Semiconductor Devices and Basic Circuits
This domain covers diodes, rectifiers, Zener regulators, bipolar transistors and field-effect devices - the building blocks of every power supply and amplifier you will service. The exam expects you to know bias conditions and to calculate operating points.
Core concepts
- Silicon diode: conducts above ≈0.7 V forward, blocks in reverse until breakdown. Germanium ≈0.3 V.
- Half-wave rectifier: one diode, output pulses at f. Full-wave and bridge: two pulses per cycle, ripple at 2f.
- A Zener diode is used reverse-biased in breakdown to hold a fixed voltage; it needs a series resistor to drop the excess.
- BJT: NPN conducts when base is ≈0.7 V above emitter; PNP is the mirror image. β = Ic/Ib, α = β/(β+1), Ie = Ic + Ib.
- Regions: cutoff (no Ic), active (amplifies), saturation (Vce ≈ 0.2 V, acts as a switch).
- MOSFET: voltage-driven, very high input resistance; watch gate threshold and ESD sensitivity.
Worked example
An NPN transistor with β = 150 has base current 40 µA and a 1 kΩ collector resistor on a 12 V supply. Ic = β × Ib = 150 × 40 µA = 6 mA. Voltage across Rc = 6 mA × 1 kΩ = 6 V, so Vce = 12 − 6 = 6 V. Since Vce is well above 0.2 V, the device is in the active region, not saturated. Checking the region, not just the current, is the second mark.
Traps that cost marks
- Assuming a transistor is saturated without checking Vce.
- Forgetting the diode drops in a bridge rectifier (two diodes conduct at once).
- Connecting a Zener forward-biased and expecting regulation.