Class 12 Physics Notes Chapter 6 (Semiconductor Electronics: Materials, Devices and Simple Circuits) – Physics Part-II Book

Detailed Notes with MCQs of Chapter 6: Semiconductor Electronics. This is a crucial chapter, not just for your board exams but also forms the foundation for many questions in competitive government exams. Pay close attention to the concepts, definitions, and applications.

Semiconductor Electronics: Materials, Devices and Simple Circuits

1. Introduction to Semiconductors

• Materials are classified based on their electrical conductivity (σ) or resistivity (ρ = 1/σ).
  • Conductors: High conductivity (e.g., metals like Cu, Al). ρ ~ 10⁻² - 10⁻⁸ Ωm, σ ~ 10² - 10⁸ S/m.
  • Insulators: Very low conductivity (e.g., glass, rubber). ρ ~ 10¹¹ - 10¹⁹ Ωm, σ ~ 10⁻¹¹ - 10⁻¹⁹ S/m.
  • Semiconductors: Conductivity lies between conductors and insulators (e.g., Silicon (Si), Germanium (Ge)). ρ ~ 10⁻⁵ - 10⁶ Ωm, σ ~ 10⁵ - 10⁻⁶ S/m. Their conductivity is strongly dependent on temperature and impurities.

2. Energy Bands in Solids

• In isolated atoms, electrons occupy discrete energy levels. In solids, these levels spread out into continuous bands due to interatomic interactions.
• Valence Band (VB): The energy band containing valence electrons. It may be partially or completely filled. Electrons here are not free to move.
• Conduction Band (CB): The energy band above the valence band. Electrons in this band are free to move and conduct electricity. It's usually empty or partially filled.
• Forbidden Energy Gap (E_g): The energy gap between the top of the valence band and the bottom of the conduction band. No electron can exist in this energy state.
  • Conductors: VB and CB overlap (E_g ≈ 0) or VB is partially filled. Electrons are readily available for conduction.
  • Insulators: Large energy gap (E_g > 3 eV). Electrons cannot easily jump from VB to CB.
  • Semiconductors: Small energy gap (E_g < 3 eV). For Si, E_g ≈ 1.1 eV; for Ge, E_g ≈ 0.7 eV at room temperature. Electrons can jump from VB to CB upon receiving energy (e.g., thermal energy).

3. Intrinsic Semiconductors

• A pure semiconductor (without any added impurities) is called an intrinsic semiconductor (e.g., pure Si, pure Ge).
• At absolute zero (0 K), it behaves like an insulator (VB completely filled, CB empty).
• At room temperature, some thermal energy excites electrons from the VB to the CB.
• When an electron moves from VB to CB, it leaves behind a vacancy called a hole in the VB. A hole acts as a positive charge carrier.
• In an intrinsic semiconductor, the number of free electrons (n_e) is equal to the number of holes (n_h). This is denoted by n_i (intrinsic carrier concentration). So, n_e = n_h = n_i.
• Conduction occurs due to both electrons (in CB) and holes (in VB). Total current I = I_e + I_h.
• Conductivity increases with temperature because more electron-hole pairs are generated.

4. Extrinsic Semiconductors

• The conductivity of intrinsic semiconductors is low and not very useful for practical devices.

• Doping: The process of deliberately adding a small amount of suitable impurity atoms to a pure semiconductor to increase its conductivity significantly.

• The added impurity atoms are called dopants.

• Doped semiconductors are called extrinsic semiconductors.

  • n-type Semiconductor:

    • Formed by doping a pure semiconductor (Si or Ge - Group 14) with pentavalent impurities (Group 15, e.g., Phosphorus (P), Arsenic (As), Antimony (Sb)).
    • The pentavalent dopant atom has 5 valence electrons. Four form covalent bonds with neighbouring Si/Ge atoms, and the fifth electron is loosely bound.
    • This fifth electron requires very little energy to move into the conduction band and becomes a free electron.
    • The impurity atom that donates an electron is called a donor impurity.
    • In n-type semiconductors, electrons are the majority charge carriers, and holes are the minority charge carriers. (n_e >> n_h).
    • The donor energy level (E_D) lies just below the conduction band (E_C).

  • p-type Semiconductor:

    • Formed by doping a pure semiconductor (Si or Ge) with trivalent impurities (Group 13, e.g., Boron (B), Aluminium (Al), Indium (In)).
    • The trivalent dopant atom has 3 valence electrons. It forms covalent bonds with three neighbouring Si/Ge atoms, but there is a deficiency of one electron to complete the bond with the fourth neighbour.
    • This deficiency creates a vacancy or hole. An electron from a neighbouring covalent bond can jump into this hole, creating a hole elsewhere.
    • The impurity atom that accepts an electron (or creates a hole) is called an acceptor impurity.
    • In p-type semiconductors, holes are the majority charge carriers, and electrons are the minority charge carriers. (n_h >> n_e).
    • The acceptor energy level (E_A) lies just above the valence band (E_V).

• Mass Action Law: In any semiconductor (intrinsic or extrinsic) under thermal equilibrium, the product of electron and hole concentration is constant and equal to the square of the intrinsic carrier concentration: n_e n_h = n_i².

5. p-n Junction

• A p-n junction is formed when a p-type semiconductor is brought into close contact (usually fabricated) with an n-type semiconductor.
• Formation:
  • Diffusion: Due to the concentration gradient, holes diffuse from the p-side to the n-side, and electrons diffuse from the n-side to the p-side. This constitutes the diffusion current.
  • Depletion Region: As diffusion occurs, holes leave behind negative acceptor ions (immobile) on the p-side, and electrons leave behind positive donor ions (immobile) on the n-side near the junction. This region, devoid of free charge carriers, is called the depletion region or space charge region.
  • Potential Barrier (V_B): The accumulation of immobile ions creates an electric field directed from the n-side to the p-side. This field opposes further diffusion. The potential difference across the depletion region is called the potential barrier or barrier voltage (typically ~0.7V for Si, ~0.3V for Ge).
  • Drift Current: The electric field in the depletion region causes minority carriers (electrons from p-side, holes from n-side) to drift across the junction. This constitutes the drift current.
  • In equilibrium (no external voltage), the diffusion current equals the drift current, and there is no net current flow.

6. Semiconductor Diode (p-n Junction Diode)

• A p-n junction with metallic contacts at the ends for applying external voltage. Symbol: ----|>|---- (Arrow points from p to n, indicating the direction of conventional current flow when forward biased).

  • Forward Biasing:

    • The positive terminal of the external voltage source is connected to the p-side, and the negative terminal to the n-side.
    • The applied voltage opposes the potential barrier (V_B).
    • If the applied voltage (V) > V_B, the potential barrier is significantly reduced.
    • Majority carriers (holes from p, electrons from n) can now easily cross the junction.
    • The depletion region width decreases.
    • A large current (mA range) flows. The diode offers very low resistance.
    • V-I Characteristics: Current increases slowly initially, then rapidly after the knee voltage (≈ V_B).

  • Reverse Biasing:

    • The positive terminal of the external voltage source is connected to the n-side, and the negative terminal to the p-side.
    • The applied voltage supports the potential barrier, increasing its height (V_B + V).
    • Majority carriers are pulled away from the junction.
    • The depletion region width increases.
    • Diffusion of majority carriers stops. Only a small current (μA range) due to minority carriers drifting across the junction flows. This is called the reverse saturation current, which is largely independent of the applied reverse voltage (up to a limit).
    • The diode offers very high resistance.
    • V-I Characteristics: A very small, almost constant reverse current flows.
    • Reverse Breakdown: If the reverse voltage is increased excessively, the junction breaks down, and a large reverse current flows. This can be due to:
      • Zener Breakdown: Occurs in heavily doped diodes at low reverse voltages. Strong electric field causes electrons to be pulled out from covalent bonds.
      • Avalanche Breakdown: Occurs in lightly doped diodes at high reverse voltages. Minority carriers gain enough energy to knock out more carriers through collisions, leading to an avalanche multiplication of charge carriers.

7. Application of Junction Diode: Rectifier

• Rectification: The process of converting alternating current (AC) into direct current (DC). Diodes are used because they conduct current primarily in one direction (forward bias).
  • Half-Wave Rectifier: Uses a single diode. Conducts only during one half-cycle of the AC input. Output is pulsating DC (only positive or only negative half-cycles). Efficiency is low (max 40.6%). High ripple factor.
  • Full-Wave Rectifier: Uses two (centre-tap transformer) or four (bridge configuration) diodes. Conducts during both half-cycles of the AC input. Output is pulsating DC with less ripple than half-wave. Efficiency is higher (max 81.2%).
    • Centre-Tap: Requires a centre-tapped transformer. Peak Inverse Voltage (PIV) across each diode is 2V_m.
    • Bridge Rectifier: Does not require a centre-tapped transformer. PIV across each diode is V_m. More commonly used.
• Filter Circuits: Usually capacitors (or inductors) are used after the rectifier circuit to smooth out the pulsating DC output and make it closer to pure DC.

8. Special Purpose p-n Junction Diodes

• Zener Diode:

  • A heavily doped p-n junction designed to operate in the reverse breakdown region without being damaged.
  • Symbol: Similar to a standard diode, but the bar has small 'flags' pointing inwards.
  • It maintains an almost constant voltage across it (Zener voltage, V_Z) when the reverse current varies over a wide range.
  • Application: Used as a voltage regulator to provide a stable DC voltage from a fluctuating DC input.

• Photodiode:

  • A p-n junction diode designed to detect light. Fabricated with a transparent window.
  • Operated under reverse bias.
  • When light falls on the junction, electron-hole pairs are generated in or near the depletion region.
  • These minority carriers are swept across the junction by the reverse bias field, causing an increase in the reverse current.
  • The reverse current is proportional to the intensity of incident light.
  • Applications: Light detection, optical communication, counters, smoke detectors.

• Light Emitting Diode (LED):

  • A heavily doped p-n junction diode that emits light when forward biased.
  • When forward biased, electrons and holes recombine at the junction.
  • In certain semiconductor materials (like Gallium Arsenide - GaAs, Gallium Phosphide - GaP, Gallium Arsenide Phosphide - GaAsP), the energy released during recombination is emitted in the form of photons (light).
  • The colour of the emitted light depends on the semiconductor material and its doping (i.e., the band gap energy E_g). E_g = hν = hc/λ.
  • Advantages: Low power consumption, fast switching speed, long life, rugged.
  • Applications: Indicator lamps, displays, optical communication, remote controls, traffic signals.

• Solar Cell (Photovoltaic Cell):

  • A p-n junction that generates an electromotive force (emf) when solar radiation falls on it. No external bias is applied.
  • Works on the photovoltaic effect.
  • Light generates electron-hole pairs near the junction.
  • The junction field separates these carriers (electrons to n-side, holes to p-side), creating a potential difference across the junction (photovoltage).
  • If an external load is connected, a current flows.
  • Materials: Si, GaAs, CdTe are commonly used.
  • Applications: Powering satellites, calculators, street lights, remote area power supply, solar panels.

9. Junction Transistor

• A semiconductor device consisting of two p-n junctions formed back-to-back. It can amplify weak signals or act as a switch.
• Types:
  • n-p-n: A thin layer of p-type semiconductor sandwiched between two layers of n-type semiconductor.
  • p-n-p: A thin layer of n-type semiconductor sandwiched between two layers of p-type semiconductor.
• Terminals:
  • Emitter (E): Supplies majority charge carriers. Heavily doped. Moderate size.
  • Base (B): The middle section. Controls the flow of carriers. Very thin and lightly doped.
  • Collector (C): Collects the majority carriers. Moderately doped. Largest size (to dissipate heat).
• Biasing (for active region/amplification):
  • Emitter-Base (EB) junction is Forward Biased.
  • Collector-Base (CB) junction is Reverse Biased.
• Working (n-p-n example):
  • Forward bias on EB junction pushes electrons from emitter into the base.
  • Base is thin and lightly doped, so most (~95-99%) electrons pass through it into the collector region. A small fraction (~1-5%) recombine with holes in the base, constituting the base current (I_B).
  • Reverse bias on CB junction attracts electrons arriving from the base into the collector. These constitute the collector current (I_C).
  • Emitter current I_E = I_B + I_C.
• Transistor Configurations: Based on which terminal is common to both input and output circuits.
  • Common Base (CB): Input between E & B, Output between C & B. Current gain α = ΔI_C / ΔI_E (α < 1, typically 0.95-0.99). High voltage gain, no current gain. Used for high-frequency applications.
  • Common Emitter (CE): Input between B & E, Output between C & E. Current gain β = ΔI_C / ΔI_B (β >> 1, typically 50-300). High voltage gain, high current gain, high power gain. Most widely used for amplification.
  • Common Collector (CC): Input between B & C, Output between E & C. Voltage gain ≈ 1. High current gain. Used as a buffer (impedance matching).
• Relation between α and β:
  • β = α / (1 - α)
  • α = β / (1 + β)
• Transistor Characteristics (CE Configuration):
  • Input Characteristics: Plot of I_B vs V_BE at constant V_CE. Resembles a forward-biased diode curve. Input resistance r_i = (ΔV_BE / ΔI_B)_VCE=const.
  • Output Characteristics: Plot of I_C vs V_CE at constant I_B. Shows three regions:
    • Cut-off Region: I_B = 0, I_C is very small (leakage current). Transistor is OFF. (Both junctions reverse biased).
    • Active Region: I_C increases slightly with V_CE but largely depends on I_B (I_C ≈ βI_B). Transistor acts as an amplifier. (EB forward, CB reverse biased).
    • Saturation Region: I_C becomes almost independent of I_B and reaches a maximum value. Transistor is fully ON. (Both junctions forward biased). Output resistance r_o = (ΔV_CE / ΔI_C)_IB=const.
• Transistor as an Amplifier (CE):
  • A small AC signal voltage applied to the base circuit causes variations in I_B.
  • These variations are amplified (by a factor β) in the collector current I_C.
  • The varying I_C flows through a load resistor (R_L) connected in the collector circuit, producing a large varying output voltage across R_L.
  • Voltage Gain (A_v) = ΔV_out / ΔV_in ≈ -β (R_L / r_i). The negative sign indicates a 180° phase shift between input and output signals.
  • Power Gain (A_p) = A_v × β.
• Transistor as a Switch:
  • Operated between cut-off (OFF state) and saturation (ON state) regions.
  • When input voltage is low (e.g., 0V), I_B = 0, transistor is in cut-off, I_C ≈ 0, V_out ≈ V_CC (High). Switch is OFF.
  • When input voltage is high enough, I_B is large, transistor goes into saturation, I_C is maximum, V_CE ≈ 0, V_out ≈ 0 (Low). Switch is ON.

10. Digital Electronics and Logic Gates

• Analog Signal: Varies continuously with time.
• Digital Signal: Has only discrete values (typically two levels: High/1 and Low/0). Used in computers, communication systems.
• Logic Gate: An electronic circuit that performs a logical operation on one or more input signals to produce a single output signal. Based on Boolean algebra.
  • NOT Gate (Inverter): One input, one output. Output is the complement of the input. Symbol: Triangle with a bubble at the output. Truth Table: A=0 -> Y=1; A=1 -> Y=0. Boolean Expression: Y = A̅
  • OR Gate: Two or more inputs, one output. Output is High (1) if any input is High (1). Symbol: Curved input side. Truth Table: A=0,B=0 -> Y=0; A=0,B=1 -> Y=1; A=1,B=0 -> Y=1; A=1,B=1 -> Y=1. Boolean Expression: Y = A + B
  • AND Gate: Two or more inputs, one output. Output is High (1) only if all inputs are High (1). Symbol: Straight input side. Truth Table: A=0,B=0 -> Y=0; A=0,B=1 -> Y=0; A=1,B=0 -> Y=0; A=1,B=1 -> Y=1. Boolean Expression: Y = A . B
  • NAND Gate (NOT AND): AND gate followed by a NOT gate. Output is Low (0) only if all inputs are High (1). Symbol: AND gate with bubble at output. Boolean Expression: Y = (A . B)̅
  • NOR Gate (NOT OR): OR gate followed by a NOT gate. Output is High (1) only if all inputs are Low (0). Symbol: OR gate with bubble at output. Boolean Expression: Y = (A + B)̅
• Universal Gates: NAND and NOR gates are called universal gates because any other basic gate (NOT, OR, AND) can be constructed using only NAND gates or only NOR gates.

11. Integrated Circuits (ICs)

• Complete electronic circuits (containing transistors, diodes, resistors, capacitors) fabricated on a single small chip of semiconductor material (usually Silicon).
• Advantages: Extremely small size, low cost, high reliability, low power consumption.
• Types (based on fabrication): Monolithic ICs (entire circuit on one chip), Hybrid ICs (interconnection of several individual components/chips).
• Scale of Integration: SSI (Small), MSI (Medium), LSI (Large), VLSI (Very Large Scale Integration).

Multiple Choice Questions (MCQs)

1. In an n-type semiconductor, the concentration of minority carriers mainly depends upon:
   (a) Doping concentration
   (b) Temperature
   (c) Forward bias voltage
   (d) Reverse bias voltage

2. The depletion region in a p-n junction diode consists of:
   (a) Free electrons only
   (b) Free holes only
   (c) Mobile ions
   (d) Immobile ions

3. Reverse bias applied to a junction diode:
   (a) Increases the potential barrier
   (b) Decreases the potential barrier
   (c) Increases the majority carrier current
   (d) Decreases the minority carrier current

4. Which of the following diodes is operated in reverse bias?
   (a) LED
   (b) Solar Cell
   (c) Zener Diode
   (d) Tunnel Diode

5. In a common emitter (CE) amplifier, the phase difference between the input signal voltage and the output voltage is:
   (a) 0°
   (b) 90°
   (c) 180°
   (d) 270°

6. If α = 0.98 in a transistor, what is the value of β?
   (a) 49
   (b) 50
   (c) 98
   (d) 0.02

7. A Zener diode is primarily used as a:
   (a) Rectifier
   (b) Amplifier
   (c) Voltage regulator
   (d) Oscillator

8. Which logic gate is represented by the following truth table?

   A	B	Y
   0	0	1
   0	1	0
   1	0	0
   1	1	0
   (a) AND
   (b) OR
   (c) NAND
   (d) NOR

9. The energy band gap is maximum in:
   (a) Metals
   (b) Semiconductors
   (c) Insulators
   (d) Superconductors

10. A solar cell converts:
    (a) Electrical energy into light energy
    (b) Light energy into electrical energy
    (c) Thermal energy into electrical energy
    (d) Electrical energy into thermal energy

Answer Key for MCQs:

1. (b) Temperature (Minority carriers are generated thermally, similar to intrinsic carriers)
2. (d) Immobile ions (Positive donor ions on n-side, negative acceptor ions on p-side)
3. (a) Increases the potential barrier
4. (c) Zener Diode (Also Photodiode, but Zener is specifically designed for reverse breakdown operation)
5. (c) 180°
6. (a) 49 (β = α / (1 - α) = 0.98 / (1 - 0.98) = 0.98 / 0.02 = 49)
7. (c) Voltage regulator
8. (d) NOR (Output is 1 only when both inputs are 0)
9. (c) Insulators
10. (b) Light energy into electrical energy

Make sure you revise these concepts thoroughly. Understanding the working principle of each device and its characteristics is key for solving problems in exams. Good luck!