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Photodiode Design Note

Photodiode is a type of light detector capable of converting light energy into either current or voltage, depending upon the mode of operation. Photodiodes are similar to typical semiconductor diodes except that they may be either exposed or packaged with a window to allow light to reach the sensitive part of the device.

Photodiode Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction. Some photodiodes are similar to a light emitting diode. They have two leads, coming from the bottom. The shorter end of the two is the cathode, while the longer end is the anode. Under forward bias, conventional current will pass from the anode to the cathode. Photocurrent flows in the opposite direction.

PhotoDiode Mode of operation

A photodiode is a PN junction or PIN structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the junction’s depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced.

Photovoltaic mode

When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. The diode becomes forward biased and “dark current” begins to flow across the junction in the direction opposite to the photocurrent. This mode is responsible for the photovoltaic effect, which is the basis for solar cells—in fact, a solar cell is just an array of large area photodiodes.

Photoconductive mode

In this mode, the diode is often reverse biased, dramatically reducing the response time at the expense of increased noise. This increases the width of the depletion layer, which decreases the junction’s capacitance resulting in faster response times. The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. The photocurrent is linearly proportional to the luminance.

Although this mode is faster, the photovoltaic mode tends to exhibit less electronic noise. The leakage current of a good PIN diode is so low – < 1nA

More methods

Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher reverse bias. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.

Phototransistor
Phototransistors also consist of a photodiode with internal gain. A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach the base-collector junction. The electrons that are generated by photons in the base-collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or hfe). Phototransistors have a higher responsivity for light so they are not able to detect low levels of light any better than photodiodes. Phototransistors also have slower response times.

Chemistry of Photodiode

The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's band gap will produce significant photocurrents. The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's band gap will produce significant photocurrents. Because of their greater band gap, silicon-based photodiodes generate less noise than germanium-based photodiodes, but germanium photodiodes must be used for wavelengths longer than approximately 1 µm. silicon-based photodiodes generate less noise than germanium-based photodiodes, but germanium photodiodes must be used for wavelengths longer than approximately 1 µm. Commonly used materials to produce photodiodes include:

Material Wavelength range (nm)
Silicon 190–1100
Germanium 400–1700
Indium gallium arsenide 800–2600
Lead sulfide <1000-3500

Performance of Photodiode

Noise-equivalent power
The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth.

Dark current
The current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction.

Responsivity
The ratio of generated photocurrent to incident light power, typically expressed in A/W when used in photoconductive mode. The responsivity may also be expressed as quantum efficiency, or the ratio of the number of photogenerated carriers to incident photons and thus a unit less quantity.

Types of Photodiodes

There are PN and PIN Photodiodes. They differ in their mode of performance. Due to the intrinsic layer, a PIN photodiode must be reverse biased. The reverse biasing increases the depletion region allowing a larger volume for electron-hole pair production, and reduces the capacitance thereby increasing the bandwidth. Reverse biasing also introduces noise current, which reduces the S/N ratio. Therefore, a reverse bias is recommended for higher bandwidth applications and/or applications where a wide dynamic range is required.
A PN photodiode is more suitable for lower light applications because it allows for unbiased operation.

Photodiode array

This has Hundreds or thousands photodiodes of typical sensitive area 0.025mmx1mm arranged as a one-dimensional array. It can be used as a position sensor. One advantage of photodiode arrays (PDAs) is that they allow for high speed parallel read out.

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