Advertisements
Advertisements
Question
The conductivity of a semiconductor increases with increase in temperature because ______.
Options
number density of free current carriers increases.
relaxation time increases.
both number density of carriers and relaxation time increase.
number density of current carriers increases, relaxation time decreases but effect of decrease in relaxation time is much less than increase in number density.
Advertisements
Solution
The conductivity of a semiconductor increases with increase in temperature because number density of current carriers increases, relaxation time decreases but effect of decrease in relaxation time is much less than increase in number density.
Explanation:
The conductivity of Semiconductor:
(1) In intrinsic semiconductors ne = nh. Both electrons and holes contribute to current conduction.
(2) When some potential difference is applied across a piece of intrinsic semiconductor current flows in it due to both electrons and holes, i.e. `i = i_e + i_h` ⇒ `eA[n_ev_e + n_hv_h]`
(3) As we know `σ = J/E = i/(AE)`. Hence conductivity of semiconductor is `σ = e[n_eu_e + n_hu_h]`; where `v_e` = drift velocity of electron, `v_h` = drift velocity of holes, E = Applied electric field, `mu_e = v_e/E` = mobility of electron and `mu_h = v_h/E` = mobility of holes.
(4) Motion of electrons in the conduction band and of holes in the valence band under the action of electric field is shown below:
(5) At absolute zero temperature (0 K) conduction band of the semiconductor is completely empty, i.e., σ > 0. Hence the semiconductor behaves as an insulator.
We know that `σ = (n e^2τ)/m`,
So, `σ ∝ nτ`
Where n = number density and τ = relaxation time
In semiconductors, conductivity increases with increase in temperature, because the number density of current carries increases, relaxation time decreases but effect of decrease in relaxation is much less than increase in number density.
APPEARS IN
RELATED QUESTIONS
Distinguish between 'intrinsic' and 'extrinsic' semiconductors
In a p-type semiconductos, which of the following statement is true:
The number of silicon atoms per m3 is 5 × 1028. This is doped simultaneously with 5 × 1022 atoms per m3 of Arsenic and 5 × 1020 per m3 atoms of Indium. Calculate the number of electrons and holes. Given that ni= 1.5 × 1016 m−3. Is the material n-type or p-type?
In a p-type semiconductor, the acceptor impurity produces an energy level ______
In n-type semiconductor, the fifth electron ______.
In n-type semiconductor majority carriers and minority carriers are respectively ______.
In p-type semiconductor, the dopant is ______.
State how a p-type semiconductor will be obtained from a pure crystal of a semiconductor.
Explain the following term:
Extrinsic semiconductor
Why are elemental dopants for Silicon or Germanium usually chosen from group XIII or group XV?
Sn, C, and Si, Ge are all group XIV elements. Yet, Sn is a conductor, C is an insulator while Si and Ge are semiconductors. Why?
Suppose a ‘n’-type wafer is created by doping Si crystal having 5 × 1028 atoms/m3 with 1 ppm concentration of As. On the surface 200 ppm Boron is added to create ‘P’ region in this wafer. Considering n i = 1.5 × 1016 m–3, (i) Calculate the densities of the charge carriers in the n and p regions. (ii) Comment which charge carriers would contribute largely for the reverse saturation current when diode is reverse biased.
The figure shows a piece of pure semiconductor S in series with a variable resistor R and a source of constant voltage V. Should the value of R be increased or decreased to keep the reading of the ammeter constant, when semiconductor S is heated? Justify your answer
Two crystals C1 and C2, made of pure silicon, are doped with arsenic and aluminium respectively.
Identify the extrinsic semiconductors so formed.
In an extrinsic semiconductor, the number density of holes is 4 × 1020 m-3. If the number density of intrinsic carriers is 1.2 × 1015 m-3, the number density of electrons in it is ______.
What type of semiconductor is obtained when a crystal of silicon is doped with a trivalent element?
The majority charge carriers in a P-type semiconductor are ______.
