PhysicsPHY 12
Semiconductor Devices
p-n junction, transistor, logic gates, LED, photodiode, solar cell
Concept Core
Essential theory — everything NCERT tests on Semiconductor Devices
SEMICONDUCTORS BASICS
Intrinsic semiconductor: Si, Ge. n-type: donor impurity (P, As, Sb) — extra electrons. p-type: acceptor impurity (B, Al, In) — holes.
Energy gap: Si = 1.1 eV, Ge = 0.7 eV, GaAs = 1.4 eV. Conductivity increases with temperature (unlike metals).
p-n JUNCTION DIODE
Depletion region: no free carriers. Forward bias: p connected to +, depletion narrows → current flows. Reverse bias: depletion widens → only minority carriers → tiny reverse current. Breakdown at high reverse voltage.
V-I curve: ideal diode equation. Knee voltage: Si ≈ 0.7V, Ge ≈ 0.3V.
SPECIAL DIODES
Zener diode: used in reverse breakdown for voltage regulation. Sharp breakdown = Zener voltage.
LED: forward biased. Photon emitted when electron recombines with hole. Energy of photon = band gap. Photodiode: reverse biased. Light creates electron-hole pairs → reverse current increases.
Solar cell: no bias applied. p-n junction generates EMF when illuminated.
TRANSISTOR
BJT: Emitter (heavily doped), Base (thin, lightly doped), Collector. Types: n-p-n, p-n-p.
Common emitter: current gain β = I_C/I_B. Voltage gain = β × R_C/R_in. Phase reversal (180° shift).
α = I_C/I_E. β = α/(1−α). I_E = I_B + I_C.
LOGIC GATES
AND: output=1 only if all inputs=1. OR: output=1 if any input=1. NOT: inverter.
NAND: NOT AND (universal gate). NOR: NOT OR (universal gate). Both NAND and NOR can implement any Boolean function.
De Morgan: (A·B)' = A'+B'. (A+B)' = A'·B'.
Fact & Formula Vault
High-yield facts, numbers, and formulas
Energy Gaps
Si: 1.1 eV
Ge: 0.7 eV
GaAs: 1.4 eV
Knee voltage: Si=0.7V, Ge=0.3V
Transistor Relations
α = Ic/Ie
β = Ic/Ib
β = α/(1−α)
Ie = Ib + Ic
Diode Types
p-n junction: rectification
Zener: voltage regulation
LED: light emission (forward bias)
Photodiode: light detection (reverse bias)
Solar cell: EMF generation (no bias)
Worked Examples
NEET-style questions solved step-by-step
EASYZener diode is used in:▾
Zener diode is used in:
Voltage regulation. Zener operates in reverse breakdown. The breakdown voltage is sharp and well-defined, maintaining a constant voltage across it despite current changes.
MEDIUMIn a common emitter transistor, if Ib = 50 μA and Ic = 5 mA. β = ?▾
In a common emitter transistor, if Ib = 50 μA and Ic = 5 mA. β = ?
β = Ic/Ib = 5 mA / 50 μA = 5000/50 = 100. Also, Ie = Ib + Ic = 50 + 5000 = 5050 μA. α = Ic/Ie = 5000/5050 ≈ 0.99.
HARDNAND gate is called a universal gate because:▾
NAND gate is called a universal gate because:
Any logic gate (AND, OR, NOT, NOR, XNOR, etc.) can be constructed using only NAND gates. Similarly for NOR. This makes fabrication simpler — only one type of gate needed on a chip.
Mistake DNA
Common NEET traps for this chapter
⚠ Photodiode vs Solar cell bias
Photodiode operates under REVERSE bias. Solar cell has NO external bias — it generates its own EMF from light. Common confusion.
✓ Fix: Photodiode = reverse bias. Solar cell = no bias (self-generating).
⚠ LED bias
LED requires FORWARD bias to emit light. Photodiode needs reverse bias to detect light.
✓ Fix: LED = forward. Photodiode = reverse.
⚠ n-type vs p-type majority carriers
n-type: majority = electrons, minority = holes. p-type: majority = holes, minority = electrons.
✓ Fix: n-type = extra electrons (donor). p-type = extra holes (acceptor).
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
2–3 Q/year. Zener voltage regulation, transistor β calculation, logic gate truth tables, LED vs photodiode bias, diode types.
High-Yield
β = Ic/Ib. Zener = voltage regulator (reverse breakdown). LED = forward bias. Photodiode = reverse. Solar = no bias. NAND/NOR = universal gates.
Strategy
Memorise all 5 special diode types and their bias conditions. Logic gates: make truth tables for AND/OR/NAND/NOR. β problems are formula substitution.
🧪 Practice with Full Mock Tests
7 full-length NEET mocks · 180 Qs · +4/−1 · Mock 1 FREE
Open Mock Tests →
