13 Jul 2026

Energy Gap of a Semiconductor Using a PN Junction

practical ug-vi solid-state semiconductor energy-gap

Experimental arrangement

The reverse-biased semiconductor junction is heated in a controlled bath and its reverse saturation current is measured.

Aim

To determine the energy gap of a semiconductor from the variation of reverse saturation current with temperature.

Apparatus

PN junction diode, heating arrangement, thermometer, regulated supply, microammeter, and voltmeter.

Theory

The reverse saturation current varies approximately as

\[I_s=I_0e^{-E_g/(kT)}.\]

Therefore, a plot of $\log_{10}I_s$ against $1/T$ is a straight line with slope $-E_g/(2.303k)$.

Observations

Temperature (K)	Reverse current ($\mu$A)	$1/T$ (K$^{-1}$)	$\log_{10}I_s$
303	2.1	0.003300	0.322
313	3.5	0.003195	0.544
323	5.8	0.003096	0.763
333	9.4	0.003003	0.973
343	15.1	0.002915	1.179

Graph

Graph used to obtain the semiconductor energy gap from the slope.

Calculation

The slope of the $\log_{10}I_s$ versus $1/T$ graph is approximately $-2.28\times10^3\,\text{K}$. Hence,

\[E_g=2.303k(2.28\times10^3)=0.452\,\text{eV}.\]

Result

\[\boxed{E_g=0.45\,\text{eV}}.\]

Precautions

1. Keep the reverse voltage constant.
2. Allow the junction to reach thermal equilibrium.
3. Use a microammeter of suitable range.
4. Do not heat the diode beyond its rated temperature.

Viva Questions

1. What is energy gap? It is the energy separation between the valence and conduction bands.
2. Why is reverse saturation current measured? Its temperature dependence contains the energy-gap information.
3. Why is the reverse voltage kept constant? To ensure that current changes are mainly due to temperature.

Maxima Code

Download the Maxima calculation file.