How does a Photodiode Work?

Catalog

I Photodiode Parameters

A photodiode is a special diode (light-receiving device) that converts optical signals into electrical signals. Its symbol is shown in the figure.

FIG 1. Photodiode Symbol

Similar to ordinary diodes, its basic structure is also a pn junction. Its shell has a window with glass for light penetration as shown below.

Figure 2. Photodiode Structure

The photodiode works in the reverse state and its reverse current increases with the increase of light intensity.

When there is no light, the reverse current is very small (generally less than 0.1μa), called the dark current. And the reverse resistance at this time is tens of mΩ.

When there is light, a reverse current is formed, called photocurrent. At this time, the reverse resistance of the photodiode drops to several kΩ.

1. Dark Current

In the photoconduction mode, when there is no light, the current through the photodiode is a dark current. Dark current in photodiode includes radiation current and saturation current of the semiconductor junction. It must be measured in advance. Especially in precision optical power measurement, the error caused by dark current must be carefully considered and corrected.

2. Photodiode Response Time

The response rate is the ratio of the photocurrent in the photoconduction mode to the emergency light, in A/W. The response characteristic can also be expressed as quantum efficiency of the photodiode, which is the ratio of the number of carriers generated by light to the number of emergency light photons.

3. Noise Equivalent Power(NEP)

Noise equivalent power refers to the minimum optical power required to generate photocurrent, which is equal to the RMS of noise power at 1 Hz. It is approximately equal to the minimum detectable input power of the photodiode.

A related property is detectivity (D), which is the inverse of the noise equivalent power.

4. Frequency Response Characteristic

It is mainly determined by three factors:

(1) The diffusion time of photogenerated carriers near the depletion layer;

(2) The drift time of photogenerated carriers in the depletion layer;

(3) The circuit time constant determined by the load resistance and the parallel capacitor.

II Photodiode Characteristics

1. Volt-ampere Characteristics

It refers to the relationship between the photocurrent on the photodiode and the voltage applied across it.

Figure 3. Photodiode Volt-ampere Characteristics

2. Illumination Characteristics

It refers to the relationship between luminous flux and photocurrent when the photodiode voltage between the cathode and anode is constant. The slope of the light characteristic curve is called the photodiode sensitivity.

3. Spectral Characteristics

The relationship between the photocurrent and the incident light wavelength is called the spectral characteristic.

The photon energy is related to the light wavelength: the longer the wavelength, the smaller the photon energy; the shorter the wavelength, the greater the photon energy.

III Photodiode Types

1. PN Photodiode

Features: small dark current and low response speed

Uses: illuminometer, color photodiode sensor, phototransistor, linear image photodiode sensor, spectrophotometer, camera exposure meter.

2. PIN Photodiode

Features: It's a kind of large dark current photodiode, with low junction capacity and fast response

Uses: high-speed light photodiode detector, optical communication, optical fiber, remote control, photoelectric transistor, fax.

3. Launch Key Photodiode

Features: Use Au film and N-type semiconductor junction instead of P-type semiconductor

Uses: detection of short-wave light as ultraviolet rays

4. Avalanche Photodiode

Features:  It's a kind of weak light photodiode detector with a fast response speed

Uses: High-speed optical communication, high-speed optical detection

IV How Does a Photodiode Work？

Photodiode works according to the following principle:

When there is light, the photon with energy enters the PN junction and transfers the energy to the electrons on the covalent bond. Some electrons break away from the covalent bond, and electron-hole pairs are produced, also called photo-generated carriers.

The number of photo-generated carriers is limited, and the number of majority carriers before light is much greater than that of photo-generated carriers. Therefore, the photo-generated carrier has a small effect on the number of carriers, but the number of minority carriers has a large impact. This is why the photodiode works under reverse voltage instead of forward voltage.

So, under reverse voltage, the minority carriers increased by the photogenerated carriers participate in the drifting motion. In the P region, the photogenerated electrons diffuse to the PN junction. If the P region thickness is less than the electron diffusion length, then most photogenerated electrons will pass through the P region to the PN junction. This is the same in the N region.

Figure 4. Photodiode Working Principle

Therefore, in photodiode manufacture, the PN junction is very shallow to promote the drift of minority carriers.

Generally speaking, the work of a photodiode is an absorption process, which converts the light change into a reverse current change. The combination of the current and the dark current is the photocurrent, so the dark current is minimized as much as possible to improve the photodiode sensitivity to light.

The light intensity is proportional to the photocurrent. The greater the light intensity, the greater the reverse current. This characteristic is called photoconductivity, and the current caused by it is called photocurrent.

V Function of Photodiode

Photodiodes are widely used for:

1. Light Control

The photodiode can be used as a photoswitch, and its circuit is shown in the figure below. When there is no light, the photodiode VD1 is cut off due to the reverse voltage. The transistor VT1 and VT2 are also cut off due to no base current. The relay is in the released state.

When light is emitted on VD1, it transits from cut-off to conduction. So, VT1 and VT2 are turned on successively, and the relay K pulls in to connect the control circuit.

Figure 5. Photodiode for Light Control

2. Light Signal Reception

Photodiodes can be used to receive light signals. The figure below shows the light signal receiving and amplifying photodiode circuit. The light signal is received by the photodiode VD, amplified by VT, and output through the coupling capacitor C.

Figure 6. Photodiode for Light Signal Reception

VI Photodiode Applications

Specific photodiode application is:

1. Photocell

The photocell is essentially a large-area PN junction. When light is emitted on one PN junction surface, such as the P region surface, if the photon energy is greater than the forbidden bandwidth of the semiconductor material, each photon in the P region will produce a free electron-hole pair.

Figure 7. Photocell

Electron-hole pairs diffuse rapidly inward, and, and an electromotive force related to the light intensity is formed under the junction electric field.

At this time, if we use it as a power source and connect it to an external circuit, it will continuously supply power as long as there is light, which is the photocell. In other words, a photocell is a PN junction photoelectric device without a bias voltage. It and can directly convert light energy into electrical energy.

According to the purpose of the photocell, it can be divided into solar photocells and measuring photocells.

Difference between photodiodes and photocells

Photocell: used for energy conversion. Advantage: increase the conversion rate.

Photodiode: used for detection. Advantages: high sensitivity, fast response, and high quantum efficiency.

They're different in:

2. Solar Cell

A solar cell is a semiconductor device, as shown in the Figure below. When sunlight is emitted on the semiconductor, part of it is reflected, and the rest is absorbed or penetrates the semiconductor.

Some absorbed light turns into heat, while other photons collide with the valence electrons making up the semiconductor, thus generating electron-hole pairs. In this way, light energy is converted into electrical energy.

Therefore, after the sunlight, a DC voltage will be generated at both ends of the solar cell, thereby directly converting sunlight energy into a DC current. If we weld metal leads to the P layer and the N layer, and connect the load, the current will flow through the external circuit.

In this way, if we connect the photocells in series and parallel, a certain voltage and current can be generated, thus outputting power.

Figure 8. Solar Cell

3. Photovoltaic Power Generation Lighting System

The photovoltaic power generation system is a power generation system that uses solar cells to convert solar energy into electrical energy. It uses the photovoltaic effect.

The main components are solar cells, storage batteries, controllers, and inverters. It has high reliability, long service life, no pollution, independent power generation, and grid-connected photodiode operation.

Because the photodiode photovoltaic mode is greatly affected by external environmental factors such as light and temperature, the operating point changes rapidly.

There are independent power generation systems and grid-connected power generation systems.

(1) Independent photovoltaic power generation system

An Independent photovoltaic power generation system is a power generation method not connected to the power grid. It needs batteries to store energy for the night. Independent solar photovoltaic power generation is mainly used in remote villages and homes

Figure 9. Structure Diagram of Photovoltaic Power Generation System

(2) Grid-connected photovoltaic power generation system

The grid-connected photovoltaic power generation system is connected to the national grid to supply the power grid. It does not need batteries. The residential photovoltaic power generation system is mostly in households. They are also used for public facilities, lighting systems in night landscaping, and solar farms.

VII Detection Method

1. Resistance Measurement Method

Use the "1k" of a multimeter to test the photodiode. The forward resistance of the photodiode is about 10KΩ.

When there's no light, if the measured reverse resistance is ∞, the diode is good, or there is a large leakage current.

When there is light, the reverse resistance decreases with the increase of light intensity. If the resistance can reach a few kΩ; or below 1kΩ, the diode is good; if the reverse resistance is ∞ or zero, the diode is broken.

2.Voltage Measurement Method

Use the "1V" gear of a multimeter. Connect the red test lead to the positive pole of the photodiode, and the black test lead to the negative pole. Under the light, the voltage is proportional to the light intensity, generally up to 0.2V-0.4V.

3.Short-circuit Current Measurement Method

Use the "50μA" gear of multimeter. Connect the red test lead to the photodiode positive pole, and the black test lead to the negative pole. Under incandescent lamps (not fluorescent lamps), if the short-circuit current increases with the light increases, the diode is good. The short-circuit current can reach tens to hundreds of μA.

Sometimes, it is necessary to distinguish between infrared light-emitting diodes and infrared photodiodes.

If they are both packed with transparent resin, we can see their die installation. There is a shallow plate under the infrared LED die, but there is no photodiode die.

Figure 10. Infrared Photodiode

if it's a small photodiode or it's packed with black resin, you can use a multimeter (set at 1k gear) to measure resistance.

First, make sure the diode is not exposed to light. If the measured forward resistance is 20-40kΩ, and the reverse resistance is greater than 200kΩ, the forward resistance is about 10k and the reverse resistance is close to ∞, it is the photodiode.

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