Semiconductor electronics has transformed modern life through the transistor, integrated circuits, and digital devices. Semiconductors have conductivity between that of conductors and insulators, and this conductivity can be precisely controlled by doping. This chapter begins with the energy band theory that classifies solids into conductors, insulators, and semiconductors.
We study intrinsic and extrinsic semiconductors, the formation of p-type and n-type materials, and the p-n junction - the basic building block of electronic devices. The junction's behaviour under forward and reverse bias gives the diode its current-voltage characteristics, which are exploited in rectifiers.
The chapter then introduces the transistor, the fundamental amplifying and switching device, and its use as an amplifier and a switch. Finally, we study digital logic gates - AND, OR, NOT, NAND, NOR - which form the basis of all digital computers and information processing.
In a solid, the energy levels of individual atoms split into bands due to the proximity of the atoms. The allowed energy levels form the conduction band and the valence band, separated by an energy gap (forbidden zone).
In conductors, the valence and conduction bands overlap, so electrons are free to move, giving high conductivity. In insulators, the energy gap is large (more than 3 eV), so electrons cannot easily jump to the conduction band. In semiconductors, the gap is small (about 1 eV), so a few electrons can be thermally promoted to the conduction band.
For silicon, the gap is 1.1 eV, and for germanium 0.7 eV. Semiconductors have negative temperature coefficient of resistance - their resistance decreases with temperature because more electrons cross the gap. The conductivity of semiconductors is between 10^-5 to 10^6 siemens per metre.
An intrinsic semiconductor is a pure semiconductor like silicon or germanium, in which the number of free electrons equals the number of holes. At room temperature, a small number of covalent bonds break, creating electron-hole pairs. Holes are vacancies that behave as positively charged mobile carriers.
The conductivity of intrinsic semiconductors is low and increases with temperature. To control conductivity, semiconductors are doped - small amounts of impurity atoms are added. This creates extrinsic semiconductors with two types:
n-type semiconductors are formed by doping with pentavalent impurities like phosphorus, arsenic, or antimony. Four of the impurity's valence electrons form covalent bonds, and the fifth becomes a free electron. Electrons are the majority carriers and holes the minority carriers.
p-type semiconductors are formed by doping with trivalent impurities like boron, aluminium, or gallium. The impurity has only three valence electrons, leaving a hole in the bond. Holes are the majority carriers and electrons the minority carriers.
A p-n junction is formed by joining p-type and n-type semiconductor regions in a single crystal. When the junction is formed, electrons from the n-region diffuse into the p-region and recombine with holes, and holes diffuse into the n-region. This leaves immobile positive and negative ions on either side, forming a depletion region with no free carriers.
An electric field develops across the depletion region, creating a potential barrier (about 0.7 V for silicon and 0.3 V for germanium) that opposes further diffusion. In equilibrium, the diffusion current equals the drift current, and no net current flows.
Under forward bias (p connected to positive, n to negative), the applied voltage reduces the barrier, allowing current to flow. Under reverse bias (p negative, n positive), the barrier increases and only a small reverse saturation current flows, until breakdown occurs at high reverse voltage.
The current-voltage characteristic of a p-n junction diode shows that: 1. In forward bias, the current increases exponentially once the applied voltage exceeds the barrier potential. 2. In reverse bias, a very small reverse saturation current flows. 3. Beyond the reverse breakdown voltage, a large current flows.
The diode conducts in one direction only, which makes it a rectifier. A half-wave rectifier uses a single diode and conducts only during one half of the AC cycle, producing a pulsating DC with large ripple. A full-wave rectifier uses two diodes (or a diode bridge) to conduct during both halves, giving smoother DC with higher average output.
A full-wave rectifier uses a centre-tapped transformer with two diodes, while a bridge rectifier uses four diodes. The efficiency of a half-wave rectifier is 40.6 percent and of a full-wave rectifier 81.2 percent.
Zener diode: A heavily doped diode designed to operate in the reverse breakdown region. In reverse bias, beyond the Zener voltage, the voltage across it remains nearly constant even when the current changes. It is used as a voltage regulator to maintain a constant output voltage.
LED (Light Emitting Diode): A diode that emits light when forward biased, as electrons recombine with holes and release energy as photons. LEDs are energy-efficient, long-lasting, and used in displays, indicators, and lighting.
Photodiode: A diode that produces a photocurrent when light falls on the junction. It is operated in reverse bias, where the current increases with the intensity of incident light. Photodiodes are used in light detectors, optical communication, and solar cells.
Solar cell: A photodiode that converts light energy into electrical power. It operates without any bias and produces a voltage when illuminated, using the photovoltaic effect. Solar cells are used in calculators, satellites, and solar power plants.
A transistor is a three-terminal device formed by sandwiching a thin semiconductor layer between two layers of the opposite type. The NPN and PNP transistors have emitter, base, and collector regions. The base is very thin and lightly doped.
In normal operation (for an NPN transistor), the base-emitter junction is forward biased and the base-collector junction is reverse biased. Most of the electrons injected from the emitter cross the thin base into the collector, so the collector current is nearly equal to the emitter current:
I_E = I_B + I_C
The current gain in the common emitter configuration is:
beta = I_C / I_B
Typical values of beta range from 20 to 200. The transistor has three regions of operation: the active region (used for amplification), the saturation region, and the cutoff region (used for switching).
In the common emitter amplifier, a small base current controls a much larger collector current. The voltage gain of the CE amplifier is:
A_V = -beta R_L / r_i
The negative sign indicates a phase reversal of 180 degrees between input and output. The amplifier is biased to operate in the active region, and the input signal rides on the DC bias.
As a switch, the transistor operates in cutoff (no collector current, output high) or saturation (collector current maximum, output low). In digital circuits, the transistor switch is the basis of logic circuits and memory.
The transistor also provides current gain, voltage gain, and power gain, making it the fundamental amplification device in electronics, from audio systems to radios and computers.
Logic gates are the building blocks of digital systems. They have one or more inputs and produce a single output of 0 (low) or 1 (high). The basic gates are:
OR gate: output is 1 if any input is 1. The Boolean expression is Y = A + B.
AND gate: output is 1 only if all inputs are 1. The expression is Y = A dot B.
NOT gate (inverter): output is the complement of the input, Y = A'.
NAND gate: the NOT of AND, Y = (A dot B)'. It is a universal gate.
NOR gate: the NOT of OR, Y = (A + B)'. It is also a universal gate.
NAND and NOR gates are universal because any logic circuit can be constructed using only these gates. Integrated circuits package thousands of gates on a single chip, enabling modern computers and digital devices.
| Quantity | Formula | Remark |
|---|---|---|
| Energy gap (Si) | Eg = 1.1 eV | Semiconductor |
| Energy gap (Ge) | Eg = 0.7 eV | Semiconductor |
| Transistor relation | I_E = I_B + I_C | Kirchhoff's law |
| Current gain (CE) | beta = I_C/I_B | 20 to 200 |
| Voltage gain (CE) | A_V = -beta R_L/r_i | 180 degree phase reversal |
| Rectifier efficiency (HW) | 40.6 percent | Half-wave |
| Rectifier efficiency (FW) | 81.2 percent | Full-wave |
| Gate | Symbol expression | Output = 1 when |
|---|---|---|
| OR | Y = A + B | Any input is 1 |
| AND | Y = A B | All inputs are 1 |
| NOT | Y = A' | Input is 0 |
| NAND | Y = (A B)' | Any input is 0 |
| NOR | Y = (A + B)' | All inputs are 0 |
This chapter built the foundation of semiconductor electronics from energy band theory to digital logic. Semiconductors with small energy gaps are doped to form n-type and p-type materials, whose junction creates a depletion region and a barrier potential. Diodes conduct in forward bias and block in reverse, enabling rectifiers, voltage regulators (Zener), light emitters (LED), detectors (photodiode), and solar cells. The transistor amplifies through beta = I_C/I_B and switches between cutoff and saturation, and the CE amplifier provides voltage gain with phase reversal. Finally, logic gates OR, AND, NOT, NAND, and NOR form the digital building blocks of all computing systems.