Question

Question 4 (10 Marks) (i) You have a P-I-N photodetector that you cool to -20° C. You reverse bias it, and measure a current (with no illumination) of 0.5 nA. It is connected to a load resistor of 100 kQ, and input amplifier. The photodetector can be assumed to be ideal in that to an each incident photon is converted into an electron-hole pair that is then collected as photocurrent. The measurement bandwidth is 100 MHz, and the average received photon flux is 1012 photons per second. Calculate the signal-to-noise ratio at the amplifier input (5 marks) Briefly explain the process of impact ionization that occurs in an avalanche photodiode (APD) (ii) (2 marks) (iii)The ability of electrons and holes to impact ionize are characterized by ionization coefficients ae and a. The ionization ratio is defined as k = ah/ae. Explain why it is generally desirable to have k 0 or k z co in an avalanche photodiode (APD) Include in your answer a description of the physical process that can occur if we do not have k 0 or k z co. Provide at least two reasons why this process leads to APD performance characteristics that are detrimental for many applications of APDS (3 marks)

Answer 1

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