Delhi Public School B.S.
City (Physics Practical Class XII)
Aim: To draw the I-V characteristic curve of a p-n junction in forward bias and reverse bias.
Materials Required: -
A p-n junction diode
A 3-volt battery
A 50-volt battery
A high resistance rheostat
One 0-3-volt voltmeter
One 0-50-volt voltmeter
One 0-100 mA ammeter
One 0-100 μA ammeter
One-way key
Connecting wires
Piece of sandpaper
Theory
Forward bias characteristics
The junction is said to be forward biased when the p-section of the diode is connected to the
positive terminal of the battery and the n-section of the diode is connected to the negative terminal
of the battery. With an increase in the voltage, the current also increases. For Si diode, at 0.7 V the
current increases suddenly.
Reverse bias characteristics
The junction is said to be reverse biased when the p-section of the diode is connected to the
negative terminal of the battery and the n-section of the diode is connected to the positive terminal
of the battery. With an increase in the voltage, there is a small change in the current but the reverse
current increases to a higher value with an increase in the voltage.
Diagram: -
Procedure
For forward-bias
The circuit connections should be as shown in the diagram.
All the connections should be neat, clean and tight.
For voltmeter (V) and milli-ammeter (mA), least count and zero error should be noted.
To get the zero reading from the voltmeter and milli-ammeter, rheostat should be brought
near the negative end by inserting the key K.
To apply the forward bias voltage (VF) of 0.1V, the contact should be moved towards the
positive end. The current remains zero.
Keeping current zero, increase the forward bias voltage up to 0.3 V for Ge diode.
To record a small current using milli-ammeter, increase the VF to 0.4 V.
Increase the VF by 0.2 V and record the corresponding current. When the VF becomes 0.7 V,
the current will increase rapidly.
When VF = 0.72 V, the current increases suddenly and this is known as forward breakdown
stage.
Take out the key if forward current won’t change as VF increased beyond forward
breakdown.
Record the observations.
For reverse bias
The circuit connections should be as shown in the diagram.
All the connections should be neat, clean and tight.
Note the least count and zero error of voltmeter (V) and micro-ammeter (μA).
To get zero reading from the voltmeter V and micro-ammeter μA, insert the key K and bring
the rheostat near the positive end.
To apply reverse bias voltage (VR) of 0.5 V, move the rheostat to the negative end so as to
flow the reverse current.
Increase VR by 0.2 V and record the corresponding current. When VR becomes 20 V, the
current will increase rapidly.
When VR = 25 V, the current increases suddenly and this is known as reverse breakdown
stage. Record the current reading and take off the key.
Record the observations.
Observations:
Table for forward bias voltage and forward current
Sl.no Forward bias voltage VF in Volt Forward current IF in mA
01 0.2 V 0 mA
02 0.3 V 0 mA
03 0.4 V 1 mA
04 0.7 V 2.5 mA
05 02 V 20 mA
06 2.4 V 30 mA
Table for reverse bias voltage and reverse current
Sl.no Reverse bias voltage VR in Volt Reverse current IR in μA
01 5V 1 μA
02 7V 2 μA
03 15 V 7 μA
04 23 V 15 μA
05 25 V 25 μA
06
Calculations:
For forward bias
Plot a graph between VF and IF taking VF on the x-axis and IF on the y-axis. The graph obtained is
known as forward bias characteristic curve
For reverse bias
Plot a graph between VR and IR taking VR on the x-axis and IR on the y-axis. The graph obtained is
known as reverse bias characteristic curve.
Precautions
1. The connections should be neat, clean and tight.
2. Key should be used when the circuit is being used.
3. Beyond breakdown, forward bias voltage should not be applied.
4. Beyond breakdown, reverse bias voltage should not be applied.
Sources of Error
Faulty junction diode might be supplied.
