Basic Electronics and Semiconductor Devices
Basic Electronics and Semiconductor Devices tests how charge behaves in doped silicon, how a PN junction rectifies, and how three-terminal devices amplify or switch. Expect characteristic curves, bias regions, and simple DC calculations rather than memorised slogans.
Core rules
- Silicon has four valence electrons; pentavalent donors make n-type (electrons majority), trivalent acceptors make p-type (holes majority).
- The depletion region widens under reverse bias and narrows under forward bias. A silicon diode conducts appreciably above about 0.7 V; germanium near 0.3 V.
- The diode equation is I = Is(e^(V/(nVT)) − 1), where VT ≈ 26 mV at room temperature and n is 1–2.
- A BJT has β = IC/IB in forward-active mode. Cut-off: IB≈0; saturation: VCE small; active: base-emitter forward, collector-base reverse.
- An enhancement MOSFET needs VGS above threshold; gate current is negligible at DC.
Worked example
A 5 V source feeds a silicon diode and 220 Ω resistor in series. Use VF = 0.7 V. Current I = (5 − 0.7)/220 = 19.5 mA. Resistor power = I²R = (0.0195)² × 220 ≈ 84 mW; diode dissipation ≈ 0.7 × 0.0195 ≈ 13.7 mW. The resistor must be rated above 84 mW, so a 0.25 W part is suitable.
Traps
Treating the diode as a closed switch with no knee voltage overestimates current. Assuming β is constant ignores base-width modulation and temperature. Confusing saturation with cut-off reverses switching logic. Using the MOSFET gate as a current input is wrong: it is a voltage-controlled channel.