Semiconductor Devices: Diodes, Rectifiers, Zener Diodes, BJTs and FETs
Semiconductor devices use doped silicon junctions to control current. The exam tests diode behaviour, rectifier action, Zener regulation, transistor construction, characteristics, and biasing.
Core facts
- A silicon PN diode conducts in forward bias above about 0.7 V and blocks in reverse bias until breakdown.
- A rectifier converts AC to pulsating DC; a Zener diode is operated in reverse breakdown to hold a stable voltage.
- BJT terminals are emitter, base, and collector. In the active region, I_C ≈ βI_B; saturation and cutoff are switching states.
- FET terminals are gate, source, and drain. It is voltage-controlled and has very high gate input resistance.
- Bias circuits include base resistor, voltage divider, and JFET self-bias.
Worked example
A 9 V supply feeds a 220 Ω series resistor and a 5.1 V Zener diode. The resistor current is (9 − 5.1)/220 = 3.9/220 = 17.7 mA. If the load takes 10 mA, the Zener carries 7.7 mA. Zener power is 5.1 × 0.0077 ≈ 39 mW. The series resistor must be low enough to keep the Zener conducting but high enough to limit current; removing the load raises Zener current to 17.7 mA, so the resistor’s power rating must cover that condition.
Traps
- Confusing BJT base current with collector current.
- Assuming β is fixed; it varies with current, temperature, and device.
- Reversing Zener polarity in a regulator calculation.
- Mixing the pin order of NPN and PNP transistors.