Page 1 of 4
P – N Junction diode
Aim :- To study the Volt- ampere (V-I) characteristics of a junction diode, both in
forward bias and reverse bias. And also to find out the following physical quantities.
1. knee voltage or cut-in voltage or threshold voltage (Vo).
2. Static or dc forward resistance (Rdc).
3. Dynamic or ac forward resistance (Rac).
4. Reverse saturation current (Io).
5. Reverse static resistance (Rr).
Apparatus :- Junction diode, battery (0-10 V), rheostat, plug key, dc voltmeter (0-10V),
milli-ammeter (0-100mA) and micro-ammeter (0-200µA).
Formula :- 1. Static or dc forward resistance Rdc =
Where
Vf = Forward voltage (V)
Vf
If
Ω
= OB
If = Forward current (mA) = OE
2. Dynamic or ac forward resistance Rac =
Where
∆Vf = Change in the forward voltage (V)
∆V f
∆I f
Ω
= AC
∆ If = change in the forward current (mA) = DF
3. Reverse static resistance
Where
Vr = Reverse voltage (V)
Rr =
Vr
Ir
Ω
= OG
Ir = Reverse current (mA) = OH
Theory :- A P–N junction is formed from a piece of a semi-conductor (Ge or Si) by
diffusing third group material to one half and fifth group material to the other half. The
first half becomes P–type and the second half becomes N–type. The plane dividing the
Page 2 of 4
two is known as P–N junction. The P–region has holes and N–region has electrons as
majority charge carriers. Also, a few electrons are present in P–region and a few holes in
N–region as minority charge carriers.
The majority charge carriers near the junction cross over the junction to the other
side and combine with the opposite charge carriers and become neutral. This region is
called depletion region or space charge region. This region lies on the two sides of the
junction. In the depletion region, on P-side immobile negative ions and on N-side
immobile positive ions lie, with no charge carriers. This gives rise a potential called
potential barrier (VB), which prevents respective majority charge carriers from crossing
the barrier region. As the reverse bias increases the width of the depletion increases and
the barrier potential (VB) also increases. In the forward bias the VB value decreases.
Procedure :- In the forward bias the battery positive terminal is connected to p-region
and negative terminal is connected to N-region. The connections are made as shown in
the figure-1. The voltmeter is connected across the diode to measure the voltage applied,
the milli-ammeter is connected in series to the diode, to measure the current flowing
through the diode. The voltage applied is increased in steps of 0.1 V or 0.2 V, starting
from zero, the corresponding current from the milli-ammeter is noted in the table-1.
Graph-1 :- A graph is drawn, in the 1st quadrant, by taking voltage on X-axis and
current on Y-axis. From the graph the threshold voltage Vo is noted. The values Vf, If,
and ∆Vf, ∆If are also noted as shown in the graph, to measure the static and dynamic
resistances respectively.
In the reverse bias the battery positive terminal is connected to N-region and negative
terminal is connected to P-region. The connections are made as shown in the figure-2.
The voltmeter is connected across the diode to measure the voltage applied, the microammeter is connected in series to the diode, to measure the current flowing through the
diode. The voltage applied is increased in steps of 1 V or 2 V, starting from zero, the
corresponding current from the micro-ammeter is noted in the table-2.
Page 3 of 4
Graph-2 :- A graph is drawn, in the 3rd quadrant, by taking voltage on negative X-axis
and current on negative Y-axis. Here, in the 3rd quadrant, deferent scale may be taken for
convenience. From the graph the reverse saturation current Io is noted. The values Vr, Ir,
are also noted as shown in the graph, to measure the reverse static resistance.
Precautions :- 1) For silicon diode the applied voltage should be more than for
Germanium diode.
2) Very high voltages should not be applied in reverse bias which cause damage to
the diode.
Results :-
Table – 2
Table –1
Forward bias
S.No.
Applied
Voltage
Vf (V)
Reverse bias
Current
Through diode
I f (mA)
S.No.
1.
1.
2.
2.
3.
3.
4.
4.
5.
5.
6.
6.
7.
7.
8.
8.
9.
9.
10.
10.
Applied
Voltage
Vr (V)
Current
Through diode
I r (µA)
Page 4 of 4
*****