Question :

a) State six desirable requirements of a generic photo-detector.

b) A gallium arsenide pin photo-diode has an intrinsic layer of width 1 μm. The diode is reverse-biased by a voltage of 3 V and then illuminated with short optical pulse of wavelength 800 nm.

i) Deduce the penetration depth to know if absorption will occur across the whole width.
ii) Calculate the electric field across the intrinsic layer width.
iii) Estimate the electron drift velocity.
iv) Estimate the duration of the photo-current across the whole width of the diode.

c) A photo-diode has an active circular area with radius 2 mm. When radiation of wavelength 800 nm and intensity 0.25 mW/cm² is incident it generates a photo-current of 0.3 μA.

What is the responsivity and quantum efficiency of the photo-diode at 800 nm?
d) A 0.5 cm² photo-detector is shone with 850 nm light of intensity 75 mW/cm². Assume that each photon generates one EHP, calculate the number of EH pairs created per second.

e) What is the thickness of a Germanium crystal layer that is needed for absorbing 85% of the incident radiation at 1 µm?