In this What is a photodiode? , types of photodiode, photodiode symbol, photodiode circuit, photodiode applications, photodiode characteristics, photodiode construction, photodiode sensor, photodiode applications.
What is a photodiode?
Photodiode is a reverse-biased special p-n junction diode having a transparent window in which reverse current varies linearly with the light flux when the diode is illuminated with light. For operating a photo-diode, a reverse bias is kept below the breakdown voltage. The conductivity of a photodiode change with the absorption of light around the depletion layer.
Explain the construction and working of photodiode
Construction. A reverse-biased p-n junction diode is enclosed in a transparent envelope. Light is allowed to fall on the transparent surface facing the diode. All other faces of the envelope may be enclosed in a metallic case.
A symbolic representation of a photodiode is shown in the figure.
Volt-ampere characteristics of photodiode and action of photodiode.
The variation of photo-diode current with the variation of reverse voltage is known as volt-ampere characteristics of a photodiode
The circuit diagram to study volt-ampere characteristics of photodiode is shown in figure. When the photodiode is reverse biased, then a constant current known as saturation current l, flows in the circuit. This current is due to thermally generated minority carriers, This current is also known as dark current.
When light of energy (hy) more than the energy gap (E) of a semiconductor falls on the photo-diode, additional electron-hole pairs are formed near the depletion region of the diode. These electrons and holes are separated before they combine by the electric field across the junction. The electric field makes electrons to diffuse through the junction to reach 4-region and holes are diffused through the junction to reach the p-region. Hence, an emf is developed across the junction. When external resistance R is connected across the photodiode, current I, flows in addition to the dark current In. The electron-hole pairs formed are proportional to the intensity of the incident light the number of incident photons. Thus, the electric current I, is proportional to the intensity of incident light. Hence, the total reverse current in the circuit is given by,
I = lo + Is
Total reverse current in a photo-diode increases with the increase in the intensity of the incident light
The volt-ampere characteristics of a photo-diode are shown in figure 27.
What are the uses of photo diodes?
1. Photo diodes are used as photodetectors to detect intensity of radiation or light.
(ii) They are used as light operated switches.
(iii) They are used in optical communication equipments. (iv) They are used in fast reading of soundtracks and tapes.
(y) They are used in logic circuits.
(vi) They are used as optical modulators.
Light Emitting Diode (LED)
What is light emitting diode (LED) ?
A special heavily doped p-n junction diode which emits spontaneous radiation when forward biased is known as light emitting diode (LED).
A light emitting diode converts electrical energy into light energy. The symbolic representation of LED is shown in the figure 28. Different types of LEDs emit different types LED made of Ga As of E, = 1:4 eV emits infrared radiations.LED made of Ga As 0-6 P0-4 of E, = 1-9 eV emits red light. LEDs made of compound semiconductors like GaAs and GaAsP emit energy in different forms.
Explain its working and draw V-I characteristics of LED
Action: When a p-n junction diode is forward biased (Figure 29), the electrons injected to p-side of the junction diode fall from the conduction band to the valence band and recombine with the holes in the valence band. This is equivalent to the jumping of electrons from higher energy state (i.e. conduction band) to lower energy state (i.e., valence band). Hence, energy is released in the process of recombination of electron-hole in the form of visible light. The process of giving light by applying an electrical source of energy is known as electroluminescence. The energy of the photon of visible light is given by hv = E, where E, is the energy gap between conduction band and valence band and v is the frequency of emitted visible radiation.
V-I characteristics of LED
V-I characteristics of LED are the same as that of elemental semiconductor say silicon diode. The Knee voltage of LEDs is the higher than that of Si diode. LEDs get damaged at low reverse voltages say 5 V.
Advantages of LED
(1) Light emitting diodes are easy to manufacture.
(i) LEDs are low cost diodes.
(ii) LED works at lower voltage as compared to the incandescent bulb.
(iv) They consume very less power as compared to the incandescent bulb.
(v) LED has longer life than that of incandescent bulb.
(vi) They can be switched on and off very fast so that they can be used as blinkers.
(vii) No warm-up time is required for them
(viii) They can emit monochromatic light as well as white light.
Uses of LEDs (Light-emitting diodes)
LEDs are widely used in CD players, laser printers, and remote-controlled devices. These days LEDs are commonly used in optical communication, (0) They are used as indicator lamps.
(i) They are used in digital displays in watches and calculators
(iii) Light-emitting diodes that emit infra-red light are used in a burglar alarm.
(iv) They are used in remote control schemes.
(v) They are used as blinkers
(vi) They are used as decorating lights.
(vii) They are used as night lamps, flashlamps, and torch lamps.
Solar Cell or Photo-Voltaic Device
What is a solar cell?
A special p-n junction diode that converts solar energy (sunlight) into electrical energy is known as a solar cell or photo-voltaic device.
Explain the construction and working of Solar Cell Or Photo-Voltaic Device
Construction A simple solar cell consists of a p-n junction diode of which-region is very thin and p-region is thick. If the n-region is about 0 3 x 10-6 m, the p-region will be 300 x 10-6 m thick. The junction surface of these diodes is kept largo so that a large amount of radiation may fall on it. Nickel-plated contacts are connected through a load resistance as shown in the figure.
Construction. The material used to manufacture a solar cell should have (1) low cost (1) easy availability. (ii) desirable conductivity. (iv) optical absorption as high as 10 per emf and (v) forbidden energy gap of 1 to 1-8 eV Action.
(i) When solar energy or light energy falls on the solar cells, electron-hole pairs are generated in both the n-region and p-region of the junction diode.
(ii) The electrons from the p-region diffuse through the junction to the n-region and holes from M-region diffuse through the junction to the p-region due to the electric field across the depletion layer: Thus, holes and electrons get separated out. For better hole electron separation under the effect of junction field, it is required that photoemission takes place in the junction area only.
() If the p-n junction diode is open-circuited, then holes and electrons will collect on both sides of the junction. This gives rise to an open-circuit voltage Vo. When an external resistance (RD) is connected across the junction diode, electric current flows through the circuit. The electric current in the circuit increases with the increase in the intensity of sunlight as shown in the figure.
Draw V-I characteristics of the solar cell
V-I Characteristics: A typical V-I characteristic of a solar cell is shown in the figure. Here, Vo is an open-circuit voltage of the solar cell and I, is the maximum current i.e., short circuit current that can be drawn from the cell. The curve is available in the IV quadrant because the solar cell does not draw the current, but it supplies the current to the load resistance.
Uses of solar cell
(1) Solar cells are used in streetlights
(2) They are used in solar heaters
(3) They are used in the power supply of satellites and space vehicles.
(4) They are used in calculators.
What is an optoelectronics’ junction device?
The Opto electronic junction device is the junction diode which conducts when light is incident on it. Charge carrier in the opto electronic junction device are generated by the photons, Junction diodes conduct when an electric field is applied across the junction of the diode. However, some special junction diodes can also conduct when charge carriers are generated by exposing the junction diode to the light of audible frequency Various op to electronic junction devices are discussed ahead.