Photodiode

What’s a Photodiode?

 

A Photodiode is a semiconductor p–n junction device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas. Photodiodes usually have a slower response time as their surface area increases. The common, traditional solar cell used to generate electric solar power is a large area photodiode.

 

 

Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specially as a photodiode use a PIN junction rather than a p–n junction, to increase the speed of response. A photodiode is designed to operate in reverse bias.

 

Principle of Operation

 

A photodiode is a PIN structure or p–n junction. When a photon of sufficient energy strikes the diode, it creates an electron–hole pair. This mechanism is also known as the inner photoelectric effect. 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 electric field of the depletion region. Thus, holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. The total current through the photodiode is the sum of the dark current (current that is generated in the absence of light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device.

To first order, for a given spectral distribution, the photocurrent is linearly proportional to the irradiance.

 

• Photovoltaic Mode: In photovoltaic mode (zero bias), photocurrent flows out of the anode through a short circuit to the cathode. If the circuit is opened or has a load impedance, restricting the photocurrent out of the device, a voltage builds up in the direction that forward biases the diode, that is, anode positive with respect to cathode. If the circuit is shorted or the impedance is low, a forward current will consume all or some of the photocurrent. This mode exploits the photovoltaic effect, which is the basis for solar cells – a traditional solar cell is just a large area photodiode. For optimum power output, the photovoltaic cell will be operated at a voltage that causes only a small forward current compared to the photocurrent.
• Photoconductive mode: In photoconductive mode the diode is reverse biased, that is, with the cathode driven positive with respect to the anode. This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction’s capacitance and increases the region with an electric field that will cause electrons to be quickly collected. The reverse bias also creates dark current without much change in the photocurrent. Although this mode is faster, the photoconductive mode can exhibit more electronic noise due to dark current or avalanche effects. The leakage current of a good PIN diode is so low (<1nA) that the Johnson–Nyquist noise of the load resistance in a typical circuit often dominates.

 

Types of Photodiodes

Although there are numerous types of photodiodes available in the market and they all work on the same basic principles, though some are improved by other effects. The working of different types of photodiodes works in a slightly different way, but the basic operation of these diodes remains the same. The types of photodiodes can be classified based on their construction and functions as follows.

• PN Photodiode: The first developed type of photodiode is the PN type. As compared with other types, its performance is not advanced, but at present, it is used in several applications. The photodetection mainly happens in the depletion region of the diode. This diode is quite small, but its sensitivity is not great as compared with others. Please refer to this link to know more about the PN diode.
• PIN Photodiode: At present, the most commonly used photodiode is a PIN type. This diode gathers the light photons more powerfully as compared with standard PN photodiode because the wide intrinsic area between the P and N regions allows for more light to be collected, and in addition to this, it also offers a lower capacitance. Please refer to this link to know more about the PIN diode.
• Avalanche Photodiode: This kind of diode is used in low light areas due to its high gain levels. It generates high levels of noise. So this technology is not appropriate for all applications. Please refer to this link to know more about the Avalanche diode.
• Schottky Photodiode: The Schottky photodiode uses the Schottky diode, and it includes a small diode junction that means, there is small junction capacitance so, it operates at high speeds. Thus, this kind of photodiode is frequently utilized in high bandwidth (BW) optical communication systems like fiber-optic links. Please refer to this link to know more about the Schottky diode.

Each type of photodiode has its own benefits and drawbacks. The selection of this diode can be done based on the application. The different parameters to be considered while selecting photodiode include noise, wavelength, reverse bias constraints, gain, etc. The performance parameters of photodiode include responsivity, quantum efficiency, transit time or response time.

These diodes are widely used in applications where the detection of the presence of light, color, position, the intensity is required. The main features of these diodes include the following.

• The linearity of the diode is good with respect to incident light
• Noise is low.
• The response is wide spectral
• Rugged mechanically
• Lightweight and compact
• Long life

The required materials to make a photodiode and the range of electromagnetic spectrum wavelength range includes the following

• For silicon material, the electromagnetic spectrum wavelength range will be (190-1100) nm
• For Germanium material, the electromagnetic spectrum wavelength range will be (400-1700) nm
• For Indium gallium arsenide material, the electromagnetic spectrum wavelength range will be (800-2600) nm
• For Lead (II) sulfide material, the electromagnetic spectrum wavelength range will be <1000-3500) nm
• For Mercury, cadmium Telluride material, the electromagnetic spectrum wavelength range will be (400-14000) nm
  Because of their better bandgap, Si-based photodiodes produce lower noise than Ge-based photodiodes.

 

How to Select a Photodiode for Your PCB?

 

There are a number of operational parameters to consider when selecting a photodiode for your PCB design and assembly. Here are some of the most important photodiode parameters to consider for your printed circuit board (PCB):

• Responsivity: this is a measure of the current produced per Watt of optical power that falls on the device. Responsivity is actually a spectrum, meaning that it is a function of the wavelength of incident light. This is the range of wavelengths to which the photodiode will produce a current. As an example, silicon photodiodes are typically sensitive to wavelengths ranging from approximately 200 nm (UV) to approximately 1100 nm (IR). The responsivity will vary throughout this wavelength range.
• Shunt resistance: this is the slope of the current-voltage curve of the photodiode at zero applied bias. An ideal photodiode should have infinite shunt resistance, but the actual shunt resistance can range from 10’s to 1000’s of mega Ohms, thus the best photodiodes have the highest shunt resistance.
• Saturation current: this is related to the shunt resistance and is equal to the current when run at reverse bias (between 0 V and the reverse breakdown voltage). The shunt resistance is normally defined with reference to a 10mV source voltage and the saturation current using Ohm’s law.
• Terminal capacitance: this determines the transient response time of a photodiode. When combined with the load resistance, the rise time is equal to the RC time constant.
• Responsivity temperature coefficient: as the temperature of a photodiode changes, the responsivity at specific wavelengths will also change. The peak in the responsivity spectrum will exhibit a redshift as temperature increases due to band narrowing.