V-I Characteristics of PN Junction Diode in Reverse Bias

Aim

To study the V-I characteristics of a PN junction diode in reverse bias and to observe the small, nearly constant reverse saturation (leakage) current.

Apparatus / Tools Required

Virtual Circuit Simulator

PhET Semiconductor / Circuit Construction Kit, Tinkercad Circuits, or Falstad Circuit Simulator — free and browser-based.

Web Browser

A modern browser such as Chrome, Firefox, or Edge with JavaScript enabled.

Spreadsheet Software

Excel, Google Sheets, or LibreOffice Calc for recording readings and plotting the V-I graph.

Diode Component

Silicon PN junction diode (e.g. 1N4007) and a microammeter (or a milliammeter capable of resolving microamp-range readings in the simulator).

Theory & Principle

In reverse bias, the positive terminal of the source is connected to the n-side and the negative terminal to the p-side of the diode. This widens the depletion region and prevents majority carriers from crossing the junction. However, a very small current still flows due to the drift of minority carriers — this is called the reverse saturation current (I₀).

Key Insight: The reverse saturation current is almost independent of the applied reverse voltage (it "saturates") and depends mainly on temperature and the semiconductor material. For silicon diodes, I₀ is typically in the nanoampere to low microampere range at room temperature — several orders of magnitude smaller than the forward current for the same magnitude of voltage.

If the reverse voltage is increased beyond the diode's rated breakdown voltage, the current rises sharply (avalanche or Zener breakdown). This experiment stays within the safe reverse voltage range and does not reach breakdown.

Circuit / Setup Diagram

Figure 1: Diode reverse-biased with a microammeter in series and a voltmeter across the diode.

Procedure

Open the simulator and connect a battery, a protective resistor, and a silicon diode in series.
Connect the diode in reverse bias: cathode (n-side) to the positive terminal, anode (p-side) to the negative terminal.
Insert a microammeter in series with the diode and a voltmeter in parallel across it.
Increase the reverse voltage in steps of 1 V from 0 V to 10 V, staying well within the diode's rated reverse breakdown voltage.
Record the voltmeter and microammeter readings at each step in the observation table.
Plot the graph of reverse current (I_r) against reverse voltage (V_r) and observe how flat the curve is.

Tip: Because the reverse current is extremely small, use the microamp (µA) range on the meter in the simulator for accurate readings — on the milliamp range the current would appear to read almost zero.

Observation Table

S. No.	Reverse Voltage, V_r (V)	Reverse Current, I_r (µA)
1	0	0.00
2	1	0.50
3	2	0.80
4	4	1.10
5	6	1.25
6	8	1.32
7	10	1.40

V-I Characteristic Graph

_r (volts) → 0246810 Reverse Current, I_r (µA) → LegendSi Diode 1N4007 Reverse current is very small and remains almost constant (saturation current) as reverse voltage increases.

Figure 2: Reverse V-I characteristic showing a nearly flat, saturated leakage current across the tested voltage range.

Calculations

Average Reverse Saturation Current

I₀(avg) = (0.50 + 0.80 + 1.10 + 1.25 + 1.32 + 1.40) / 6 ≈ 1.06 µA

This value is thousands of times smaller than the forward current (tens of mA) observed at a similar magnitude of applied voltage in the forward-bias experiment.

Result

Result: The V-I characteristic of the PN junction diode in reverse bias was plotted. The reverse current was found to be very small (in the microampere range) and remained nearly constant as the reverse voltage was increased from 0 V to 10 V, confirming the concept of reverse saturation current.

Viva Voce Questions

Q1: What causes the small current in reverse bias?

In reverse bias, majority carriers cannot cross the widened depletion region, but a small number of thermally generated minority carriers (electrons in the p-region, holes in the n-region) drift across the junction under the reverse field, producing the reverse saturation current.

Q2: Why does the reverse current not increase much with reverse voltage?

The reverse current is limited by the rate of thermal generation of minority carriers, not by the applied voltage. Once all available minority carriers near the junction are being swept across, further increases in reverse voltage do not significantly increase the current — hence the current 'saturates'.

Q3: What happens if the reverse voltage exceeds the diode's rated value?

Beyond the rated reverse breakdown voltage, the diode enters avalanche or Zener breakdown, and the reverse current rises sharply. Unless limited by an external resistor, this can permanently damage an ordinary rectifier diode; Zener diodes are specifically designed to operate safely in this region.

Q4: How does temperature affect the reverse saturation current?

The reverse saturation current roughly doubles for every 10°C rise in temperature, because more electron-hole pairs are thermally generated at higher temperatures, increasing the number of minority carriers available to cross the junction.

Q5: Why is the reverse current of a diode important in circuit design?

The reverse (leakage) current determines how 'ideal' a diode's blocking behaviour is. In precision or low-power circuits (such as sample-and-hold circuits or battery-powered devices), a lower reverse leakage current is essential to minimise unwanted current drain when the diode is meant to be non-conducting.

Related Experiments

Continue exploring semiconductor devices with the following experiment:

← V-I Characteristics of PN Junction Diode in Forward Bias Mode

V-I Characteristics of PN Junction Diode in Reverse Bias Mode

Aim