Semiconductor Devices
Easy Overview
Semiconductors are materials whose conductivity lies between that of conductors and insulators. But that simple definition hides their true importance — semiconductors are the backbone of modern electronics. Silicon, germanium, and gallium arsenide are the most important. By adding tiny amounts of impurities (doping), we can precisely control their electrical properties. This control gives us diodes, transistors, and integrated circuits — the building blocks of every electronic device. Pure (intrinsic) silicon has a conductivity of about 4.4 × 10⁻⁴ S/m. Doping it with phosphorus (which has 5 valence electrons) creates an n-type semiconductor with excess electrons. Doping with boron (3 valence electrons) creates p-type with excess holes. The hole is a conceptual convenience — it is the absence of an electron, behaving like a positive charge carrier. When p-type and n-type materials are joined, a p-n junction forms. At the junction, electrons and holes diffuse across, creating a depletion region with an internal electric field. This field prevents further diffusion — equilibrium is reached. The crucial point: applying external voltage can either widen or narrow this depletion region. A p-n junction diode allows current to flow easily in forward bias (positive voltage to p-side) but blocks current in reverse bias. This is rectification — converting AC to DC. Half-wave and full-wave rectifiers power our electronics. Zener diodes are specially designed to break down at a precise voltage without damage — used for voltage regulation. Transistors are three-terminal semiconductor devices that amplify signals or act as switches. The bipolar junction transistor (BJT) has three layers: n-p-n or p-n-p. A small base current controls a much larger collector current — current gain β = I_C/I_B ≈ 100-300. Field-effect transistors (FETs) use voltage to control current, consuming almost no gate current. Integrated circuits pack millions of transistors on a single chip. Logic gates (AND, OR, NOT, NAND, NOR) built from transistors form the basis of digital electronics — from calculators to supercomputers.
Classification of Solids — Conductors, Semiconductors, Insulators
Based on energy bands: conductors (no band gap or overlapping bands), semiconductors (small gap ~1 eV), insulators (large gap >3 eV). Elementary band theory: valence band (filled at 0 K) and conduction band (empty at 0 K). For semiconductors: E_g(Si) = 1.12 eV, E_g(Ge) = 0.67 eV, E_g(GaAs) = 1.43 eV.
Intrinsic and Extrinsic Semiconductors
Intrinsic: pure semiconductor, n_i = p_i = 1.5 × 10¹⁰ cm⁻³ (Si at 300 K). Extrinsic: doped with impurities. n-type: pentavalent dopant (P, As, Sb). Majority carriers = electrons. p-type: trivalent dopant (B, Al, In). Majority carriers = holes. Doping level ~ 10¹⁵-10¹⁸ cm⁻³.
Formation of p-n Junction
p-type and n-type joined. Diffusion: electrons → p-side, holes → n-side. Recombination near junction. Depletion region (space charge region) forms. Built-in potential V₀ ≈ 0.7 V (Si), 0.3 V (Ge). Width ~ 0.1-1 µm. No external bias: net current = 0 (drift and diffusion currents balance).
Forward and Reverse Bias of p-n Junction
Forward bias: p to +ve, n to −ve. Depletion width decreases, barrier height reduces. Current increases exponentially: I = I₀[e^(eV/kT) − 1]. Reverse bias: p to −ve, n to +ve. Depletion width increases. Very small current I₀ (saturation current). Break-down at high reverse voltage.
Rectification — Half-Wave and Full-Wave
Half-wave rectifier: single diode, conducts only one half-cycle. Ripple factor = 1.21. Efficiency = 40.6%. Full-wave: centre-tap or bridge (4 diodes). Conducts both half-cycles. Ripple factor = 0.48. Efficiency = 81.2%. Filter capacitor smooths output. Zener diode regulates voltage.
Zener Diode and Voltage Regulation
Zener diode operates in reverse breakdown region (Zener or avalanche). Zener voltage V_Z precisely controlled by doping. Breakdown is sharp and non-destructive. Used as voltage regulator: series resistor limits current, Zener maintains constant voltage across load. V_out = V_Z regardless of input voltage or load changes (within limits).
Photodiode and Solar Cell
Photodiode: reverse-biased p-n junction. Light generates electron-hole pairs → reverse current increases. Current ∝ light intensity. Fast response (~ns). Applications: light sensors, optical communication. Solar cell: no external bias. Light creates electron-hole pairs separated by built-in field → photovoltage. Open circuit voltage ~0.5-0.6 V (Si). Efficiency ~15-25%.
Light-Emitting Diode (LED)
Forward-biased p-n junction: electrons and holes recombine, emitting photons. Energy of photon = E_g (approximately). Colour determined by band gap: GaAs (infrared), GaAsP (red, yellow), GaP (green), GaN (blue, white). Advantages: low power, long life, fast switching. Used in displays, indicators, lighting.
Bipolar Junction Transistor (BJT)
Three layers: n-p-n or p-n-p. Three terminals: emitter (E), base (B), collector (C). Emitter heavily doped, base thin and lightly doped, collector moderately doped. Common base and common emitter configurations. Active mode: emitter-base forward biased, collector-base reverse biased. Current gain: α = I_C/I_E ≈ 0.95-0.99 (common base). β = I_C/I_B = α/(1−α) ≈ 50-200 (common emitter).
Transistor as an Amplifier (Common Emitter)
Small base current I_B controls large collector current I_C = βI_B. Input signal at base. Output at collector. Voltage gain A_V = −R_C/R_E (approximately). Input resistance: r_be ≈ 1-2 kΩ. Output resistance: r_ce ≈ 50-100 kΩ. CE amplifier needs proper biasing (voltage divider bias) and coupling capacitors.
Transistor as a Switch
Cut-off: V_BE < 0.7 V (Si), I_B = 0, I_C = 0 → switch OFF. Saturation: V_CE ≈ 0.2 V, I_C = (V_CC − V_CEsat)/R_C → switch ON. Very fast switching (ns). Application: digital logic gates. Input 0 V → output V_CC. Input 5 V → output 0 V (inverter).
Logic Gates — NOT, AND, OR, NAND, NOR
NOT (inverter): output = NOT input. AND: output = 1 only if all inputs 1. OR: output = 1 if any input 1. NAND: AND followed by NOT (universal gate). NOR: OR followed by NOT (universal gate). Truth tables and Boolean expressions. NAND and NOR are universal — any logic can be built from either one.
Integrated Circuits (ICs)
Many transistors, resistors, capacitors on a single chip (silicon). SSI (<100 gates), MSI (100-3000), LSI (3000-100000), VLSI (100000+). Moore's law: transistor count doubles ~2 years. Fabrication: photolithography, doping, metallisation. Modern CPU: billions of transistors. ICs reduced size, cost, and power consumption dramatically.
Field Effect Transistor (FET) — JFET and MOSFET
JFET: voltage-controlled device. Gate-source voltage controls channel width → drain current. Input impedance ~10⁸-10⁹ Ω. MOSFET: gate insulated by SiO₂. Very high input impedance ~10¹⁵ Ω. Enhancement and depletion types. CMOS (complementary MOS): low power consumption. Used in most modern digital ICs.
Key Points
- •Band gap: Si 1.12 eV, Ge 0.67 eV, GaAs 1.43 eV.
- •n-type: pentavalent dopant (excess electrons). p-type: trivalent (excess holes).
- •p-n junction: depletion region, built-in potential V₀ ≈ 0.7 V (Si).
- •Diode: I = I₀[e^(eV/kT) − 1]. Forward: conducts. Reverse: blocks.
- •Zener diode: voltage regulator in reverse breakdown.
- •Transistor: β = I_C/I_B ≈ 100-300. α = I_C/I_E.
- •CE amplifier: high voltage gain. Transistor switch: cut-off/saturation.
- •Logic gates: NOT, AND, OR, NAND, NOR. NAND/NOR are universal.
- •MOSFET: high input impedance, CMOS for low power.
Practice Questions
- Explain formation of p-n junction. Describe forward and reverse bias with V-I characteristic.
- What is rectification? Explain working of a full-wave bridge rectifier with circuit diagram and input-output waveforms.
- Explain working of a Zener diode as voltage regulator. Draw circuit. A 12 V Zener with series resistor 200 Ω, input 18 V, load 500 Ω. Find Zener current and load voltage.
- Explain working of n-p-n transistor in common emitter configuration as an amplifier. Derive voltage gain.
- Explain transistor as a switch. A CE circuit with V_CC = 12 V, R_C = 1 kΩ, β = 100. Find I_B and V_CE when V_BE = 0.7 V and input is 5 V through 10 kΩ base resistor. Is transistor saturated?
- What are logic gates? Draw truth tables and logic symbols for AND, OR, NOT, NAND, NOR. Show that NAND is a universal gate.
- Distinguish between intrinsic and extrinsic semiconductors. Explain doping and how n-type and p-type are formed.
- Explain working of LED and photodiode. What determines the colour of an LED?
- Describe construction and working of a MOSFET. State advantages over BJT.