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can you help me to do the problem ( some hints how to solve those problems based on experimental method course)

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    \$\begingroup\$ What have you tried so far and what results did you get? Why don't you think those are the correct results? \$\endgroup\$ Commented Dec 15, 2014 at 22:40
  • \$\begingroup\$ You have a current source proportional to light input. This feeds a capacitor and two resistors in parallel. If you don't know how to calculate (in general terms) the impulse and frequency response of an RC network, I'm afraid there's not much we can do for you. \$\endgroup\$ Commented Dec 16, 2014 at 19:55

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The RC limitation of the frequency (-3dB) depends by the junction capacitance and load resistance RC.

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The junction capacitance is analogous with permitivity of the semiconductor material, the diode junction area and the width of the depletion layer. Increasing the width of the depletion layer reduces the junction capacitance and thereby the RC time constant. A high speed PD normally operates under the condition that Ri>>RL (Ri is 1~100MΩ)

However if you want to see the real performance of a PD, you have to enetr in the atomic level of the semiconductor structure, to define the frequency limitation as a function of transit time of the carriers. That is indicate the PD response to a square optical pulse and especially the fall edge of the pulse. In simple case and if diffusion mechanism eliminated, the frequency response it is limited by the average transit time of electrons and holes at saturation drift velocity

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For example in a depletion width of 1μm with a fieldstrength of 2V across (max Vd), gives a saturated (max) carriers velocity of about 10^7/cms, and transient time of 10ps. This is about 5 times more that the RC limitation.

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