NUST NET (Engineering) · Physics · Solids and electronics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
A pure semiconductor such as silicon conducts poorly. Doping it with a tiny amount of impurity adds charge carriers: a group V atom gives a spare electron (n-type) and a group III atom leaves a hole (p-type). Both types stay electrically neutral overall.
Where p-type meets n-type, a depletion region with a potential barrier forms. Forward bias shrinks it and current flows; reverse bias widens it and almost no current flows. This one-way behaviour lets a diode turn AC into DC, which is rectification.
A transistor uses a small base current to control a much larger collector current. An op-amp is a very high gain amplifier whose working gain is set by two resistors. Logic gates give an output of 0 or 1 that depends on their inputs.
Thinking an n-type semiconductor is negatively charged. It is neutral; the n only tells you that its majority carriers are electrons.
Answer: −1.5 V
Try each one, then open the answer.
Answer: B. In forward bias the applied voltage opposes the junction's own potential barrier, so the depletion region narrows and majority carriers flow easily. In reverse bias it widens and only a tiny minority-carrier current flows.
Answer: D. NOR is OR followed by NOT: the OR output is 0 only when both inputs are 0, so the NOR output is 1 only then. A NAND gate gives 1 for three of the four input combinations.
Answer: A. For silicon the barrier is about 0.7 V (about 0.3 V for germanium). A forward bias larger than this is needed before an appreciable current flows.
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