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Which of the following statements about intrinsic semiconductors are incorrect?

(1) In intrinsic semiconductors, the number of free electrons, is greater than the number of holes

(2) An intrinsic semiconductor will behave like an insulator at T = 0 K

(3) The conductivity of an intrinsic semiconductor depends on its temperature, and at room temperature its conductivity is very high

(4) C, Si and Ge all have the same lattice structure and are all intrinsic semiconductor materials.

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$\text{ANSWER: }$ Only (2) is TRUE, (1), (3) and (4) are all FALSE.
(1) In intrinsic semiconductors, the number of free electrons, is greater than the number of holes
$\quad \quad \text {FALSE:}$ In intrinsic semiconductors, the number of free electrons, ne is equal to the number of holes, $n_h$, i.e, $n_e = n_h = n_i $, where $n_i$ is called intrinsic carrier concentration.
(2) An intrinsic semiconductor will behave like an insulator at T = 0 K
$\quad \quad \text {TRUE:}$ An intrinsic semiconductor will behave like an insulator at T = 0 K. It is the thermal energy at higher temperatures (T > 0K), which excites some electrons from the valence band to the conduction band.
(3) The conductivity of an intrinsic semiconductor depends on its temperature, and at room temperature its conductivity is very high
$\quad \quad \text {FALSE:}$ At room temperature its conductivity is very low. Hence there is a necessity of improving their conductivity. This can be done by making use of impurities.
(4) C, Si and Ge all have the same lattice structure and are all intrinsic semiconductor materials.
$\quad \quad \text{FALSE:}$ The 4 bonding electrons of C, Si or Ge lie, respectively, in the second, third and fourth orbit. Hence, energy required to take out an electron from these atoms (i.e., ionisation energy) will be least for Ge, followed by Si and highest for C. Hence, number of free electrons for conduction in Ge and Si are significant but negligibly small for C. C is a conductor, while Si and Ge are intrinsic.
answered Jul 13, 2014 by balaji.thirumalai
 

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