Semiconductor Electronics

Semiconductor Electronics introduces the materials and devices at the heart of every phone, computer and charger. In this chapter you will learn how solids are classified as metals, semiconductors and insulators on the basis of their conductivity and their energy bands: the valence band, the conduction band and the energy gap between them. You will study intrinsic semiconductors such as pure silicon and germanium, where electrons and holes are created in equal numbers, and extrinsic semiconductors made by doping: n-type with pentavalent impurities and p-type with trivalent impurities, obeying ne nh = ni^2. You will then study the p-n junction: how diffusion and drift of charge carriers create a depletion region and a barrier potential. You will learn how the junction behaves under forward and reverse bias, draw and interpret the I-V characteristics of a junction diode, and find its dynamic resistance. Finally, you will see how diodes are used as rectifiers, converting alternating current into direct current in half-wave and full-wave rectifier circuits.

Detailed Worksheet: Semiconductor Electronics Section A - Definitions (10 marks) 1. Distinguish between conductors, semiconductors and insulators on the basis of energy bands. (2 marks) 2. What is an intrinsic semiconductor? Why does its conductivity increase with temperature? (2 marks) 3. What is doping? Name one dopant each for making n-type and p-type silicon. (2 marks) 4. Define depletion region and barrier potential of a p-n junction. (2 marks) 5. What is rectification? Why is a diode suitable for it? (2 marks) Section B - Calculations and Applications (15 marks) 6. A pure silicon crystal has an intrinsic carrier concentration of 1.5 x 10^16 m^-3. It is doped with arsenic at 5 x 10^22 atoms per m^3. Find the electron and hole concentrations. Is the material n-type or p-type? (3 marks) 7. The energy gap of silicon is 1.1 eV. Find the maximum wavelength of light that can create electron-hole pairs in silicon (hc = 1240 eV nm). (3 marks) 8. In the forward characteristic of a diode, the current increases from 10 mA to 20 mA when the voltage increases from 0.7 V to 0.8 V. Find the dynamic resistance of the diode in this region. (3 marks) 9. A 12 V rms, 50 Hz AC supply is connected to a rectifier. Find the peak voltage. What is the frequency of the output for (i) a half-wave rectifier and (ii) a full-wave rectifier? (3 marks) 10. A silicon diode (drop 0.7 V) is connected in forward bias with a 215 ohm resistor across a 5 V battery. Find the current and the power dissipated in the diode. (3 marks) Section C - Diagrams (10 marks) 11. Draw the circuit diagrams for obtaining the forward and reverse bias characteristics of a p-n junction diode, and draw the I-V characteristic curve. (4 marks) 12. Draw energy band diagrams of (i) a metal, (ii) a semiconductor and (iii) an insulator. (3 marks) 13. Draw the circuit diagram of a half-wave rectifier and its input and output waveforms. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the formation of the depletion region and barrier potential in a p-n junction. Explain how forward and reverse bias change the width of the depletion region and the barrier height. (5 marks) 15. Draw the circuit diagram of a full-wave rectifier using a centre-tap transformer and explain its working with input and output waveforms. Why is its output smoother than that of a half-wave rectifier? (5 marks) 16. Compare n-type and p-type semiconductors on the basis of dopant, majority and minority carriers, and energy band diagrams. Explain why a doped semiconductor is still electrically neutral. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw neat circuit diagrams with correct symbols and show waveforms clearly.