Essay Example on Design avalanche photodiodes using III V semiconductors for optical fibre communications








Optical Fibre communication less than 20 pages about 2500 words ish 1 Description and Aims of the Project The purpose of this project is to design avalanche photodiodes using III V semiconductors for optical fiber communications Optical fiber communication is a means of transmitting information by converting electrical signals into light via a medium 1 Avalanche photodiodes APDs are the raw materials that can be found in optical detectors which are used in optical fiber communications APD is a reversed biased p n junction that is operated at voltages close to breakdown voltage 2 Breakdown voltage is the voltage reached when an avalanche breakdown has occurred Avalanche breakdown is affected by the ionization of electron hole pairs and occurs under reverse bias in PN diodes it is a continuous process where the electric field across the depletion region increases due to the reverse bias which in turn causes the collision of the charge impurities in the material to increase 2 Avalanche breakdown controls the maximum reverse bias voltage that can be applied to a PN diode 3 Avalanche multiplication and quantum mechanical tunneling of carriers are two mechanisms that can cause safe avalanche breakdown without destroying the diode In this project the avalanche multiplication mechanism will be applied

This project will look at modeling of some of these raw materials and will determine the best design which meets the design specifications and goes beyond the transmission speed rate of 10Gb sec Figure 1 System diagram of an optical fiber communication 4 Figure 1 shows how an electrical signal is converted into optics and vice versa When the electrical signal enters into the transmitter stage the transmitter drives the light wave The regenerator stage has the photonic detectors which helps to amplify signals LASERS are used as the optical source which is carrying the light wave passes through the medium into the receiver stage The optical detector demodulates the optical signal and then produces an electrical output 4 One of the major problems of using APD devices in optical communications is the prolonged device stressing that leads to an increase in band to band tunneling which makes the device unreliable 5 This is why tunneling current is taken into consideration when selecting APD devices In the following sections we will discuss the model to calculate the gain and tunneling current of InP and InAlAs both individually latticed matched to InGaAs following the project specifications in section

2 3 Background theory For long communications Indium Phosphide InP lattice matched to Indium Gallium Arsenide InGaAs are most widely used InP being the avalanche region and InGaAs being the absorption region InP is used as the avalanche region because it is a direct energy gap material with a wide band gap InAlAs is another III V APD material with a much wider band gap when compared to InP and it that has been theoretically proven to have improved breakdown characteristics while maintain similar lattice spacing Band to band tunnelling current decreases with wider band gap 6 InGaAs is used as the absorption layer in this project because it helps detects longer wavelength and absorption coefficient InGaAs has a much thicker layer for the absorption of incident light and for pure electron injection PN and PIN photodiodes are old technologies used in optical communications as recent development towards longer lasting and higher transmission rates which can also provide a significant improvement in receiver sensitivity has created a need for avalanche photodiodes 6 Currently there is little information that discusses AlGaAsSb reliability This is mostly due to the fact that studies of this new semiconductor device have only just begun We speculate however that the AlGaAsSb wwww 5 Results and Analysis 5 1 Modelling the gain and tunnelling current in InAlAs Figure 2 Comparison of Gain and tunnelling current with respect to the voltage in InAlAs at a width of 100nm Figure 3 Comparison of Gain and tunnelling current with respect to the voltage in InAlAs at a width of 300nm Figure 4 comparison of Gain and tunnelling current with respect to voltage in InAlAs at a width of 500nm 5 2 Modelling the gain and and tunnelling current in InP Figure 5 Comparison of Gain and tunnelling current with respect to the voltage in InP at a width of 100nm Figure 6 Comparison of Gain and tunnelling current with respect to the voltage in InP at a width of 300nm Figure 7 Comparison of Gain and tunnelling current with respect to the voltage in InP at a width of 500nm Since no lab experiments were carried out theoretical models for calculating the electron and hole ionisation coefficients will be used Discussions and Conclusions Avalanche noise measurements on a range of GaAs p i n and n i p diodes show reduced excess noise in the thinner devices even though the ionization ratio approaches unity in the associated high fields This effect follows from the increased importance of dead space which results in a narrower ionization probability distribution function for ionization path length and in turn a more deterministic multiplication process The results suggest that the use of thin avalanche regions is advantageous for low noise APD s although the onset of tunnelling in such thin devices will ultimately limit their performance 8

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