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
- 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
Semiconductors concept map
Semiconductors
Semiconductors quiz
Semiconductors, quiz
1. Adding a pentavalent impurity to silicon produces:
N-type material with free electronsP-type material with holesAn insulator2. The depletion layer of a PN junction is a region that is:
Free of mobile charge carriersFull of free electronsA perfect conductor3. A silicon diode begins to conduct in the forward direction at about:
0.7 V0.3 V5 V4. In reverse bias, a normal diode:
Blocks current until breakdownConducts freelyBehaves as a resistor5. The approximate barrier potential of a forward-biased silicon diode is:
0.7 V0.3 V1.4 V6. The majority carrier in N-type material is:
ElectronsHolesProtons7. P-type semiconductor is produced by adding a:
Trivalent impurityPentavalent impurityTetravalent impurity8. A block of N-type semiconductor material is:
Electrically neutral overallNegatively charged overallPositively charged overall9. Applying forward bias to a PN junction causes the depletion region to:
NarrowWidenRemain the same10. The number of valence electrons in an intrinsic silicon atom is:
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