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Electrical Response Time of Diodes
66 Photonic Devices
Laser pulse
duration = 140 psec
Photodiode diffusion
time = 180 psec
Ga 0.47 In 0.53 As/InP heterophotodiode, V B = –14 V
Figure 4.2. The measured time response for a GaInAs photodiode. The intrinsic re-
sponse time for the photodiode is about 200 psec, as measured by the fall time in this
oscillograph. The response time is determined mostly by the diffusion of minority carri-
ers, and by the resistance–capacitance product of the diode, which is discussed in the
next section.
4.5 The Resistance–Capacitance Response Time
The extrinsic response is determined by the time is takes for the pho-
todiode to charge up the first stage of the amplifier that it is driving.
This time is simply the product of the capacitance of the photodiode
times the input resistance of the amplifier:
= RC (4.4)
The n and p regions of a p-n junction diode form a capacitor. Capac-
itance is defined as:
A
C = 0 (4.5)
W
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