Semiconductors
Intrinsic and doped semiconductor material; N-type and P-type majority carriers; the PN junction, depletion region and barrier potential; forward and reverse bias.
Semiconductors — summary notes
Free- Intrinsic (pure) semiconductors — silicon and germanium — have 4 valence electrons and conduct poorly at room temperature.
- Doping adds impurities to control conduction. N-type uses a PENTAVALENT donor (5 valence electrons), giving free electrons as the majority carrier.
- P-type uses a TRIVALENT acceptor (3 valence electrons), giving holes as the majority carrier.
- Both N-type and P-type material remain electrically NEUTRAL overall — doping changes the carrier type, not the net charge.
- Where P and N material meet, diffusion creates a depletion region containing no free carriers, and a barrier potential across it — about 0.7 V in silicon and 0.3 V in germanium.
- Forward bias (P to positive) narrows the depletion region; above the barrier potential the junction conducts. Reverse bias widens it and the junction blocks, apart from a tiny leakage current, until breakdown.
- ⚠ Exam trap: majority carriers are electrons in N-type and holes in P-type, but neither material carries a net charge.
- Silicon barrier potential
- ≈ 0.7 V
- Germanium barrier potential
- ≈ 0.3 V
Antimony (5 valence electrons) is added to silicon. What type of material results and what is the majority carrier?
A pentavalent donor produces N-type material, in which electrons are the majority carrier. The material is still electrically neutral overall.
Why does a silicon diode not conduct at 0.4 V forward bias?
The applied voltage has not yet overcome the 0.7 V barrier potential of the depletion region, so the junction remains effectively non-conducting.
LED & photodiode
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