Semiconductor Devices and Basic Circuits
The Semiconductor Devices and Basic Circuits domain covers PN junctions, diodes, rectifiers, Zener regulation, BJT and MOSFET operation, and simple amplifier/switching circuits. The exam tests both device behavior and DC bias calculations.
Core rules and concepts
- A silicon diode drops about 0.7 V when forward-biased; germanium about 0.3 V. Reverse bias blocks current until breakdown.
- Half-wave rectifier uses one diode; full-wave bridge uses four. A reservoir capacitor smooths the pulsating DC.
- A Zener diode is operated in reverse breakdown; its series resistor must limit current: R = (V_in − V_Z)/I_Z.
- BJT relations: I_E = I_C + I_B; I_C = βI_B in the active region; V_BE ≈ 0.7 V for silicon. Saturation occurs when V_CE is near 0.2 V; cutoff when I_B = 0.
- An LED needs a series resistor: R = (V_supply − V_LED)/I_LED.
Worked example
An NPN silicon transistor has β = 100, I_B = 20 µA, R_C = 1 kΩ and V_CC = 5 V. I_C = 100 × 20 µA = 2 mA. V_RC = 2 mA × 1 kΩ = 2 V. V_CE = 5 − 2 = 3 V, so the device is in the active region. If R_C were 2.2 kΩ, V_RC would be 4.4 V and V_CE only 0.6 V, indicating saturation.
Traps that cost marks
Assuming β is exact or constant; ignoring V_BE in bias calculations; operating an LED without a resistor; fitting a Zener without a current-limiting resistor; confusing saturation with cutoff; reversing diode or electrolytic capacitor polarity.