Questions about Silicon Diode Detectors

[CL39]

I have some questions about how silicon diode works, specifically as a ionizing radiation measurement instrument. I will write what my understanding is so far from different textbooks/websites (please comment whether I have it correct) and also write question where I get confused. Thank you.


- For radiation measurement, can be used in non-biased and reverse-biased mode.

CONSTRUCTION:

- When p-type silicon and n-type silicon come into contact

The majority carrier (hole) of p-side diffuse to n-side, leaving negatively-charged acceptor ion.

The majority carrier (e-) of n-side diffuse to n-side, leaving positively-charged donor ion (the minority carrier).

This forms the space charge region or depletion layer, with built-in potential (positively charged on the n-side and negatively charged on the p-side).

This built-in potential prevents any further movement of majority carriers.

The space charge region is a zone with a net charge provided by the fixed ions (donors or acceptors) that have been left uncovered by majority carrier diffusion

There are two types of p-n junction diode detectors, p-type diode (made with p-type silicon substrate doped with small amount of n-type silicon) and n-type diode (made with n-type silicon substrate doped with small amount of p-type silicon)


- For radiation measurement, can be used in non-biased and reverse-biased mode.

FOR OPERATION IN NON-BIAS MODE

- Due to the built-in potential, electrons from the electron-hole pair (created in the depletion zone by ionizing radiation) moves to the n-side, where the connection to the electrometer allows measurement of charge

FOR OPERATION IN REVERSE-BIAS MODE
- Because the p-type material is now connected to the negative terminal of the power supply, the 'holes' in the P-type material are pulled away from the junction, causing the width of the depletion zone to increase. Similarly, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction. Therefore the depletion region widens, and does so increasingly with increasing reverse-bias voltage. This increases the voltage barrier causing a high resistance to the flow of charge carriers thus allowing minimal electric current to cross the p–n junction.

When ionizing radiation creates electron-hole pairs in the depletion zone, the electrons are swept to the n-side by the positive external potential on the n-side and the charges are collected by the electrometer.


QUESTIONS

- in reverse-bias mode, “there is no current, except some leakage”. But isn’t this also true for non-bias mode (once equilibrium is established, there’s no more migration, isn’t it?)

- Between p-type diode detector (with p-type silicon substrate doped with small amount of n-type silicon) and n-type diode detector (with n-type silicon substrate doped with small amount of p-type silicon), why is p-type detector better for collecting electrons at the n-side. Wouldn’t electrons (liberated by ionizing radiation migrate to the n-side in p-type or n-type just the same? I believe most radiation measurement diodes are p-type.

- I read that p-type diode suffers less damage over time than n-type, why is this true?

- why does operating in reverse-bias mode prolong the life of diode (as compared to non-biased mode)?

- what is the significance of the increased width of the depletion zone in reverse-bias mode? Does it give you better detection efficiency? Better signal to noise ratio?

 

[anorlunda]

in reverse-bias mode, “there is no current, except some leakage”. But isn’t this also true for non-bias mode (once equilibrium is established, there’s no more migration, isn’t it?)

Yes, that's also true.

It is difficult to reply without links to the sources of the assertions you are asking about.