Question

What happens to the bulk resistance of a semiconductor when the doping in the semiconductor increases?
A. Remains the same
B. Increases
C. Decreases
D. None of the Above

Answer 1

Hint: Doping is the process by which an intrinsic semiconductor with a lower conductance is changed to an extrinsic semiconductor of higher conductance by adding impurities. The conductance is the inverse of resistance and therefore, an extrinsic semiconductor has larger conductance.

Complete answer:
As we know, there are two types of semiconductors – intrinsic (or the pure) and the extrinsic. Extrinsic semiconductors are developed by doping of the pure semiconductors. Doping is the process of adding impurities in a semiconductor to improve its conductivity. There are, again, two types of extrinsic semiconductors – the p-type and the n-type.
 The p-type semiconductors are those which are doped by a trivalent impurity such as Al, B or In. When Boron family impurities, which possess three valence electrons are added, an extra hole is produced in the semiconductor, i.e., each added impurity acts as a positive charge being added to the system.
The n-type semiconductors are those which are doped by a pentavalent impurity such as Arsenic (As), Antimony (Sb) or Phosphorus (P). When an impurity of \[ + 5\] is added to a four-valency element like Si or Ge, an extra electron is produced in the semiconductor. Each added atom of the impurity contributes to a negative charge to the system.
From the above explanations it is clear that addition of impurities either trivalent (p-type) or pentavalent (n-type) results in the increase of the number of charge carriers in the system.
The intrinsic number of charge carriers to the additional holes or electrons is related by - \[n_i^2 = {n_e}.{n_h}\] .
Where, \[{n_i}\]is the number of intrinsic charge carriers
\[{n_e}\] is the number of electrons
\[{n_h}\] is the number of holes
Now, we know that the number of charge carriers increases with doping. The increase in number of charge carriers is directly responsible for the increase in conductance of the semiconductor, which, in turn implies that the bulk resistance of the semiconductor is reduced.
Therefore, it is quite clear that the bulk resistance or the resistance across the semiconductor reduces with the increase in doping.

The answer to the question is option C.

Additional Information:
The temperature is also important in considering the resistance of the semiconductor. They have a positive coefficient of conductance, i.e., the resistance decreases as the temperature increases.

Note:
The principle involved in conduction of conductors and semiconductors are entirely different. Any impurity in metal conductors can lead to contradictory results as compared to the semiconductors. Students should take care to compare and understand the difference between the mechanisms.