L-28: Semiconductors and Semiconducting Devices

Semiconductors and Semiconducting Devices

Solid-state electronics rests on understanding energy bands, doped semiconductors, p-n junction diodes, and bipolar junction transistors. Module 8: Semiconductors Devices and Communication.

28.1 Energy Bands in Solids

In crystals, interacting atoms form quasi-continuous energy bands from discrete levels. Conduction band (CB) — unfilled levels; valence band (VB) — filled levels.

28.2 Intrinsic and Extrinsic Semiconductors

Intrinsic (pure Si/Ge): electrons and holes generated in pairs by thermal energy; equal concentrations; low conductivity at room temperature (Ge: ~2.5×10¹⁹ m⁻³ pairs at 300 K).

Extrinsic (doped): ~1 dopant atom per 10⁸ host atoms. Group V (P, As, Sb) → n-type (donor impurities, excess electrons). Group III (B, Al, Ga) → p-type (acceptor impurities, holes). Neither n-type nor p-type is net charged — ions balance carriers.

Current convention: direction of hole motion = conventional current direction.

28.3 p-n Junction

Joining n-type and p-type: electrons diffuse to p-side, holes to n-side → recombination → depletion region (~0.5 μm) of immobile donor/acceptor ions → barrier potential opposes further diffusion (Si ~0.7 V, Ge ~0.3 V). Symbol: arrow shows conventional current direction (p → n).

28.4–28.5 Forward and Reverse Bias; I-V Characteristics

Forward bias: p to +, n to −. Barrier overcome when V > knee voltage (Si ~0.7 V, Ge ~0.3 V). Low forward resistance (~10–30 Ω); current ~mA. Depletion width decreases.

Reverse bias: p to −, n to +. Majority carriers move away; small reverse saturation (leakage) current from minority carriers (μA for Ge, nA for Si). High resistance. Depletion width increases. At breakdown voltage: avalanche or Zener effect → sharp current rise.

28.6 Special Diodes

• Zener diode: heavily doped; thin depletion; operates in reverse breakdown — constant V_Z for voltage regulation
• LED: forward bias; electroluminescence (GaAsP, InP); Group III–V materials
• Photodiode: reverse bias; photovoltaic effect; photocurrent ∝ light intensity
• Solar cell: photovoltaic; generation, separation (depletion field), collection of e⁻–h⁺ pairs; I–V in 4th quadrant (supplies current)

28.7–28.8 Transistors

BJT (1948, Bardeen/Brattain/Shockley): three regions — Emitter (heaviest doping), Base (thinnest, lightest doping), Collector (largest, moderate doping). Types: npn and pnp. Arrow in symbol shows conventional emitter current direction.

Active region biasing: emitter-base junction forward biased; collector-base junction reverse biased. >95% emitter carriers reach collector (thin lightly doped base).

Configurations: Common Emitter (CE) — most used (voltage + current gain); Common Base (CB) — constant current source; Common Collector (CC) — impedance matching.

CE characteristics: input (V_BE vs I_B) like forward diode; output (V_CE vs I_C at fixed I_B). Input resistance R_ie ≈ 20–100 Ω (npn) or ~kΩ (pnp CE).

         SEMICONDUCTORS — KEY POINTS
         ==============================
    Conductor       :  CB overlaps VB
    Semiconductor   :  Eg ≤ 3 eV ; insulator Eg > 3 eV
    n-type          :  pentavalent donor (extra e⁻)
    p-type          :  trivalent acceptor (holes)
    Barrier pot.    :  Si 0.7 V ; Ge 0.3 V
    Forward bias    :  low R, knee voltage, depletion shrinks
    Reverse bias    :  high R, leakage current, depletion grows
    Zener           :  reverse breakdown, voltage regulator
    Photodiode      :  reverse bias · Solar cell: photovoltaic
    Transistor      :  E-B forward + C-B reverse (active)
    α, β            :  β = α/(1−α) ; CE config most common

Quick Revision

• Intrinsic: e⁻ and holes in equal numbers; extrinsic: majority carriers from dopants.
• Depletion region = immobile ions; not depleted of charge, but of mobile carriers.
• Diode: unidirectional — rectification, battery protection, detection.
• Two back-to-back diodes ≠ transistor (four doped regions, wrong base).
• Transistor naming: AC125 = Ge AF transistor; BY127 = Si rectifier diode.