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Extended InGaAs Biased Detector
Material
Silicon (Si)
Germanium (Ge)
Gallium Phosphide (GaP)
Indium Gallium Arsenide
(InGaAs)
Extended Range: Indium
Gallium Arsenide (InGaAs)
4.5.
Junction Capacitance
Junction capacitance (C
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
): Time for electron-hole pairs to move from the depletion zone to the
df
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
): Diffusion time for electron-hole pairs generated outside of the
diff
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
): Rise time of the RC circuit formed by the junction capacitance of the
RC
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
response (t
) can be approximated using the junction capacitance and the load
r
resistance (R
LOAD
1
Approximate values, actual wavelength values will vary.
Page 6
) is an important property of a photodiode as this can
J
):
Dark
Speed
Current
Low
High
High
Low
Low
High
Low
High
High
High
Chapter 4: Operation
1
Sensitivity
(nm)
400 – 1000
900 – 1600
150 – 550
800 – 1800
1200 – 2600
) and the rise time
BW
TTN134284-D02
Cost
Low
Low
Med
Med
High
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