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Semiconductors

Ch. 14Std 11

Easy Overview

Every single electronic device you own - your phone, laptop, TV - is built from tiny semiconductors. These materials are neither good conductors nor good insulators; they are in between. But with a little clever doping (adding impurities), they become the most versatile materials in the world. This chapter is about the building blocks of modern electronics. You will learn about intrinsic and extrinsic semiconductors, PN junction diodes and their V-I characteristics, rectifiers that convert AC to DC, special diodes like Zener diodes and LEDs, and the basics of logic gates - the fundamental building blocks of digital circuits. The transistor, invented in 1947, revolutionized the world. Today, billions of transistors fit on a fingernail-sized chip. Understanding semiconductors is understanding how modern technology works.

Intrinsic Semiconductors

Pure silicon (Si) or germanium (Ge) are intrinsic semiconductors. At absolute zero (0 K), they are perfect insulators - all electrons are in the valence band, the conduction band is empty. As temperature rises, some electrons gain enough energy to jump from the valence band to the conduction band, leaving behind holes (vacant spots) in the valence band. Each electron-hole pair is created thermally. In an intrinsic semiconductor, the number of electrons in conduction band (n_i) equals the number of holes in valence band (p_i), and both increase with temperature. At 300 K, silicon has about 1.5 x 10^10 carriers/cm^3 - much less than a conductor (about 10^22/cm^3). This is why pure silicon does not conduct well.

Extrinsic Semiconductors - Doping

Doping is adding a tiny impurity (about 1 part in 10^7) to dramatically change conductivity. N-type semiconductor: add a pentavalent atom (phosphorus, arsenic, antimony - 5 valence electrons). Four electrons bond with silicon, one is extra - it becomes a free electron. The majority carriers are electrons, minority are holes. P-type semiconductor: add a trivalent atom (boron, aluminum, indium - 3 valence electrons). Three electrons bond, leaving one bond unfilled - a hole. Majority carriers are holes, minority are electrons. The doping concentration determines conductivity. Heavier doping = more carriers = higher conductivity. The extrinsic carrier concentration far exceeds the intrinsic concentration - n_n >> n_i for n-type, p_p >> n_i for p-type.

PN Junction Diode

Join a p-type and n-type semiconductor together. At the junction, electrons from n-side diffuse into p-side, recombining with holes. This creates a depletion region - a layer with no free carriers, only fixed ions (positive on n-side, negative on p-side). This sets up a built-in potential barrier (about 0.7 V for silicon, 0.3 V for germanium). Forward bias: p-side connected to positive, n-side to negative. The applied voltage reduces the barrier, current flows easily. Reverse bias: p-side to negative, n-side to positive. The barrier increases, no current flows (except a tiny leakage current). The diode acts as a one-way valve for current - the basis of rectification.

V-I Characteristics of PN Junction

The voltage-current relationship of a diode is nonlinear. In forward bias: current is negligible until the voltage exceeds the knee voltage (about 0.7 V for Si, 0.3 V for Ge). After that, current increases rapidly with small voltage increase - approximately I = I_0 (e^(eV/kT) - 1). In reverse bias: a very small reverse saturation current I_0 flows (microamperes for Si, milliamperes for Ge), which is almost independent of voltage. If reverse voltage exceeds the breakdown voltage, the diode breaks down and current suddenly increases. The V-I characteristic clearly shows the rectifying behavior - the diode conducts in one direction only.

Half-Wave Rectifier

A rectifier converts AC to DC. A half-wave rectifier uses a single diode. During the positive half-cycle of AC input, the diode is forward biased and conducts - current flows through the load. During the negative half-cycle, the diode is reverse biased and blocks current. The output is a pulsating DC with only positive half-cycles. The average (DC) voltage V_dc = V_m/pi, where V_m is the peak voltage. The ripple factor (measure of smoothness) is about 1.21 for half-wave - very poor. Efficiency is about 40.6%. Half-wave rectifiers are simple but inefficient - they waste half the input power.

Full-Wave Rectifier

A full-wave rectifier uses both half-cycles. There are two types: center-tap rectifier (uses two diodes and a center-tapped transformer) and bridge rectifier (uses four diodes, no center tap needed). In a bridge rectifier: during positive half-cycle, diodes D1 and D2 conduct, D3 and D4 are reverse biased. During negative half-cycle, D3 and D4 conduct, D1 and D2 are reverse biased. The output current flows through the load in the same direction for both half-cycles. Average voltage V_dc = 2V_m/pi. Ripple factor is about 0.48 - much smoother than half-wave. Efficiency is about 81.2%. Adding a filter capacitor across the output smoothes the pulsating DC further.

Zener Diode

A Zener diode is designed to operate in reverse breakdown - it conducts in reverse bias when voltage exceeds the Zener voltage V_z. The key property: the voltage across a Zener diode remains nearly constant at V_z even when current varies widely (within limits). This makes it perfect for voltage regulation. A Zener voltage regulator: connect a Zener diode (in reverse bias) with a series resistor R_s. The input voltage V_in drops across R_s and the Zener. As V_in fluctuates, the current through the Zener changes, but the voltage across it stays at V_z. The load across the Zener gets a stable voltage. Zener diodes are available with various breakdown voltages from about 2 V to hundreds of volts.

Light Emitting Diode (LED)

An LED emits light when forward biased. When electrons and holes recombine at the PN junction, energy is released as photons (light). The color depends on the band gap energy E_g of the semiconductor: E_g = hf = hc/lambda. Different materials give different colors: GaAs (infrared), GaAsP (red, yellow), GaP (green), GaN (blue, white). LEDs have many advantages: low power consumption, long life, fast switching, small size, ruggedness. They replaced incandescent bulbs in many applications. White LEDs are made by coating a blue LED with yellow phosphor - the combination produces white light. LEDs require proper current limiting (a series resistor) - too much current destroys them.

Logic Gates

Logic gates are the simplest digital building blocks. They perform Boolean operations on binary inputs (0 and 1). AND gate: output 1 only if ALL inputs are 1. OR gate: output 1 if AT LEAST one input is 1. NOT gate: inverts the input (0?1, 1?0). NAND gate: AND followed by NOT - universal gate (any logic gate can be built using only NANDs). NOR gate: OR followed by NOT - also universal. XOR gate: output 1 if inputs are different. The truth table lists all input combinations and the corresponding output. Logic gates are implemented using transistors (diodes and transistors in RTL, DTL, TTL, or CMOS technology). Millions of gates fit on a single chip in modern processors.

Key Points

  • •Intrinsic semiconductor: pure Si/Ge. Electrons and holes in equal numbers (n_i = p_i).
  • •Extrinsic: n-type (pentavalent dopant ? extra electrons) and p-type (trivalent dopant ? extra holes).
  • •Doping dramatically increases conductivity. Majority carriers dominate conductivity.
  • •PN junction: depletion region forms at junction. Built-in potential ~0.7 V (Si).
  • •Forward bias: reduces barrier ? current flows. Reverse bias: increases barrier ? no current.
  • •V-I characteristic: nonlinear. Knee voltage ~0.7 V (Si). Breakdown in reverse at high voltage.
  • •Half-wave rectifier: 1 diode, V_dc = V_m/pi, efficiency 40.6%, ripple factor 1.21.
  • •Full-wave bridge rectifier: 4 diodes, V_dc = 2V_m/pi, efficiency 81.2%, ripple factor 0.48.
  • •Zener diode: operates in reverse breakdown. Used as voltage regulator - maintains constant V_z.
  • •LED: emits light when forward biased. Color depends on band gap. Requires current limiting resistor.
  • •Logic gates: AND, OR, NOT, NAND, NOR, XOR. NAND and NOR are universal gates.
  • •Truth table lists all input-output combinations for a logic gate.
  • •Transistors are the building blocks of logic gates and all digital circuits.

Practice Questions

  • Distinguish between intrinsic and extrinsic semiconductors. Explain n-type and p-type doping.
  • Explain the working of a PN junction diode in forward and reverse bias with V-I characteristics.
  • Draw and explain the working of a full-wave bridge rectifier. How is ripple reduced?
  • Distinguish between half-wave and full-wave rectifiers.
  • What is a Zener diode? Explain its use as a voltage regulator with a circuit diagram.
  • Explain the working of an LED. What determines the color of light emitted?
  • Realize the logic gates AND, OR, NOT using NAND gates only.
  • Draw the truth tables for AND, OR, NOT, NAND, and NOR gates.
  • A half-wave rectifier has input 230 V (rms) AC. Find the DC output voltage and ripple factor.
  • Explain the formation of depletion region in a PN junction. What is barrier potential?