Photodetectors: SAQ Solutions

Listed below are the solutions to the SAQ on Photodetectors:

1. Insulators > Semiconductors > Conductors
    The semiconductor band gap is smaller than that of insulators, but larger than that of conductors. This means that it is more conductive than insulators, since it requires less energy to bridge the band gap, but less conductive than conductors.
2. positive; negative
   See the video for p and n junction orientation in a circuit. In forward biasing, the p junction faces the positive side of the circuit. Vice versa for reverse biased (which is opposite what the letters in the bolded pneumonic would suggest).
3. p-side has electron holes; n-side has electrons
   This makes sense for a forward-biased circuit in that the electrons will move across the junction (from the n-side) to pair with the holes on the p-side as they are attracted to the positive side of the circuit (see above question). Reverse biasing will create the opposite effect, thus making a more sensitive photodetector.
4. open circuit; short circuit
   The short circuit condition in photoconductive mode makes it so that more excitation energy input from light creates more current flow (short circuit), while the photovoltaic mode creates a larger voltage difference in an open circuit condition with the same conditions.