In a bipolar junction transistor
- A.all the three regions (the emitter, the base and the collector) have equal concentrations of impurity
- B.the emitter has the least concentration of impurity
- C.the collector has the least concentration of impurity
- D.the base has the least concentration of impurity
Correct answer
Explanation
The correct answer is D, the base has the least concentration of impurity. The base is made thin and lightly doped so that most charge carriers from the emitter cross it to the collector.
A bipolar junction transistor has three regions, in the order emitter, base and collector, forming either an n-p-n or a p-n-p device. The emitter is heavily doped because its job is to inject a large number of charge carriers. The base is very thin and lightly doped, so only a few of those carriers recombine there and the small base current controls a much larger collector current; this is what lets a transistor amplify. The collector is moderately doped and is the largest region, because it has to collect the carriers and get rid of the heat produced.
A is wrong because the three regions are deliberately doped to different levels. B is wrong because the emitter is the most heavily doped region. C is wrong because the collector is moderately doped, more than the base.
Exam tip: Doping: emitter heavy, base light, collector moderate; size: collector largest, base thinnest.
Practice Questions
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- B.Coke
- C.Fullerene
- D.Carbon black
Show answer
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- A.g₁
- B.2g₁
- C.g₁/2
- D.g₁/4
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- A.reflection of light
- B.scattering of light
- C.refraction of light
- D.polarization of light
Show answer
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- A.Most of the space in the atom is empty.
- B.The radius of the atom is about 10⁵ times the radius of the nucleus.
- C.Electrons move in a circular path of fixed energy called orbits.
- D.Nearly all the mass of the atom resides in the nucleus.
Show answer
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