Semiconductors
Easy Overview
Every electronic device around you — phone, laptop, TV, calculator — runs on semiconductors. These materials are neither good conductors nor good insulators. By adding tiny impurities (doping), we can control how they conduct electricity. This chapter is about diodes, transistors, and the physics behind the chips that power our world. It's pretty much magic... but it's real.
Conductors, Insulators, and Semiconductors
Conductors (copper, aluminum) have lots of free electrons and low resistivity. Insulators (rubber, glass) have almost no free electrons — they don't conduct. Semiconductors (silicon, germanium) are in between. Their conductivity increases with temperature (opposite of metals!) and can be controlled by doping — adding tiny amounts of impurities. At absolute zero, silicon is an insulator. At room temperature, some electrons jump to the conduction band and it conducts a little.
Intrinsic and Extrinsic Semiconductors
Pure silicon is an intrinsic semiconductor — equal numbers of electrons and holes (missing electrons act as positive charge carriers). By doping, we get extrinsic semiconductors. N-type: add phosphorus (5 valence electrons) → extra free electrons. P-type: add boron (3 valence electrons) → extra holes. In n-type, electrons are majority carriers; in p-type, holes are majority carriers. The doping concentration determines conductivity.
The P-N Junction — Where Magic Happens
When you bring p-type and n-type silicon together, a depletion region forms at the junction — electrons from n-side diffuse to p-side and recombine with holes. This creates an internal electric field that prevents further diffusion. In forward bias (p connected to +, n to −), the depletion region shrinks and current flows. In reverse bias (p to −, n to +), the depletion region widens and almost no current flows. This one-way behavior is the basis of the diode.
Diodes and Their Applications
A diode allows current in only one direction. It's like a one-way valve for electricity. Rectification: converting AC to DC using diodes. A half-wave rectifier uses one diode and passes only one half of the AC cycle. A full-wave rectifier uses four diodes (bridge rectifier) and converts both halves — much more efficient. Zener diodes are designed to work in reverse breakdown — they maintain constant voltage, making them great voltage regulators.
Transistors — The Amplifier and Switch
A transistor is essentially a sandwich of semiconductors — either n-p-n or p-n-p. The three terminals: emitter, base, collector. A small current at the base controls a much larger current between collector and emitter — that's amplification. In a common-emitter configuration, current gain β = I_C/I_B. Transistors can also act as switches (on/off) — that's how logic gates and computers work. The base input controls whether the transistor is in cutoff (off) or saturation (on).
Key Points
- •Semiconductors: conductivity between conductors and insulators
- •Intrinsic: pure Si/Ge with equal electrons and holes
- •N-type: doped with pentavalent atoms (extra electrons)
- •P-type: doped with trivalent atoms (extra holes)
- •P-N junction: allows current only in forward bias
- •Forward bias: depletion region decreases; Reverse bias: depletion region increases
- •Diode used for rectification (AC → DC)
- •Transistor: three-terminal device for amplification and switching
- •Current gain β = I_C/I_B in common-emitter configuration
Practice Questions
- Explain the formation of the depletion region in a p-n junction.
- Draw and explain the circuit diagram of a full-wave rectifier.
- What is a Zener diode? Explain its use as a voltage regulator.
- In a common-emitter transistor circuit, I_B = 50 μA and I_C = 4 mA. Find β.
- Distinguish between intrinsic and extrinsic semiconductors.