Physics Lab Trainer

# Experimental Determination of :

• Reverse saturation current I0  and material constant h
• Temperature coefficient of junction voltage dV/dt
• Energy band-gap VG0
• Junction capacitance

STUDY OF THE ENERGY BAND-GAP AND DIFFUSION POTENTIAL OF P-N JUNCTIONS

Ø  Introduction:

Ø  Experiments:

### Given below is a brief description of the experiments that may be performed.

(a) Reverse saturation current I0 and material constant h

### The magnitude of I0 is too small to be Measured conveniently and further, it is a function of the applied voltage. The   direct measurement of this current is therefore both difficult and erroneous. In the present set-up, readings for the forward V-I characteristics are obtained by 3½ digit DPM for a wide range of currents. If, V and lnI are plotted on a graph paper a straight line is obtained. This line intersects the current (lnI) axis at lnI0 and its slope DV/DlnI may be solved to compute h. (fig.1)

(b) Energy band-gap and temperature coefficient of the junction voltage

### The P-N junction under test is kept in a small, fast temperature controlled oven. The temperature is adjustable in the range from room temperature to about 80°C. From the readings of the temperature and junction voltage on digital instruments provided on the panel, the temperature coefficient gap are computed. (fig.2)

(a) The Junction Capacitance

### The junction capacitance of a typical diode varies in the range 10pf-100pf approximately, as a non linear function of  the  reverse  voltage. This  parameter  though  important  in  high  frequency  circuits, is  difficult  to measure  because of  its  small value. In the present  set-up,  the output  V1  and V2 at two frequencies f1 & f2, where f2>f1, are obtained at different values of bias voltage to compute the junction capacitance. A typical graph between bias voltage and junction capacitance is shown in fig.3.The experimental set-up consists of the following

(1)  Study of P-N Junction, Model PN-1

### (a)    3½ digit DPM for current/temperature measurements.(b)    3½ digit DPM for bias voltage/junction voltage measurements.(c)    Two parts to connect the diode - one for experiment 1 & 2 and other for experiment 3.(d)    Two fixed frequency oscillators (5KHz & 20KHz) with the same output (200 mV).

(2)  Fast temperature controlled oven with sensor