Laser

Thank you Dr. Kapusta,Dr Goliney and Dr. Yao for kind reply.

According to Dr. Yao for photodetector the EQE can be greater than 100% as when bias voltage increases photocurrent also increased and corresponding responsivity and EQE increases. Though responsivity =( Iph-Id) / (P *A). When bias voltage increases the proportional dark current (Id) also increased and (Iph- Id) term is only current for illumination. Then how bias effect the current due to illumination to increase the responsivity and corresponding proportionally dependent EQE?Kindly explain this please.

If your device is a photoconductor (it has two ohmic contacts), you may get photoconductive gain if the minority carriers are either trapped or transit slowly across the device and the majority carriers have a faster transit time across the device giving the effect of a high photocurrent in the outside circuit per incident photon or a high effective external quantum efficiency. In order to maintain charge neutrality, when a majority carrier reaches the ohmic contact due to its drift in the applied field from the contacts, it exits the device and a new majority carriers is injected from the opposite contact to preserve charge neutrality. This can happen many times before the photogenerated minority carriers that are present in the device recombine by transiting to the ohmic contact (where they recombine), or recombining radiatively or non-radiatively (Auger, SRH, etc) in the device interior. If there are no traps, the photoconductive gain would roughly be the ratio of the majority carrier electron mobility to the minority carrier holes mobility in an N-type photoconductor at low applied fields since these control the drift velocity ratio.  Photoconductive gain can run up to a factor of 100X or more if minority carrier traps are involved.. In general, people like to talk about the quantum efficiency as a number less than 1 multiplied by a photoconductive gain, even though effectively it looks like a quantum efficiency higher than unity. There is also gain-bandwidth product, so high gain means slow speed. That is, if the minority carriers take a long time to recombine the impulse response will be long and the speed will be slow.

If your device is a non-avalanching photodiode (PN junction or a MSM detector with Schottky contacts), in general the majority carriers cannot be replenished in the device by injection through the blocking/rectifying contact and the quantum efficiency should be less than unity. In Si UV photodiodes, where the incident light energy is many times higher than the bandgap, it is not uncommon to observe more than one electron-hole pair generated per incident photon.

Current is mobility*carrier density* electric field. The carrier density is fixed here, so either the increased bias results in a change in mobility (perhaps change in density of filled/unfilled traps or device heating, but this is generally backward for a room temperature device-decreased mobility) or the electric field is increasing (of course), so naturally the current will go up. This is assuming that the carrier density is not depleted (rate of generation is not competing with the rate of collection). Once you reach the limit where generation matches collection, it should stay constant until you reach carrier injection at the contact or multiplication (impact ionization/avalanche).

Thank you Dr. Wojtczuk & Dr. Lukosi for kind and detailed reply.This will help me surely.

Previous respondents implied the issues, but did not go into detail on the effects of noise.  The reason for separating the quantum efficiency from the gain is that the initial detection (and the dark current) are always subject to Poisson statistics (or shot noise), and the gain mechanism will generate additional noise.   Photoconductors at high gain typically add about the same amount of noise again (ignoring fluctuation effects).  The additional noise in a photomultiplier tube depends essentially on the gain of the first multiplier stage - high gains are virtually noiseless.  The same applies to multiple carrier release in UV or Xray detectors.  The added noise in avalanche photodiodes depends on the multiplication ratio of the released carriers; if the ratio is large multiplication can again be virtually noiseless, but this is not the case for germanium, and not necessarily the case for fast silicon devices; avalanche effects in III-V detectors depend on the structure as well as the basic materials.

Can anyone explain why the QE of photodetector increases to beyond 100% when we increase the temperature.

The heat can lead to the change of mobility of charge carriers and then change the conductivity σ = n*µ*q, that is called bolometric effect. Also, a photovoltage can be created when there is the difference in temperature between two doping region, Seebeck effect. V = (S1-S2)*delta T