Semiconductor Electronics
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Direct answer
Silicon doped with phosphorus acquires spare conduction electrons; doped with boron it acquires holes; the junction between the two regions conducts one way only — and every device in this chapter grows from that junction. The band picture sets the rules (Si gap 1.1 eV, Ge 0.7 eV), the junction's one-way behaviour rectifies and regulates, and the device set — Zener, LED, photodiode, solar cell — carries the questions.
What you must remember
- Bands: the valence band fills, the conduction band sits an energy gap Eg above; insulators have Eg > 3 eV, semiconductors near 1 eV (Si 1.1 eV, Ge 0.7 eV), and heat promotes electrons across the gap, leaving mobile holes behind — both carriers conduct.
- Intrinsic silicon: electron and hole densities equal; extrinsic: pentavalent donors (P, As, Sb) give n-type with electrons majority; trivalent acceptors (B, Al, In) give p-type with holes majority; either way the crystal stays neutral overall.
- Junction formation: majority carriers diffuse across, recombine, and leave a depletion region of immobile ions with a barrier potential — about 0.7 V for silicon, 0.2-0.3 V for germanium — where drift finally balances diffusion.
- Forward bias narrows the depletion layer and lets a large diffusion current flow beyond the knee voltage; reverse bias widens it, leaving only a tiny minority-carrier saturation current until breakdown.
- Rectification: one diode gives half-wave output; two diodes with a centre-tapped transformer give full-wave, and the ripple frequency doubles to 100 Hz on 50 Hz mains; a capacitor across the load smooths the ripple.
- Zener diode: heavily doped, operated in reverse breakdown which is sharp and non-destructive — the standard voltage regulator.
- Photodiode: reverse-biased; incident light generates carriers, so the reverse current grows with illumination — light detection, smoke alarms, optocouplers.
- LED: forward-biased junction emitting photons on electron-hole recombination (GaAs, GaP and related compounds); solar cell: the same physics run as a generator, delivering an emf under light with no external bias.
Following the junction through bias
At equilibrium, majority-carrier diffusion across the fresh junction is exactly balanced by minority-carrier drift, and the depletion region's charged wall holds the barrier. Forward-bias it (positive to the p-side): the barrier lowers and the wall thins, majority electrons pour from n to p, holes from p to n, and current climbs steeply past the knee. Reverse-bias it and the wall grows taller and thicker: majority diffusion is extinguished outright, and the only current is the small, bias-insensitive drift of thermally generated minority carriers — a trickle that avalanches only at the designed Zener voltage. The asymmetry is absolute, and every application is a pose on it. The rectifier lets the forward half-cycles through and starves the reverse ones; in full-wave, the two diodes alternate by half-cycle and the ripple runs at double the mains frequency. The Zener exploits the breakdown rather than fearing it; the photodiode turns the reverse trickle into a light meter; the LED and solar cell run the same engine in opposite directions along the bias axis.
Where students slip
Majority-carrier naming trips first: an n-type sample still contains some holes — they are its minority carriers — and "n-type is negatively charged" is false, since donor ions in the lattice balance the donated electrons. Second, the depletion region contains immobile ions, not free charges; it is called depleted because its mobile carriers have recombined away. Third, the reverse saturation current flows because of minority carriers and so is almost independent of the reverse voltage — options claiming it grows steadily with reverse bias confuse the flat pre-breakdown region with the breakdown knee. Fourth, the device census: the LED is forward-biased to emit, the photodiode reverse-biased to detect, and the solar cell unbiased to generate — the exam pairs wrong biases with devices to test exactly this. Fifth, the ripple-frequency question: full-wave rectification of 50 Hz mains gives 100 Hz, and answering "50" is the planted error.
Frequently asked questions
What distinguishes n-type from p-type doping?
Pentavalent atoms (P, As, Sb) donate electrons to the conduction band, making electrons the majority carriers; trivalent atoms (B, Al, In) create acceptor levels and holes become majority — the crystal remains electrically neutral in both cases.
What is the depletion region at a p-n junction?
The region straddling the junction where mobile carriers have recombined, leaving fixed ionised donors and acceptors; it supports the barrier potential and widens under reverse bias.
Why is the reverse current of a diode so small and nearly constant?
It is carried only by thermally generated minority carriers, whose supply is fixed by temperature; it barely responds to the reverse voltage until breakdown intervenes.
How does a Zener diode regulate voltage?
Operated in reverse breakdown, it conducts whatever current the circuit offers while holding a nearly constant voltage across itself; the sharp, non-destructive breakdown comes from its heavy doping.
How do an LED and a photodiode differ in operation?
The LED is forward-biased and converts current to light by electron-hole recombination; the photodiode is reverse-biased and converts incident light into current by generating carriers — emission and detection, opposite jobs.