# Band Gap and Resistivity

6) Can electron jump from valence band to conducting band by light radiation, if gap band in semiconductor is equal to 0.3 eV, light wavelength lambda: 500 nm?

1- No   2.Yes

7) The resistance of semiconductors decreases, when temperature increases 7

l. In all cases  2. Only in intrinsic semiconductors.  3. Only in extrinsic semiconductors. 4. No correct answer

8) Find the value of band gap in intrinsic semiconductor, if afier heating from T=300K

to T=500K the resistance of semiconductor decreases from 300 ohm to 30 ohm.

l. 0.3 eV   2. 3 eV   3. 0.6eV   4. 6 eV   5. 0.3 J

1. $E =h*c/\lambda = 6.62*10^{-34}*3*10^8/500*10^{-9} =3.97*10^{-19} J =2.4825 eV$

Electron can jump! Correct answer is 2) Yes

2) With increasing temperature the number of free carriers increases both in intrinsic and extrinsic semiconductors.

$\sigma = 1/\rho =e*n*\mu_n +e*p*\mu_p$ is increasing.

Means resistivity decreasing.

Correct answer is 4) No correct answer

3) $\rho = C/n$ where $n$ is the carrier concentration, $C$ is a constant

$\rho(T_1)/\rho(T_2) = n2/n1$

$300/30 = n2/n1 =10$

$n(500)/n(300) =10$

$n=N_0*exp(-Eg/2kT)$

$n(300)= N0*exp(-Eg/2*K*300)$

$n(500) =N0*exp(-Eg/2*k*500) 10 =n(500)/n(300) = exp[(Eg/2K)*(1/300-1/500)]$

$1.33*10^{-3}*Eg/2K =2.3$

$Eg =2k*2.3/1.33*10^{-3} =0.298 eV$

Correct answer is 5). 0.3 eV