Semiconductors

Step 1: Understanding the Concept:
The temperature coefficient of resistance ( ) describes how the electrical resistance of a substance changes with a change in temperature. A positive coefficient means resistance increases with temperature, while a negative coefficient means resistance decreases with temperature.
Step 2: Detailed Explanation:
Conductors (Metals): In metals, as temperature increases, the positive ions vibrate more vigorously. This increases the frequency of collisions for the free electrons moving through the material, which in turn increases the electrical resistance. Therefore, conductors have a positive temperature coefficient of resistance.
Semi-conductors: In semi-conductors (like silicon and germanium), the electrical conductivity depends on the number of charge carriers (electrons and holes). At absolute zero, a pure semiconductor behaves like an insulator. As the temperature rises, thermal energy breaks covalent bonds, creating more electron-hole pairs. This increase in the number of charge carriers drastically increases the conductivity, and consequently, decreases the resistance.
Since the resistance of a semiconductor decreases as its temperature increases, it has a negative temperature coefficient of resistance.
Step 3: Final Answer:
The temperature coefficient of a semi-conductor is negative. Option (B) is correct.

Step 1: Understanding semiconductor behavior.
In a semiconductor, the resistance decreases as the temperature increases. This is because, with increased temperature, more charge carriers (electrons or holes) are released, which increases the electrical conductivity of the material. Therefore, the resistance of a semiconductor decreases with rising temperature.
Step 2: Conclusion. The correct answer is that the resistance of a semiconductor decreases with rising temperature.

Step 1: Type of semiconductor.
When boron, a trivalent element, is added as an impurity to silicon, it creates a "hole" or absence of an electron in the silicon lattice. These "holes" are positive charge carriers, leading to the formation of a p-type semiconductor.
Step 2: Explanation of p-type semiconductor.
In p-type semiconductors, the majority charge carriers are holes, which are created by the absence of electrons. As a result, the electrical conductivity is facilitated by the movement of holes.
Step 3: Elimination.
- (A) n-type semiconductor: Incorrect, n-type semiconductors are formed by adding pentavalent impurities (e.g., phosphorus).
- (B) p-type semiconductor: Correct, as boron creates "holes" that act as majority charge carriers.
- (C) n-type conductor: Incorrect, this refers to a conductor with excess electrons.
- (D) p-type conductor: Incorrect, semiconductors are being discussed, not general conductors.
Step 4: Conclusion.
Thus, when boron is added as an impurity in silicon, it forms a p-type semiconductor.

Step 1: Type of semiconductor.
In n-type semiconductors, the majority charge carriers are electrons, which are provided by the donor impurity atoms (usually pentavalent elements such as phosphorus). These extra electrons increase the conductivity of the material.
Step 2: Minority charge carriers.
The minority charge carriers in n-type semiconductors are holes. These are the empty spaces left when an electron moves, and they carry a positive charge. Since there are fewer holes than electrons, holes are the minority carriers.
Step 3: Elimination.
- (A) electrons: Incorrect, electrons are the majority charge carriers in n-type semiconductors.
- (B) holes: Correct, holes are the minority charge carriers in n-type semiconductors.
- (C) electron and hole: Incorrect, only holes are the minority charge carriers.
- (D) none of these: Incorrect.
Step 4: Conclusion.
Thus, in n-type semiconductors, the minority charge carriers are holes.