What is Photo Diode and How does the photodiode work?
What are the applications of Photo Diode?
A photodiode is a semiconductor device with a p-n junction and an intrinsic layer between p and n layers that convert light into current. Photo-diode produces photo-current by generating electron-hole pairs, because of the absorption of light in the intrinsic or depletion region.
The photo-current thus generated is proportional to the absorbed light intensity. Photo-diodes usually have a slower response time as their surface area increases. We can use photodiodes in two ways — in a photo-voltaic or photoconductive role. To use a photodiode in its photo-conductive mode, the photo-diode is reverse-biased; the photodiode will then allow a current to flow when it is illuminated.
Principle of operation of Photo-diode
A photodiode is a PN junction, When a photon of sufficient energy (<1.1ev) strikes the diode, it creates an electron-hole pair. The intensity of photon absorption depends on the energy of photons – the lower the energy of photons, the deeper the absorption is. It knew this mechanism as the inner photoelectric effect. If the absorption occurs in the depletion region of the p-n junction, it sweeps these hole pairs from the junction – because of the built-in electric field of the depletion region. Thus holes move toward the anode, and it produced electrons toward the cathode and a photocurrent. The total current through the photo-diode is the sum of the dark current (Dark current is a generated current in the absence of light) and the photo-current, so it must minimize the dark current to maximize the sensitivity of the device.
Operation modes of Photo-diode
Photo-diodes can be operated in different modes like Photovoltaic mode, Photoconductive mode, Avalanche diode mode.
- Photo-conductive mode–In this mode device used in reverse biased. The depletion layer increased with the help of reverse voltage, which reduces the response time and capacitance of the junction. The Photo-conductive mode is quick and exhibits electronic noise.
- Avalanche diode mode–This mode also used in a high reverse bias condition, which allows multiplication of an avalanche breakdown of each photo-generated electron-hole pair; which accumulates the responsiveness of the device.
- Photo-voltaic mode–In this mode voltage is generated by the illuminated photodiode and something also known as a zero-bias mode. It provides a small dynamic and non-linear dependence on the voltage produced.
- Surveying instrument
- Safety equipment
- Bar code scanners
- Medical devices
- Optical communication devices
- Position sensors
- Automotive devices
Two important characteristics of a photodetector are its quantum efficiency and its responsivity. These parameters depend on the material band-gap, the operating wavelength, and the doping and thickness of the P-N regions of the device.
It is defined as the ratio of the number of electron-hole pairs (ehp) generated to the total number of incident photons and is given by
η= Number of ehp’s generated/total no. of incident photons
η= Ip/q/Pop/h? or
η= Ip x h?/q x Pop
where Ip is the photo-generated current, Pop is the incident optical power, h? is the photon energy and q is the free carrier charge.
It is defined as the ratio of photo-generated current to the incident optical power. The following formula gives it:
Responsivity is related to the quantum efficiency by
This parameter is quite useful since it specifies the photocurrent generated per unit of optical power.
Problem 1: The responsivity of a photodiode is 0.85 A/W and the i/p saturation is 3.5 mW. What is the photocurrent if the incident light power is (a) 1mW?
Solution: Given that photodiode responsivity Ʀ = 0.85 A/W
Pop = 1 mW
Ʀ = IP / Pop
IP = Ʀ =Pop
Ip = 0.85 x 1
IP = 0.85 mA
Problem 2: What is the responsivity of an InGaAs photodiode if its quantum efficiency is equal to 70%? The energy gap of InGaAs is 0.75 eV.
Solution: Given that the energy gap of InGaAs is 0.75 eV; quantum efficiency = 70%.
According to Einstein energy Eg = hc/l;
l = 6.6 x 10-34 (J-s) x 3x 108 (m/s) / 0.75 (ev) x 1.6 x 10-19
l = 1664X 10-9 m
So, the reponsivity in terms of efficiency is defined by
Ʀ = (h/1248) l
Ʀ = (0.70/1248) 1664 x 10-9