V-I Characteristics of Photodiode in Reverse Bias

Aim

To study the V-I characteristics of a photodiode operated in reverse bias, and to observe how the reverse (photo)current varies with incident light intensity.

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.

Photodiode & Light Source

A silicon photodiode component and an adjustable virtual light source (or simulated illuminance control) available in the simulator's optoelectronics module.

Theory & Principle

A photodiode is a PN junction diode designed to be sensitive to light. It is normally operated in reverse bias (this is called photoconductive mode). When light (photons with energy greater than the semiconductor's bandgap) falls on the depletion region, it generates additional electron-hole pairs, which are swept across the junction by the reverse-bias electric field, adding to the small dark (leakage) current already present.

I_total = I_dark + I_photo

Total reverse current = dark (leakage) current + light-generated photocurrent

Key Insight: The photocurrent is approximately proportional to the incident light intensity and is almost independent of the reverse voltage (it saturates), just like an ordinary diode's reverse saturation current — except here the "saturation" level is set by how many photons are arriving, not just by temperature.

Circuit / Setup Diagram

Figure 1: Photodiode reverse-biased with an adjustable light source, a microammeter in series, and a voltmeter across the photodiode.

Procedure

Open the simulator and connect a battery, a protective resistor, and a photodiode in series, with a microammeter in series and a voltmeter across the photodiode.
Connect the photodiode in reverse bias: cathode to the positive terminal, anode to the negative terminal.
Set the light source to 'dark' (no illumination) and record the current at reverse voltages of 0, 2, 6, and 10 V — this is the dark current.
Set the light source to 'low intensity' and repeat the readings at the same voltage points.
Set the light source to 'high intensity' and repeat the readings again.
Plot all three curves (dark, low light, high light) of photocurrent against reverse voltage on the same graph and compare their levels.

Tip: Keep the reverse voltage well below the photodiode's rated breakdown voltage throughout — the goal is to observe the flat, voltage-independent saturation region for each light level, not the breakdown region.

Observation Table

Dark Current (No Light)
S. No.	Reverse Voltage, V_r (V)	Photocurrent, I_p (µA)
1	0	0.00
2	2	0.50
3	6	1.30
4	10	1.60

Low Light Intensity
S. No.	Reverse Voltage, V_r (V)	Photocurrent, I_p (µA)
1	0	5.00
2	2	25.00
3	6	29.00
4	10	31.00

High Light Intensity
S. No.	Reverse Voltage, V_r (V)	Photocurrent, I_p (µA)
1	0	10.00
2	2	65.00
3	6	78.00
4	10	82.00

V-I Characteristic Graph

_r (volts) → 020406080100 Photocurrent, I_p (µA) → LegendHigh Light IntensityLow Light IntensityDark (No Light) Each curve is nearly flat with voltage but shifts to a higher current level as light intensity increases.

Figure 2: Reverse V-I characteristics of the photodiode at three light levels. Each curve is nearly flat with voltage, confirming that photocurrent depends on light intensity, not reverse voltage.

Calculations

Photocurrent (Light-Generated Component) at V_r = 6 V

I_photo(low light) = I_total(low) − I_dark = 29.0 − 1.3 = 27.7 µA

I_photo(high light) = I_total(high) − I_dark = 78.0 − 1.3 = 76.7 µA

The photocurrent at high intensity is roughly 2.8 times that at low intensity, consistent with the increase in light intensity used.

Result

Result: The reverse V-I characteristics of the photodiode were plotted at three light levels. Each curve was nearly flat (voltage-independent), but the current level increased significantly with increasing light intensity — confirming that a reverse-biased photodiode's current is primarily a function of incident light intensity, not applied voltage. This is the operating principle used in photodiode-based light sensors and optical receivers.

Viva Voce Questions

Q1: Why is a photodiode operated in reverse bias rather than forward bias?

In reverse bias (photoconductive mode), the depletion region is wide and the electric field across it is strong, so light-generated carriers are swept out quickly and efficiently, giving a fast response time and a photocurrent that is linearly proportional to light intensity over a wide range. In forward bias, the diode conducts a large current on its own (making the small photocurrent hard to distinguish) and the response is much slower.

Q2: Why does the photocurrent stay almost constant as reverse voltage increases?

The number of electron-hole pairs generated by incident light is fixed by the light intensity, not by the applied voltage. Once the reverse voltage is enough to sweep essentially all the generated carriers across the junction before they recombine, further increasing the voltage does not create more carriers, so the current saturates — similar to how an ordinary diode's reverse saturation current is roughly constant.

Q3: What is 'dark current' in a photodiode?

Dark current is the small reverse current that flows even with no incident light, caused by thermally generated minority carriers — exactly the reverse saturation current of an ordinary PN junction diode. It sets the lower limit of light intensity a photodiode can reliably detect, since any signal has to be distinguished from this baseline current.

Q4: What are some practical applications of photodiodes?

Photodiodes are used in optical fibre communication receivers, light meters and camera exposure sensors, barcode scanners, smoke detectors, solar cells (in photovoltaic mode), infrared remote control receivers, and as position/proximity sensors in industrial automation.

Q5: What is the difference between photoconductive mode and photovoltaic mode of a photodiode?

In photoconductive mode, the photodiode is reverse biased by an external source and the current through it is measured — this gives a faster, more linear response, as used in this experiment. In photovoltaic mode, no external bias is applied; the photodiode itself generates a small voltage/current from incident light, similar to how a solar cell works, which is used for very low-power or zero-bias light sensing applications.

Related Experiments

Continue exploring semiconductor devices with the following experiment:

← V-I Characteristics of LED in Forward Bias Mode

V-I Characteristics of Photodiode in Reverse Bias Mode

Aim