Semiconductor Electronics

1. Conductors: conduction and valence bands overlap, free carriers always available.
2. Insulators: large gap (E_g ≳ 3 eV), thermal energy can’t bridge it.
3. Semiconductors: small gap ($E_g\approx 0.2\text{–}3$ eV; Si ≈ 1.1 eV), thermally bridgeable, carrier count rises with T.

1. n-type: pentavalent (donor) impurity, e.g. P, As — free electrons are the majority carriers.
2. p-type: trivalent (acceptor) impurity, e.g. B, Al — holes are the majority carriers.

Diffusion across the junction leaves fixed ionised donor/acceptor atoms behind, a carrier-free depletion region with its own built-in field, which halts further diffusion at equilibrium.

1. Reverse bias: depletion region widens; current stays essentially flat at $-I_0\approx -1\times10^{-12}\text{ A}$, no matter how reverse you go. V=−1 V and V=−5 V give the same current, non-Ohmic.
2. Forward bias: depletion region narrows; past ≈0.5–0.7 V, current explodes exponentially. V=0.5 V → 0.2248 mA; V=0.6 V → 10.52 mA; V=0.7 V → 0.4927 A. Each +0.1 V multiplies current by ≈47×.

See it live: interactive diode I-V simulation →

Half-wave rectifier: a diode passes only the half-cycle that forward-biases it, blocking the reverse half-cycle, converting AC into a one-directional, pulsing output.

1. Zener diode: operated in reverse breakdown at a fixed voltage — used for voltage regulation.
2. LED: forward-biased junction emits light on carrier recombination.
3. Photodiode: reverse-biased; light increases reverse current — used to detect light.
4. Solar cell: unbiased junction; light generates its own usable voltage and current.