Photodiodes and phototransistors are useful in numerous applications. By converting visible, infrared, or ultraviolet light into electrical signals, photodetectors serve as a bridge between the optical realm and the electronic realm.
In many applications, the performance of a photodetector is not particularly important. A photodiode-based proximity sensor, for example, may be designed so that the light source is either very intense or completely obstructed. In such cases it is not difficult to achieve reliable operation.
Sometimes, though, you’re trying to push the system to the limit. Two examples that come to mind are a long-range optical communication system and an IR-photodiode-based device that attempts to detect thermal events across long distances. In situations such as these, the sensitivity of the detector will be an important factor in the design process.
What Is D-Star?
The capabilities of different photodetectors can be conveniently compared using a parameter referred to as D-star (or D*). We cannot, of course, capture every detail of a photodetector’s performance using one parameter, but D-star is especially useful when your application requires high sensitivity because it gives you a way to directly compare different detectors that are all more or less acceptable for a given application.
D-star tells you a detector’s sensitivity for a fixed active detector area (because not all detectors are the same size) and at a specific optical wavelength (because detectors react differently according to the nature of the incident radiation).
As you can see in this plot for an indium arsenide detector made by Teledyne Judson Technologies, the responsivity is greatly affected by the wavelength of the incident radiation. Temperature is also an important factor at the higher wavelengths.
The formal definition of D-star is the square root of the active area (A, in cm2) divided by the noise equivalent power (NEP):
From NEP to D-Star
NEP is the light intensity that is equivalent to a detector’s noise floor. In other words, the detector itself generates a certain amount of noise, and NEP tells you the quantity of light that would produce the same amount of signal. Thus, if you illuminate the detector with a quantity of light corresponding to the NEP, the SNR will be one. Another way of thinking about NEP is as follows: it is the smallest optical power that can be detected, because the signal does not emerge from the noise until the incident quantity of light has reached the NEP. This means that a lower NEP corresponds to higher sensitivity.
Remember that the amount of noise that you see (in general, not just from a photodetector) depends on “how fast you look.” In other words, the quantity of noise is influenced by the bandwidth of the system. NEP is defined relative to a specific noise bandwidth.
It’s important to understand NEP because D-star is actually just an extension of NEP; it uses the inverse of the NEP of a given detector and normalizes it to a 1 cm2 active area. If detector size is not a significant concern in your application, you could compare detectors using NEP: lower NEP means more sensitivity. If you want a metric that accounts for detector area, you need D-star, and note that since D-star uses the inverse of NEP, higher D-star means better sensitivity.
This plot conveys typical D-star values for an indium antimonide (InSb) sensor made by Hamamatsu. InSb does better when the detector is in a cold environment, and this is why the D-star value decreases as temperature increases.