hz; both half cycles are delivered to the load in the same direction, so pulses arrive twice per input cycle.
How many diodes does a bridge rectifier use and why four?
Four, so that one diagonal pair conducts in each half cycle, keeping load current unidirectional without needing a centre-tapped transformer.
Why is a photodiode reverse biased while an LED is forward biased?
The photodiode's reverse current grows with illumination, which is how it detects light; the LED needs forward injection of carriers to recombine and emit — one junction, opposite jobs.
What forward voltage turns a silicon diode on?
About 0.7 V (0.2 V for germanium); below the threshold the depletion barrier is not overcome and conduction is negligible.
Does a solar cell need an external battery?
No; incident light generates electron-hole pairs at the junction and drives current through the load directly — a photovoltaic cell with no bias at all.