Both Indium Gallium Arsenide (InGaAs) and Germanium (Ge) photodiode detectors are commonly used to measure optical power in the near IR (NIR) range. Newport offers various versions of NIST traceable InGaAs and Ge detectors (see Low-Power Calibrated Photodiode Sensors 918D series and Low-Power Calibrated Photodiode Sensors 818 Series). The specifications of these detectors are very similar, except for only a few key areas: shunt resistance and die capacitance, for example. However, these differences make a huge performance difference and hence the price difference.
Shunt resistance is desired to be infinite, however, every photodiode has a finite value, leading to Johnson noise. The resultant noise, which has to do with the random thermal fluctuations in the charge carriers, is inversely related to the shunt resistance. Therefore, in order to minimize the noise, the largest possible resistance value is desired and designed into a measurement system. There comes the difference between Germanium and Indium Gallium Arsenide. The shunt resistance of an InGaAs detector is on the order of 10 MΩ, while that of a Ge detector is on the order of kΩ, several orders of magnitude smaller.
Thus, a Ge detector will exhibit a much higher level of thermally induced noise than an InGaAs detector. One thing to note is that at Newport we use the special high shunt resistance Ge photodiodes from suppliers, which is up to twice bigger resistance value than a standard Ge diode.
The Larger the Active Area, the Better?
Some competitors boast that their germanium detector active area is large. The users should be warned, though, that the larger active area will lead to a larger dark current as well as a lower shunt resistance. The smaller shunt resistance results in higher thermal noise. And the larger dark current results in higher shot noise. The dark current of Ge is already much higher than that of InGaAs, and the large active area can potentially introduce high noise current. Newports Ge detector active area is 3 mm. In comparison with a 5 mm diode, if all else is identical, a 3 mm diode can have up to 3 times larger shunt resistance value and 3 times smaller dark current.
Specifications Comparison between 918D Series InGaAs and Ge Detector
|Spectral Range (µm)
|Calibration Uncertainty (Without Attenuator)(5)
||2% @ 780-1700nm, 4% @ 1701-1800nm
||2% @ 800-1650nm
|Calibration Uncertainty (With Attenuator)(5)
||5% @ 780-910nm, 2% @ 911-1700nm, 4% @ 1701-1800nm
||5% @ 800-900nm, 2% @ 901-1650nm
|Rise Time (µs)
|Shunt Resistance (MΩ) (typ)
|Die Capacitance (pF)
||14 ( nF)
|Reverse Bias, Maximum (V)
||0.7 x 10-12
||3.0 x 10-14
||Indium Gallium Arsenide
|Active Area (cm2)
|Active Diameter (cm)
1) Applies to entire spectral response, max w/ attenuator is for OD3. For OD2 derate listed value by 10x, & for OD1, derate value by 100x.
2) 15 ns pulse width, max w/ attenuator is for OD3. For OD2 derate listed value by 10x, & for OD1, derate value by 100x.
3) Uniformity specification applies to detector only.
4) Selected at time of ordering.
5) Calibration uncertainty can be varied depending on the NIST transfer standard uncertainty variation.