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Si Biased Detector Chapter 4: Operation
Page 6 TTN134189-D02
Material
Dark
Current
Speed
Sensitivity 1
(nm)
Cost
Silicon (Si) Low High 400 – 1000 Low
Germanium (Ge) High Low 900 – 1600 Low
Gallium Phosphide (GaP) Low High 150 – 550 Med
Indium Gallium Arsenide
(InGaAs)
Low High 800 – 1800 Med
Extended Range: Indium
Gallium Arsenide (InGaAs)
High High 1200 – 2600 High
4.5. Junction Capacitance
Junction capacitance (C
J
) is an important property of a photodiode as this can
have a profound impact on the bandwidth and the response of a photodiode. It
should be noted that larger diode areas encompass a greater junction volume
with increased charge capacity. In a reverse bias application, the depletion width
of the junction is increased, thus effectively reducing the junction capacitance
and increasing the response speed.
4.6. Bandwidth and Response
The response time of a photodiode is a function the speed at which these
charges move inside the semiconductor as well as the external circuitry
connected to the photodiode. The three main contributions to the response time
are drift time, diffusion, and rise time of the RC circuit.
Drift time (t
df
): Time for electron-hole pairs to move from the depletion zone to the
electrodes. The depletion zone has an intrinsic electric field that accelerates the
charges so that they are collected quickly. Hence, the drift time is generally in the
ps time regime.
Diffusion (t
diff
): Diffusion time for electron-hole pairs generated outside of the
depletion zone to be collected. Depending on the wavelength used, a larger
proportion of electron-hole pairs are generated in the N and P layers of the
semiconductor, which do not have an electric field to accelerate the charges.
Therefore, the charges are only collected after slowly diffusing through the
semiconductor.
RC (t
RC
): Rise time of the RC circuit formed by the junction capacitance of the
photodiode, its series resistance (generally negligible), and the load resistance.
This is generally the dominating factor on the bandwidth of the PDs and
detectors and is the reason why the bandwidth is specified for a particular load
resistance. For best frequency response, a 50
Ω terminator should be used in
conjunction with a 50 Ω coaxial cable. The bandwidth (f
BW
) and the rise time
response (t
r
) can be approximated using the junction capacitance and the load
resistance (R
LOAD
):
1
Approximate values, actual wavelength values will vary.