Basic Electronics and Semiconductor Devices
What this domain tests
RRB JE CBT 2 checks semiconductor physics and device behaviour: doping, p-n junctions, diode circuits, BJT/FET characteristics, and device identification. Questions are usually short numerical or conceptual items, so know the models and their limits.
Core concepts
- Intrinsic silicon has equal electron and hole concentrations. Pentavalent doping gives n-type material (majority electrons); trivalent doping gives p-type material (majority holes).
- At a p-n junction, diffusion creates a depletion region and a barrier potential: about 0.7 V for silicon and 0.3 V for germanium.
- Forward bias narrows the depletion region and lowers the barrier. Reverse bias widens it and leaves a small reverse saturation current from minority carriers.
- A Zener diode is designed for controlled reverse breakdown and is used for voltage regulation. Ordinary rectifier breakdown is not a normal operating condition.
- In a BJT the emitter injects carriers, the base is thin and lightly doped, and the collector collects them. beta = IC/IB, alpha = IC/IE, and alpha = beta/(beta+1).
Worked example
A silicon diode is forward biased in series with a 1 kohm resistor and a 10 V DC supply. Using the constant-drop model, I = (10 - 0.7)/1000 = 9.3 mA. The resistor drops the remaining 9.3 V; the diode does not drop 10 V.
Traps
Do not use 0.7 V for germanium or for an ideal diode. Real reverse current is small but not zero. Remember that beta is current gain, not voltage gain, and that a Zener diode is normally reverse biased.
Key points
- Silicon barrier ≈ 0.7 V; germanium ≈ 0.3 V.
- n-type majority carriers are electrons; p-type majority carriers are holes.
- Forward bias lowers the barrier; reverse bias raises it.
- Zener breakdown is used for regulation.
- beta = IC/IB; alpha = beta/(beta+1).