InGaAs Amplified Detector
Chapter 4: Operation
Rev F, July 15, 2017
Page 5
4.3. Dark Current
Dark current is leakage current which flows when a bias voltage is applied to a
photodiode. The PDA with Transimpedance Amplifier does control the dark
current flowing out. Looking at the figure above, it can be noted that Point B is
held at ground and the amplifier will try to hold point A to “Virtual Ground”. This
minimizes the effects of dark current present in the system.
The dark current present is also affected by the photodiode material and the size
of the active area. Silicon devices generally produce low dark current compared
to germanium devices which have high dark currents. The table below lists
several photodiode materials and their relative dark currents, speeds, sensitivity,
and costs.
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
4.4. Bandwidth and Response
A load resistor will react with the photodetector junction capacitance to limit the
bandwidth. For best frequency response, a 50
Ω
terminator should be used in
conjunction with a 50
Ω
coaxial cable. The gain of the detector is dependent on
the feedback element (R
F
). The bandwidth of the detector can be calculated
using the following:
3
4
Where GBP is the amplifier gain bandwidth product and C
D
is the sum of the
photodiode junction capacitance and the amplifier capacitance.
1
Approximate values, actual wavelength values will vary from unit to unit
