Chapter 14 — Semiconductor Electronics: Materials, Devices and Simple Circuits
Why does silicon — neither metal nor insulator — run the modern world? Energy bands explain it, doping controls it, and the p–n junction turns it into the diode that converts AC into DC.
Open any phone, laptop, television or electric car and almost everything inside is made of one element: silicon. A modern processor holds tens of billions of transistors on a chip the size of a postage stamp; the camera sensor turns light into charge pixel by pixel; the charger turns the AC of the wall socket into the steady DC a battery needs. All of it rests on a family of materials, the semiconductors, whose conductivity can be set — over a range of a million or more — by temperature, by light, and above all by adding a few impurity atoms in the right places.
This chapter explains how. We begin with a puzzle from Chapter 3: why do some solids conduct a hundred billion billion times better than others? The answer is that the energy levels of atoms in a crystal merge into energy bands, and what matters is whether the highest occupied band is full and how wide the gap is to the next. That picture explains metals, insulators and semiconductors, and introduces the second carrier of current in a semiconductor — the positively charged hole. Doping with donors or acceptors makes n-type and p-type material, and joining the two inside one crystal creates a p–n junction, with its depletion region and barrier potential. The junction is a diode: it lets current through in one direction only, and a pair of them makes a rectifier — the first step in every DC power supply.
- classify solids as metals, semiconductors or insulators by resistivity and by their energy bands, and relate the band gap to the absorption of light ($\lambda_{\max}=hc/E_g$);
- explain intrinsic conduction by electrons and holes, and why a semiconductor's resistivity falls as it is heated;
- describe n-type and p-type doping, identify majority and minority carriers, and calculate carrier densities with $n_en_h=n_i^2$ and the conductivity with $\sigma=e(n_e\mu_e+n_h\mu_h)$;
- explain how the depletion region and barrier potential form at a p–n junction and how they change in forward and reverse bias;
- draw and interpret the I–V characteristic of a diode, find dynamic resistance, and solve circuits containing diodes;
- explain half-wave and full-wave rectifiers and capacitor filters, and calculate their output;
- solve board, JEE Main and NEET problems on all of the above.
Sections in this chapter
Metals, Semiconductors and Insulators: Energy Bands
Classifying solids by resistivity, how atomic levels merge into bands, the band gap, and why heat and light free electrons in a semiconductor.
Intrinsic and Extrinsic Semiconductors
Electrons and holes, doping with donors and acceptors, n-type and p-type material, the law of mass action and conductivity.
The p–n Junction, the Semiconductor Diode and Rectifiers
Depletion region and barrier potential, forward and reverse bias, the diode's I–V characteristic, half-wave and full-wave rectifiers and filters.