Semiconductor Devices and Communication Systems
What this domain tests
Semiconductor Devices and Communication Systems tests intrinsic and extrinsic semiconductors, p-n junctions, diodes, rectifiers, Zener regulators, transistors, logic gates and basic communication ideas such as modulation.
Core rules
- Intrinsic semiconductors have equal electron and hole concentrations; pentavalent donors give n-type, trivalent acceptors give p-type.
- A p-n junction has a depletion layer and barrier potential; forward bias narrows it, reverse bias widens it.
- A diode conducts in forward bias above about 0.7 V for silicon; a half-wave rectifier uses one diode, a full-wave bridge uses four.
- A Zener diode works in controlled reverse breakdown as a voltage regulator.
- Transistor: I_E = I_B + I_C; β = I_C/I_B; α = I_C/I_E; α = β/(1+β).
- Modulation superimposes a signal on a high-frequency carrier for transmission.
Worked example
A transistor has β = 100 and base current 20 μA. Collector current I_C = βI_B = 100 × 20 μA = 2 mA. With a 2 kΩ collector load, the voltage across it is I_C R = 2 mA × 2 kΩ = 4 V.
Traps
- Reversing forward and reverse bias.
- Thinking doping increases both carrier types; it increases one and reduces the other.
- Confusing α and β.
- Believing a Zener diode is damaged in breakdown; it is designed for controlled reverse breakdown.