Review Questions

Theory of SiPM operation: review questions

1. What is a semiconductor? How is it differ from a conductor? from an insulator?
2. Sketch a diagram of a semiconductor diode and label the parts. What is meant by the deletion region? Where is the depletion region in a diode?
3. What is meant by the term bias voltage in a diode circuit? What does it mean to say that a diode is reverse biased?
4. What is diode breakdown and how does it occur? What normally happens when a diode is reverse-biased above the breakdown voltage?
5. Describe a way that a reverse-biased diode be connected in a circuit so that it can survive breakdown. What modification to the circuit is required to make the diode produce avalanche pulses?
6. What is a photodiode? How does it differ from an ordinary diode?
7. What is an avalanche photodiode? How does it differ form an ordinary photodiode?
8. What role does temperature play in the performance of an avalanche photodiode?

1. Semiconductor has some properties of a conductor as well as some properties of an insulator. It has a critical temperature T0 below which the semicondutor becomes an insulator and above it it acts as a conductor. Conductors are materials with many free electrons and/or holes that allow electric charge to freely move in response to an electric field. An insulator has its electrons firmly bound therefore does not allow electric charge to move in response to an electric field.
2. A depleted region is a region where there are no holes and no electrons in a semiconductor, therefore making it a insulator at that condition.
3. Bias voltage is the voltage it takes to get the diode to start conducting. Below a certain voltage, the diode will no work even in the forward direction. It is the voltage which allows the electrons and holes to overcome the depleted zone and meet with each other, creating current. Reverse Biased means that the positive end of the battery is connected with the n-type semiconductor and the negative end is connected with the p-type semiconductor. This sucks all the electrons from the n-type region and filled in all the holes in the p-type region, creating a large depleted zone.
4. Diode breakdown happens when a reverse biased diode is subjected to so much voltage that it conducts in the reverse direction. When a diode is biased past the bias voltage, it stays in a metaphysical state, so by itself, it would stay an insulator, but anything at all (such as a temperature fluctuation) would cause the diode to draw so much current that would cause excessive heat that will destroy a diode.
5. If a resister is placed in the circuit, it prevents the amperage to get too high and stops the diode from being fried. If the amps do get too high, then the resister would lower the voltage and the amps by creating resistance and turning the excess energy to heat. To make an avalanche-like pulse, you would also a capacitor that would be connected to the diode. It serves as a temporary battery that allows the voltage and amperage to get high and create a pulse, then the capacitor will become drained and the resister would lower the voltage to below breakdown and the system would reset itself.
6. A Photodiode is a diode that is especially sensitive to light. It is usually made with a clear glass or plastic case that allows light through to maximize the effect light has upon the diode
7. An avalanche photodiode is a diode designed in such a way that it releases a pulse of electrons per photon instead of just one. In the process mentioned beforehand, the avalanche photodiode uses the energy from the photon to throw the photodiode off its metastable state and create a pulse.
8. Temperatures are caused by vibrations of atoms and a higher temperature would cause more random thermal fluctuations. Some of these thermal fluctuations have enough energy to throw the photodiode off its metastable state also, causing misfiring.