PhysicsCBSE Class 12 · Practical, Section B, experiment 9

I-V characteristic curve of a p-n junction diode in forward and reverse bias

Updated 16 October 2026 · Fact-checked

In forward bias (p-side to positive), the current stays almost zero until the cut-in voltage (about 0.7 V for silicon), then rises steeply. In reverse bias, only a tiny, nearly constant reverse saturation current of a few microamperes flows. Plot both on one graph: forward in the first quadrant, reverse in the third.

Aim

To draw the I-V characteristic curve for a p-n junction diode in forward and reverse bias.

Apparatus and material required

p-n junction diode (for example 1N4007 silicon or OA79 germanium)
Variable voltage power supply (0 to 12 V)
Voltmeter (0 to 12 V); a low-range voltmeter (0 to 1 V or 0 to 3 V) helps in forward bias
Milliammeter (0 to 200 mA) for forward bias
Microammeter (0 to 200 μA) for reverse bias
Resistor of 3 Ω, ½ W in series with the diode (the value NCERT's lab manual lists), to limit the current
Plug key, connecting wires and sandpaper

Principle

A p-n junction has a depletion region and a barrier potential across it.

Forward bias: the p-side is at a higher potential than the n-side. The applied voltage opposes the barrier, the depletion region narrows, and majority carriers cross the junction. The current is negligible until the voltage crosses the threshold or cut-in voltage (about 0.2 V for germanium and 0.7 V for silicon, NCERT); after that it rises very steeply (exponentially).

Reverse bias: the n-side is at a higher potential. The barrier rises, the depletion region widens, and only minority carriers flow, giving a very small current that soon becomes constant: the reverse saturation current (a few microamperes). At a very large reverse voltage (breakdown), the current rises suddenly; a general-purpose diode is not used there.

The dynamic resistance in the steep part is $r_{d} = \frac{Δ V}{Δ I}$ .

p-n junction diode: I-V characteristics: formulas and key facts

Dynamic (a.c.) resistance: $r_{d} = \frac{Δ V}{Δ I}$; From the straight, steep part of the forward curve.
Static (d.c.) resistance at a point: $R = \frac{V}{I}$
Forward and reverse barrier: $forward: V_{0} - V; reverse: V_{0} + V$; Effective barrier height with an applied voltage V; V₀ is the barrier potential.
Key facts: Forward bias: p to +, n to −
Reverse bias: p to −, n to +
Milliammeter in forward bias, microammeter in reverse
Cut-in voltage: about 0.7 V (Si), 0.2 V (Ge)
Forward resistance low, reverse resistance very high
Raise the voltage in small steps near the knee

Procedure

1Note the range and least count of the voltmeter, milliammeter and microammeter. Clean the ends of the wires and the diode leads.
2Forward bias: connect the power supply, key, resistor, milliammeter and diode in series, with the p-side (anode) towards the positive terminal. Connect the voltmeter across the diode.
3With the key open, check that no current flows. Close the key and raise the supply voltage gently from zero.
4Note the voltage across the diode and the current in steps. Near the cut-in voltage, use steps of about 0.02 V to 0.1 V, as the current rises fast. Stop before the current reaches the diode's or the meter's limit.
5Reverse bias: reverse the diode so the p-side goes to the negative terminal, and replace the milliammeter with the microammeter.
6Raise the reverse voltage in steps of 1 V or 2 V and note the voltage and current each time, staying well below the diode's maximum reverse voltage.
7Plot forward V on the positive x-axis and forward current (mA) on the positive y-axis; plot reverse V on the negative x-axis and reverse current (μA) on the negative y-axis. Mark the different scales.

Observation table

Draw the table like this in your record. The numbers are sample readings to show how it is filled in; use your own.

Forward bias (silicon diode; voltmeter 0 to 1 V, least count 0.01 V; milliammeter least count 1 mA)
S. No.	Forward voltage $V_{f}$ (V)	Forward current $I_{f}$ (mA)
1	0.00	0
2	0.20	0
3	0.40	0
4	0.50	0
5	0.60	1
6	0.65	3
7	0.70	8
8	0.72	13
9	0.74	20
10	0.76	29
11	0.78	40

Reverse bias (voltmeter 0 to 12 V, least count 0.1 V; microammeter least count 1 μA)
S. No.	Reverse voltage $V_{r}$ (V)	Reverse current $I_{r}$ (μA)
1	0.0	0
2	1.0	1
3	2.0	1
4	4.0	2
5	6.0	2
6	8.0	2
7	10.0	2

Sample readings to show how the tables are filled in; they are not real measurements. A silicon diode's reverse current can be too small for some microammeters to show; record what your meter reads.

Calculation

Read the cut-in voltage and the reverse saturation current from the graph; the resistances are optional but often asked.

1.Cut-in voltage: extend the straight, steep part of the forward curve down to the voltage axis. The line through (0.74 V, 20 mA) and (0.78 V, 40 mA) meets I = 0 at 0.74 − 0.020 × 2.0 = 0.70 V, so the cut-in voltage is 0.70 V.
2.Dynamic resistance: $r_{d} = \frac{Δ V}{Δ I} = \frac{0.78 - 0.74}{( 40 - 20 ) \times 1 0 ^{- 3}} = \frac{0.04}{0.020} = 2.0 Ω$
3.Static resistance at 0.78 V: $R = \frac{0.78}{40 \times 1 0 ^{- 3}} = 19.5 Ω$ , much larger than $r_{d}$ , because the curve is not a straight line through the origin.
4.Reverse saturation current (flat part, from 4.0 V to 10.0 V): 2 μA, read to the microammeter's least count of 1 μA. Reverse resistance at 10.0 V: $\frac{10.0}{2 \times 1 0 ^{- 6}} = 5 \times 1 0^{6} Ω$ (one significant figure, because the current is read only to 1 μA).

Result

1. The cut-in (threshold) voltage of the given diode = ____ V (sample: 0.70 V, silicon). 2. The reverse saturation current = ____ μA (sample: 2 μA). 3. The diode conducts easily in forward bias and hardly at all in reverse bias.

Precautions

Do not exceed the maximum forward current given by the manufacturer; watch the milliammeter.
Do not exceed the maximum reverse voltage of the diode.
Raise the voltage gradually in small steps, especially near the knee.
Use a milliammeter in forward bias and a microammeter in reverse bias, with correct polarity.
Keep the key open when not taking readings, so the diode does not heat up.
Use different, clearly marked scales for the forward and reverse parts of the graph.

Sources of error

The diode heats up if current flows for long, and its characteristic shifts.
The voltmeter draws a little current, which the milliammeter also reads.
Least counts of the meters are large compared with the small reverse current.
Loose connections change the readings.

Viva questions with answers

Say your answer out loud first, then open the card. Examiners want one or two clear sentences, not a paragraph.

1.What is a p-n junction diode?Show answer
A device made by joining p-type and n-type semiconductor in one crystal. It conducts much more easily in one direction than the other.
2.What is forward bias?Show answer
When the p-side is connected to the positive terminal and the n-side to the negative terminal of the supply.
3.What is the cut-in (threshold) voltage? Give its value for Si and Ge.Show answer
The forward voltage after which the current rises sharply. About 0.7 V for silicon and 0.2 V for germanium.
4.What is the depletion region?Show answer
A thin region on both sides of the junction with no free charge carriers, only immobile ions, set up by diffusion of electrons and holes across the junction.
5.What happens to the depletion region in forward and reverse bias?Show answer
It narrows in forward bias and widens in reverse bias.
6.Why does a small current flow in reverse bias?Show answer
It is due to minority carriers, which the field at the junction sweeps across. It is the reverse saturation current.
7.Why does reverse current stay almost constant as the reverse voltage increases?Show answer
Even a small reverse voltage sweeps all the available minority carriers across the junction. So the current is limited by how many minority carriers there are on either side, not by the size of the applied voltage, until breakdown. These carriers come from thermal generation of electron-hole pairs, so the reverse current rises if the diode warms up.
8.Why is a microammeter used in reverse bias and a milliammeter in forward bias?Show answer
The reverse current is only a few microamperes, while the forward current is several milliamperes.
9.What is dynamic resistance?Show answer
The ratio of a small change in voltage to the small change in current it causes, r_d = ΔV/ΔI, in the steep part of the forward curve.
10.Does a diode obey Ohm's law?Show answer
No. Its I-V graph is not a straight line through the origin, so it is a non-ohmic device.
11.What is breakdown?Show answer
At a very high reverse voltage, the reverse current rises suddenly. Ordinary diodes are not used in this region; a Zener diode is designed for it.
12.Name one use of a p-n junction diode.Show answer
As a rectifier, which changes alternating current to direct current because the diode conducts only in forward bias.

Viva practice: these questions one at a time, with the answer hidden until you ask, and a tally of what to revise.

Common mistakes in the practical exam

The slips examiners see most often, and what to write instead.

Mistake: Connecting the voltmeter across the diode and resistor together.
You then plot the supply voltage, not the voltage across the diode.
Do this: Connect the voltmeter directly across the diode leads.
Mistake: Using the same scale for forward mA and reverse μA.
Do this: Plot reverse current on the negative y-axis with its own μA scale, and label both scales.
Mistake: Taking forward readings only in 0.5 V steps.
The whole rise happens within about 0.1 V, so the knee is missed.
Do this: Use steps of 0.02 V to 0.05 V once the current starts to flow.
Mistake: Drawing the reverse curve in the first quadrant.
Do this: Reverse voltage goes on the negative x-axis and reverse current on the negative y-axis.
Mistake: Reading the cut-in voltage where the current first becomes non-zero.
Do this: Extend the straight steep part of the curve to the voltage axis and read where it meets.

p-n junction diode: I-V characteristics: frequently asked questions

Why does my silicon diode show zero reverse current?

The reverse current of a silicon diode can be smaller than the least count of a 0 to 200 μA meter. Record zero honestly and say so in your result; a germanium diode shows a larger reverse current.

Based on: CBSE Physics (042) curriculum 2026-27, Class XII practicals, Section B experiment 9; NCERT Laboratory Manual Physics Class XII, Experiment 16 (p-n junction characteristics); NCERT Physics Part II, Textbook for Class XII, Chapter 14, section on the p-n junction diode