Wednesday, August 13, 2008


AIM: To determine the forbidden energy gap of a semiconductor.
APPARATUS: P-N diode, DC regulated power supply, Voltmeter, Milliammeter, 1 KW resistor, Beaker, Thermometer, Heater.
PRINCIPLE: The forbidden energy gap of a material is a collection of energy levels above the top of the valence band and below the bottom of the conduction band. For the determination of the energy gap of a semiconductor, a semiconductor in the form of P-N diode is used. The forward current in such a diode is given by,
If = Is[exp(eV/hkBT)-1]
Where V = forward voltage across the junction, e = electronic charge, kB = Boltzmann constant, T = absolute temperature, Is = reverse saturation constant and h = a constant called emission co-efficient.
Is is given by,
Is = BT3ex(Eg/kBT) where B is a constant.
For a low constant forward current, the above equations can be approximated to yield an equation,
eV = Eg - hkBT
Hence a plot of V versus T gives a straight line with the V-intercept equal to Eg/e at T=0K. From this, the energy gap of the semiconductor can be obtained.
PROCEDURE: The circuit is built up as shown in figure (1). The forward biased voltage is kept at room temperature. A constant forward current is passed through the diode and the voltage developed across the diode at this temperature is noted. Then the diode is immersed in hot water bath. Voltage across the diode is noted down for different temperatures as water bath cools down. The readings are noted down till the water bath attains room temperature.
A graph with voltage along Y-axis and temperature along X-axis is drawn. The Y-intercept of the graph at 0K is found. From this the energy gap of the semiconductor is calculated.
Constant forward current through the diode =
Temperature ( 0C ) Temperature ( K ) Junction Voltage (V)

Y-intercept of the graph (Eg/e) =
\Eg =
RESULT: Energy gap of the given semiconductor =

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