Overview
This chapter covers semiconductor devices (diodes, transistors), logic gates, and the basics of communication systems.
1. Semiconductors
Intrinsic: pure (Si, Ge)
Extrinsic (doped):
n-type: pentavalent dopant (P, As) → extra electrons (majority: electrons)
p-type: trivalent dopant (B, Al) → holes (majority: holes)
2. p-n Junction Diode and Rectifiers
Forward bias: conducts (low resistance)
Reverse bias: blocks (high resistance)
Half-wave rectifier: 1 diode (uses half the AC cycle)
Full-wave / bridge: uses both halves → smoother DC
3. Transistor and Logic Gates
Transistor (npn/pnp): used as amplifier and switch
Current relation: I_E = I_B + I_C ; gain β = I_C/I_B
Basic gates: AND, OR, NOT ; universal gates: NAND, NOR
4. Communication Systems
Block: Source → Transmitter (modulation) → Channel → Receiver (demodulation) → User
Modulation types: AM (amplitude), FM (frequency)
Worked Examples
Example 1 — Transistor currents
A transistor has I_E = 10 mA and I_B = 0.1 mA. Find I_C and β.
I_C = I_E − I_B = 10 − 0.1 = 9.9 mA
β = I_C/I_B = 9.9/0.1 = 99
Example 2 — Logic gate output
Truth table of an AND gate.
A B | A·B
0 0 | 0
0 1 | 0
1 0 | 0
1 1 | 1
Example 3 — NAND as universal
Show NAND with both inputs tied gives NOT.
A A | NAND(A,A) = (A·A)' = A'
0 | 1
1 | 0 → behaves as NOT
Example 4 — Rectifier
Why does a full-wave rectifier give smoother DC than half-wave?
Half-wave uses only one half of each AC cycle (output present 50% of time).
Full-wave uses both halves → ripple is smaller and average DC is higher.
Common Mistakes
- Swapping majority carriers of p-type and n-type.
- Saying a diode conducts in reverse bias.
- Forgetting I_E = I_B + I_C.
- Mixing AM and FM definitions.
Exam Tips & Revision Checklist
- n-type → electrons, p-type → holes (n=negative carriers).
- NAND and NOR are universal gates.
- Practised: transistor currents/β, AND truth table, NAND as NOT, rectifier reasoning.
Frequently Asked Questions
n-type is doped with pentavalent atoms (extra electrons, majority carriers electrons); p-type is doped with trivalent atoms (holes, majority carriers holes).