Semiconductor Electronics — Practice

Question 1

Silicon is doped with indium. State the type of semiconductor formed and name its majority carriers.

Accepted Answer

Count indium's valence electrons against silicon's four to decide whether a bond is left incomplete. Answer: p-type; holes are the majority carriers.

Question 2

An alternating input of 60 Hz is applied to a full-wave rectifier. Find the output frequency.

Accepted Answer

A full-wave rectifier uses both halves of each input cycle. Answer: 120 Hz.

Question 3

A solid has a band gap of 4.5 eV. Classify it and justify your answer.

Accepted Answer

Compare the gap with the thermal energy available at room temperature. Answer: Insulator, since a gap above about 3 eV cannot be crossed thermally.

Question 4

Explain why no net current flows through an unbiased p-n junction even though carriers are diffusing across it.

Accepted Answer

Consider both the diffusion current and the drift current set up by the barrier field. Answer: At equilibrium the diffusion and drift currents are equal and opposite, so the net current is zero.

Question 5

State what happens to the width of the depletion region and to the barrier potential when a p-n junction is reverse biased.

Accepted Answer

The applied field either opposes or reinforces the junction's own field. Answer: The depletion region widens and the barrier potential increases.

Question 6

In a sample the intrinsic carrier concentration is 1.5 x 10^16 per cubic metre. After doping the electron concentration becomes 4.5 x 10^22 per cubic metre. Find the hole concentration.

Accepted Answer

Apply the mass action law, which holds in doped material as well as pure. Answer: 5.0 x 10^9 per cubic metre.

Question 7

Explain why germanium conducts better than silicon at room temperature, referring to their band gaps.

Accepted Answer

Compare how easily thermal energy promotes electrons across each gap. Answer: Germanium's gap of about 0.7 eV is smaller than silicon's 1.1 eV, so more carriers are thermally generated.

Question 8

Explain why a full-wave rectifier output is easier to smooth than a half-wave output, and state the role of the capacitor in the filter.

Accepted Answer

Compare how often output pulses arrive in each case and what happens between them. Answer: Full-wave pulses arrive twice as often so the gaps are shorter and the ripple smaller; the capacitor charges at each peak and discharges through the load between peaks.

Question 9

A silicon diode is connected in series with a 500 ohm resistor across a 5.0 V supply in forward bias. Estimate the current, taking the diode drop as 0.7 V.

Accepted Answer

Subtract the diode's forward voltage drop before applying Ohm's law to the resistor. Answer: About 8.6 mA.

Practice — Semiconductor Electronics

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