Why does New Focus use Schottky diodes instead of PINs or MSMs?
For our high-speed detector line, we use Schottky diodes rather than PINs or MSMs. Schottky diodes have faster responses than PINs because they have lower parasitics such as unwanted capacitances and resistances. Compared to an MSM, a Schottky photodiode typically has twice the quantum efficiency and, therefore, twice the responsivity. We use two Schottky-photodiode configurations: back-illuminated and front-illuminated. In the back-illuminated photodetectors (Models 151X, 154X, 141X, 144X, 101X, and 102X) the input light comes in through the substrate (usually InP). The benefit of this structure is that almost all the light that passes through the substrate is absorbed in the n-type layer because the top gold contact acts as a mirror. Unfortunately, these photodiodes cannot be used at wavelengths shorter than 950 nm, because the InP substrate absorbs strongly at these wavelengths.
