of the Energy Band-Gap and Diffusion Potential of P-N Junctions
is an advanced level experiment to be performed on commercially
available diodes viz. germanium or silicon diodes, various
types of LEDs and also on the base-emitter/collector-base
junctions of a transistor. The results of the experiments
not only give the device characteristics but also provide
an insight into the properties of the materials used in the
fabrication of the junction. In the set-up, all the necessary
instrumentation is integrated as a result of which a minimum
of external connections need to be made by the user. A CRO
is the only accessory that is required.
below is a brief description of the experiments that may be
(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 and energy band-gap are computed. (fig.2)
(c) 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
- 3½ digit DPM for current/ temperature
- 3½ digit DPM for bias voltage/junction
- Two parts to connect the diode - one
for experiment 1 & 2 and other for experiment 3.
Two fixed frequency oscillators (5KHz &
20KHz) with the same output (200mV).
(2) Fast temperature controlled oven with
(3) Set of samples
- Transistor BC109 (base-emitter)-Si
- Transistor AC126 (base-emitter)-Ge
- Diode 1N 5408/1N 4002-Si
The unit is supplied complete with a detailed
instruction manual. Sufficient theoretical description is
included for a proper understanding. This is followed by a
step-by-step procedure and a typical set of readings and results.
Complete in all respect, except a CRO
for capacitance measurement.
Typical results obtained are shown in
fig no 1, 2 & 3.