Semiconductors Revision Notes for NEET Physics PDF Download
💻 SEMICONDUCTOR ELECTRONICS · Materials, Devices & Circuits
Energy bands • Intrinsic/extrinsic • p‑n junction • Diodes • Transistors • Logic gates • NEET problems
⚡ ENERGY BANDS IN SOLIDS
Valence band, conduction band, forbidden gap (Eg).
Conductors: Eg = 0 (overlap).
Insulators: Eg > 3 eV.
Semiconductors: Eg ≈ 1 eV (Si: 1.1 eV, Ge: 0.7 eV).
📊 Energy band diagram
🔬 INTRINSIC & EXTRINSIC SEMICONDUCTORS
Intrinsic (pure): ne = nh = ni.
n‑type (doped with pentavalent): ne >> nh (majority = electrons). Donor level near CB.
p‑type (doped with trivalent): nh >> ne (majority = holes). Acceptor level near VB.
Mass action law: ne nh = ni²
🧬 n‑type & p‑type crystal structures
🔌 p‑n JUNCTION DIODE
Formed by joining p‑type and n‑type semiconductors. Depletion region created, built‑in potential barrier (~0.7V for Si, 0.3V for Ge).
Forward bias: p to +, n to – → barrier reduced, current flows.
Reverse bias: p to –, n to + → barrier increased, small leakage current.
I–V equation: I = I₀(eeV/kT – 1).
📈 p‑n junction I‑V characteristic
⚙️ DIODE APPLICATIONS
Rectifiers
Half‑wave: η = 40.6%
Full‑wave (bridge): η = 81.2%
Ripple factor: r = √(Vrms²/Vdc² – 1).
Zener Diode
Operates in reverse breakdown → voltage regulator.
LED & Photodiode
LED: forward bias → light emission.
Photodiode: reverse bias → light detection.
🔧 BIPOLAR JUNCTION TRANSISTOR (BJT)
Three regions: emitter (E), base (B), collector (C). Types: npn & pnp.
Transistor action: IE = IB + IC, IC = β IB (β = current gain).
Common emitter configuration: voltage gain Av = –β RC/RB.
📊 Transistor symbol & biasing
🔘 LOGIC GATES
NOT
Y = Ā
Truth table: 0→1, 1→0.
AND
Y = A·B
Output 1 only if both inputs are 1.
OR
Y = A+B
Output 1 if any input is 1.
NAND
Y = (A·B)′
Universal gate.
NOR
Y = (A+B)′
Universal gate.
XOR
Y = A⊕B = A′B + AB′
🧩 Logic gate symbols
💡 NEET TIPS & SHORTCUTS
- In intrinsic semiconductor, ni ∝ T3/2 e–Eg/2kT.
- For a diode, forward voltage drop is constant (≈0.7V Si) in simplified model.
- Transistor β = IC/IB (typical 50–300).
- NAND and NOR are universal gates (can implement any logic).
⚠️ COMMON MISTAKES
- Confusing n‑type (electrons majority) with p‑type (holes majority).
- Applying Ohm's law to p‑n junction – it's non‑ohmic.
- Forgetting that in forward bias, potential barrier reduces.
- Mixing up logic gate truth tables.
📌 QUICK REVISION CARD
ne nh = ni²
Diode I‑V: I = I₀(eeV/kT–1)
Rectifier efficiency (FW): 81.2%
Transistor: IC = β IB
Common emitter gain: Av = –β RC/RB
Logic gates: AND (·), OR (+), NOT (′)