Semiconductor Physics and Diodes
This domain tests energy bands, intrinsic and extrinsic semiconductors, PN junction behaviour, diode characteristics, and rectifier or Zener applications. CHSE numericals usually combine a diode drop with a series resistor.
Core rules
- Silicon and germanium are tetravalent; covalent bonding creates valence and conduction bands separated by a band gap.
- Pentavalent doping gives n-type material with electrons as majority carriers; trivalent doping gives p-type material with holes as majority carriers. The crystal remains electrically neutral.
- At the PN junction, diffusion leaves a depletion layer and a barrier potential: about 0.7 V for silicon and 0.3 V for germanium.
- Forward bias narrows the depletion layer and permits current above the knee voltage; reverse bias widens it and allows only small leakage.
- A practical silicon diode is modelled as a 0.7 V drop when conducting. An ideal diode is a short when forward-biased and an open when reverse-biased.
- A Zener diode is operated in reverse breakdown to regulate voltage.
Worked example
A silicon diode and a 1 kΩ resistor are in series with a 10 V DC supply, forward-biased. Using the practical model, voltage across the resistor is 10 − 0.7 = 9.3 V. Current I = 9.3 V / 1000 Ω = 9.3 mA. The diode dissipates about 0.7 × 9.3 mA = 6.5 mW.
Marks-losing traps
Do not use 0.7 V when the diode is reverse-biased; it is then approximately an open circuit. Do not treat reverse saturation current as large. In a bridge rectifier, two diodes conduct in each half-cycle, so account for two diode drops. Zener regulation requires the correct reverse polarity and a series resistor.