Why do silicon photodiodes respond to a wavelength range of 190-1100nm? Silicon photodiodes respond to a wavelength range of around 190-1100nm (source: wikipedia). I understand that photodiodes function by having a photon of sufficient energy create an electron-hole pair in a semiconductor via the photoelectric effect, which in turn increases the number of charge carriers, which in turn increases the photocurrent.
I know that the energy of a photon is proportional to its frequency as per the Planck-Einstein relation: $E=hf$. The minimum energy required to stimulate the creation of an electron-hole pair in the semiconductor material should therefore be a product of its bandgap. The bandgap for silicon at 302 Kelvin is approximately 1.14eV. The associated wavelength of the minimum energy should therefore be:
$$f = \frac E h = \frac {1.14eV} {6.62607015 \times 10^{−34}} = 2.75650774 \times 10^{14} Hz$$
$$\lambda = \frac c f = \frac {3 \times 10^8} {2.75650774 \times 10^{14}} = 1088 nm$$
This tallies with the 1100nm figure quoted on the wikipedia page.
The maximum energy is where I get a little confused. My intuition says that the ionisation energy would be an upper limit; I found 8.15168eV quoted for silicon. Repeating the equation finds the associated wavelength:
$$f = \frac E h = \frac {8.15168eV} {6.62607015 \times 10^{−34}} = 1.97106745 \times 10^{15} Hz$$
$$\lambda = \frac c f = \frac {3 \times 10^8} {1.97106745 \times 10^{15}} = 152nm$$
This is somewhat smaller than the 190nm figure claimed above.
My initial guess was that the maximum energy level is lowered as to avoid exceeding the free exciton binding energy, but the figure I found listed for that is 14.7eV (84nm) so that doesn't line up.
Where does the 190nm lower wavelength bound (upper energy bound) figure come from? Am I also right in thinking that not all wavelengths in the 190-1100nm range would elicit photocurrents, due to the requirement that photon energies match the quantum energy state transition levels?
 A: It's somebody's practical limit. There's usually some sort of thin "dead layer" on the diode surface. It's close to perfectly transparent at long wavelengths, but as you move into the UV, it becomes opaque. At x-ray wavelengths it becomes transparent again.
With careful, fussy surface treatment it is possible to reduce the dead layer for better UV and soft x-ray response.
A: I do believe the doping has an effect on the wavelength response considerably given the band gap can be widely tuned with doping, and cmos detectors are doped. However, I have yet to find a concrete upper limit of a silicon cmos spectral response. Through my own testing I have unequivocally detected light at 1153nm no problem with a Sony A7s cmos, which is above the theoretical wavelength one would derive from pure Si’s bandgap alone, it seems that based off my own spectroscopy measurements, it is sensitive up past 1180nm as well, but I’m hesitant to give that figure until further testing is done. Signal gets buried into noise there doesn’t seem to be a clear upper wavelength limit, longer integration times yield yet deeper IR wavelengths thus far so I’ve yet to determine the limit, though I’m sure one exists.
