Which one of the following statements about an n-type silicon semiconductor at room temperature is correct?
A
It is electrically neutral overall, although electrons are its majority carriers.
B
It carries a net negative charge because it has extra free electrons.
C
It is made by doping silicon with a trivalent impurity such as boron.
D
Its hole concentration is greater than that of intrinsic silicon at the same temperature.
Show the answer
The answer is A.
Pentavalent donors give free electrons and become positive ions, so the crystal stays neutral. With nₑnₕ = nᵢ², holes are fewer than in intrinsic silicon.
B.
Each free electron leaves behind a fixed positive donor ion, so the crystal has no net charge.
C.
A trivalent impurity makes p-type silicon; n-type needs a pentavalent donor such as phosphorus.
D.
Since nₑnₕ = nᵢ², the large electron concentration lowers the hole concentration below nᵢ.
NCERT: Class 12 Physics, Chapter 14, Extrinsic semiconductor
Sample 2NEET level
Assertion (A): Under reverse bias, the current through a p-n junction diode is very small and nearly independent of the applied voltage, up to the breakdown voltage. Reason (R): Under reverse bias, the depletion layer of the p-n junction becomes narrower.
A
Both A and R are true and R is the correct explanation of A
B
Both A and R are true but R is not the correct explanation of A
C
A is true but R is false
D
A is false but R is true
Show the answer
The answer is C.
Under reverse bias the applied voltage adds to the barrier potential, so the depletion layer widens and only the small drift current of minority carriers flows, nearly independent of voltage until breakdown. A is true and R is false.
A.
R is false: reverse bias widens the depletion layer.
B.
This needs R to be true, but reverse bias widens the depletion layer rather than narrowing it.
D.
A is true: the reverse current is a small minority-carrier current, almost independent of voltage below breakdown.
NCERT: Class 12 Physics, Chapter 14 (Semiconductor Electronics: Materials, Devices and Simple Circuits), p-n junction diode under reverse bias
Practise Semiconductors and the p-n junction diode
Practice at the level you choose, with the answer after every question; timed tests with the answers when you submit. Every question is verified against NCERT first.