Photonics Devices PDF

4.7 PHOTONIC DEVICES control and de. de- Photonics is that branch of Science which deals with the production, Phot...

4.7 PHOTONIC DEVICES control and de. de- Photonics is that branch of Science which deals with the production, Photonics electronics. tection of photons. It is a technology that combines optics and mechanics. Photonic quantum make usce of the idecas of optics, electromagnetism and data processing,transporta- technology has application in the fields of fcommunication, Photonics, photons have tion and traffic, medicine and biotechnologies,.lightingetc. In have a number more or less the same role as elcctrons in electronics. Photonic devices light. Sothe of advantages over electronic devices because of the very high speed of intormation transmitted photonically can travel verv long distances within a very siot time. For the transmission of light over long distances through optical fibres, several devices are required for generating, switching and amplifying the light. Solid State Lighting (SSL) is a type of lighting that uses mainly light emitung diodes (LEDs). This type of lighting has higher efficiency, reliability and environmen tal friendly technology compared to the conventional incandescent lighting. This chapter deals with the working of devices like LED, Photodiodes, Solar cells, working and applications of optic fibres etc. 4.8 PHOTO DETECTOR A photo detector is a device used to convert light signals that hit the junction into a voltage or current. Photodetectors are required at the receiving end of an optical com munication link. The essential requirements for a detector are high sensitivity, high reliability, short response time, low bias voltage and high electrical response. Exam ples of photodetectors are photodiodes and phototransistors. Other optical devices similar to photo detectors are solar cells which also absorb light and convert it into clectrical energy. Asimilar but different optical device is the LED which is basically  a onductor Devices 4.21 inverse of the converts a photodiode, voltage or instead of converting light into a current into light. voltage or current, it 4.8.1 Photo Diode A photodiode is a type of light detector, used to con- vertthelight into current or voltage based of the device. The onthe mode operation of response time of these Anode Cathode diodes decreases as their surface area todiodes are increases. Pho- similar to regular semiconductor diodes, butthey may be transparent enough to let light reach thedelicate part of the device. Symbol of aphoto diode Fig. 4.14 is shown in figure 4.14. Types of Photodiode There are different types of photodiodes available and they all work on the same basic nrinciple. The working of different types of photodiodes is slightly different, but the hasicoperation of these diodes remains the same. Photodiodes can be classified based on its construction and functions as follows. 1. PN Photodiode 2. Schottky Photo Diode 3. PIN Photodiode 4. Avalanche Photodiode 4.8.2 Junction Photodiode which is embedded in clear plastic mediun. When IIis a reverse biased pn junction, light, the current varies linearly with flux is exposed to Uas Teverse biased pn junction small of the order of 1/10 th of an inch. is very O ght. The complete unit Construction Junction photodiode semiconductor diode, which is formed by ap type eis a pn junction semiconductor material such as Phosphorous. material such as Boron and ntype 4.22 Phvsics for Ençincers Active region Anode + s,0, s,0, AR Coating pregion Depletion region nregion Cathode + Contact material Fig. 4.15 Junction photodiode the heavily doped., It is formed by the diffusion of lightly doped p region into calle region as shown in the figure 4.15. The space betweenp region and n region is which t depletion region. Here some portion of the front area is called active area coated with antireflection coating and the remaining portion is non active area which is coated with thick layer of SiO,. By adjusting the thickness of the non active area wecan control the response and speed of a photodiode to convert the light intocurrent Working Junction photodiode is connected in a circuit in reverse biased condition as shown in the figure 4.16. If the reverse biased voltage is very low, a constant current fiows through the diode. This current is called reverse saturation current, which flows due to thermallygenerated minority car riers namely electrons in the p type and holes in the n type that are at Fig. 4.16 Junction photodiode ina circuit tracted towards the junction. The  Semiconductor Devices 4.23 motion of minority carriers form a current currentis also known as dark current. It knownonas leakage current. The leakage depends the reverse biased voltage, arrnbi- ent temperature and serics resistance in the to cross the junction. circuit. It does not allow najority carriers When aphoton is absorbecd from the incident by the por nregion, an electron isreleased from the valence band and it goes tolight the conduction band. This creates a holeinthe valence band. Thus the incident light ctron hole pairs. These electron hole pairs causes the creation of alarge number of clectron pairs are called photo carriers and produce ourent known as photocurTent in the external circuit in addition to dark current. The dark currentshould be minimised increase the sensitivity ofofthe to Jonletion region should be the dedevice. Width of the increased absorb a large quantity of light. The resulting to output current or voltage can be measured. Applications of Photodiode 1. Photo diodes are used in scintillators, charge-coupled devices, photoconductors, and photomultiplier tubes. 2. These diodes are used in consumer electronic devices like smoke detectors, com pact disc players, and televisions and remote controls. 3. Photodiodes are frequently used for exact measurement of the intensity of light. 4, Photodiodes are also widely used in medical field. For example it is used in instruments to analyze samples, detectors for computed tomnography and also used in blood gas monitors. 5. These diodes are much faster & more complex than normal pn junction diodes and hence are frequently used for lighting regulation and in optical communica tions. 4.8.3 PIN Photodiode lightly doped and wide intrinsic PIN Photodiode is a type of photodiode in which a very This is to improve semiconductor is introduced between heavily doped p and n regions. sensitivity of a photodiode. Figure 4.17 shows the schematic diagram of aPIN he larger (10-200 microns) than the photodiode. The width of the intrinsic region is much (depletion region) of a normal pn junction. When reverse biased pace charge region  voltage is applied across the terminal of a PIN photodiode the space charge region extends throughout the intrinsic region. AMetal contact Metal contact Fig. 4.17 p-i-n photodiode When light is incident on the diode, electrons are excited from the valence band to conduction band. This produces a large number of electron hole pairs. Because of its large width, the intrinsic layer absorbs a very large number of incoming photons compared to the p and n regions. This increases the photo current, improves the effi ciency, speed and sensitivity compared to a pn junctionphotodiode. The output canbe measured and analyzed. Applications of PIN Diode 1. These diodes are used in the RF and microwave attenuators since they have lowcapacitance. switches andmicrowave variable 2. PIN photodiodes are used for fibre optic network cards and also 3. The PIN photodiode is also used to detect X-rays and gamma switches. gamma rays photons