Photo Diode Working

A photodiode is a semiconductor p–n junction device that converts light into electrical current. Photodiodes are commonly used in LEDs. It is the absorption of photons in the photodiode that causes the current to be generated. Photodiodes may be equipped with optical filters or built-in lenses, and their surface areas can range from large to small. As the surface area of a photodiode increases, the response time of the device becomes slower. A large area photodiode is the most common type of solar cell used to generate electric solar power in the past and is still used today.

How are Photodiodes different from diodes?

Photon-detector diodes or photodiodes are similar to regular semiconductor diodes, with the exception that they can either be exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fibre connection, which allows light to reach the sensitive part of the device. Photo-detector diodes are used in a variety of applications. Many diodes designed specifically for use as a photodiode employ a PIN junction rather than a p–n junction in order to increase the speed with which they respond. A photodiode is a semiconductor device that is designed to operate in reverse bias.

The basic operating principle

A photodiode is made up of a PIN structure, also known as a p–n junction. When a photon with sufficient energy strikes a diode, it produces an electron–hole pair, which is a semiconductor. This mechanism is referred to as the inner photoelectric effect in some contexts. If the absorption occurs in the depletion region of the junction, or one diffusion length away from it, the carriers are swept away from the junction by the built-in electric field of the depletion region, which results in the formation of a new junction. As a result, holes move toward the anode and electrons move toward the cathode, resulting in the generation of a photocurrent. The total current flowing through a photodiode is the sum of the dark current (current generated in the absence of light) and the photocurrent; therefore, the dark current must be kept to a minimum in order to maximise the sensitivity of the device, as previously stated.

With respect to a given spectral distribution, the photocurrent is linearly proportional to the irradiance, at least to the first order.

Mode of operation in photovoltaics

The I-V characteristics of a photodiode shows the important characteristic of a photodiode. The linear load lines in a typical characteristic graph of a photodiode represent the response of the external circuit, where 

I =(Applied bias voltage-Diode voltage)/Total resistance, 

In photovoltaic mode (zero bias), photocurrent flows into the anode through a short circuit to the cathode, resulting in the generation of electricity. Whenever a circuit is opened or when a load impedance is present, which prevents photocurrent from exiting the device, a voltage builds up in the direction that forward biassed the diode, i.e., anode positive with respect to cathode. If the circuit is shorted or the impedance is low, a forward current will absorb all or part of the photocurrent, depending on the circumstances.. This mode makes use of the photovoltaic effect, which is the fundamental principle behind solar cells – a traditional solar cell is simply a photodiode with a large surface area. The photovoltaic cell will be operated at a voltage that causes only a small forward current when compared to the photocurrent in order to achieve the highest possible power output.

Photoconductive mode

It is possible to operate a photoconductive diode in reverse bias mode, meaning that the cathode is biassed positively with respect to the anode. Since the additional reverse bias widens the depletion layer, it decreases the capacitance of the junction and expands the region with an electric field, which causes electrons to be collected more quickly. This reduces the response time of the device. The reverse bias also results in the generation of dark current without causing a significant change in the photocurrent.

Despite the fact that this mode is faster, the photoconductive mode can produce more electronic noise as a result of dark current or avalanche effects, among other things. With its low leakage current (less than 1 nA), a good PIN diode can often be used in a typical circuit without introducing significant Johnson–Nyquist noise into the circuit.

Applications

A p–n photodiode is used in applications that are similar to those of other types of photodetectors, such as photoconductors, charge-coupled devices (CCDs), and photomultiplier tubes, such as in medical imaging. When used to generate an output that is dependent on the illumination (for example, analogue for measurement), or to change the state of a circuitry, they can be very useful (digital, either for control and switching or for digital signal processing).

Among other things, photodiodes are used in consumer electronics devices such as compact disc players (CD players), smoke detectors, medical devices, and receivers for infrared remote control devices, which are used to control a variety of electronic devices ranging from televisions to air conditioners. Photodiodes and photoconductors are both suitable for a wide range of applications. Depending on the application, either type of photo sensor can be used for light measurement, such as in camera light metres, or to respond to light levels, such as when turning on street lighting after dark.

Conclusion

A photodiode is a type of light sensor that converts light energy into electrical energy, and it is used in electronics (voltage or current). An example of a semi conducting device with PN junction is the photodiode. An intrinsic layer exists between the p (positive) and n (negative) layers, and it connects the two layers together. The photodiode converts light energy into electrical current by accepting it as an input. This device is also referred to as a photodetector, photo sensor, or light detector. The photodiode operates in reverse bias condition, which means that the negative terminal of the battery (or the power supply) is connected to the p – side of the photodiode and the positive terminal of the battery is connected to the n – side.