In order to understand the effect of the distance between to adjacent electrodes, assume that there is an incident photon flux phi with the suitable wavelength such that each incident photon will generate an electron hole pair. If this pair is left in the material without separation and collection it will recombine again in an average lifetime of Tau seconds. To prevent this recombination it is required to collect these charges by application of an electric field via an external voltage on the two faced electrodes. Assume that the electric field is E, then it will exert adrift velocity on the charge carriers vd. Then in order to collect these charges by the electric field it takes a drift time Td = L/ vd where L is the distance between the electrodes. 

If the drift time is smaller than the the minority carrier life time Tau, then all the generated e-h pairs will be collected and the maximum collection efficiency is achieved. Other wise the collection efficiency will decrease and the sensitivities of the 

photo-detector will be smaller.

It is also required to increase the speed of the response of the photo detector . This  is achieved also by decreasing the Drift time by reducing the distance L between the two electrodes.

So, to increase the sensitivities of the photo detector  and its speed it is required to reduce the distance between any faced adjacent electrodes.

As the area of the photo detector is such that its dimentions is many times the the required distance then the solution is to configure the electrodes in a digitated form.

For more information please consult the famous reference: Physics of semiconductor devices, by S.M.Sze.

wish you success

In order to understand the effect of the distance between to adjacent electrodes, assume that there is an incident photon flux phi with the suitable wavelength such that each incident photon will generate an electron hole pair. If this pair is left in the material without separation and collection it will recombine again in an average lifetime of Tau seconds. To prevent this recombination it is required to collect these charges by application of an electric field via an external voltage on the two faced electrodes. Assume that the electric field is E, then it will exert adrift velocity on the charge carriers vd. Then in order to collect these charges by the electric field it takes a drift time Td = L/ vd where L is the distance between the electrodes. 

If the drift time is smaller than the the minority carrier life time Tau, then all the generated e-h pairs will be collected and the maximum collection efficiency is achieved. Other wise the collection efficiency will decrease and the sensitivities of the 

photo-detector will be smaller.

It is also required to increase the speed of the response of the photo detector . This  is achieved also by decreasing the Drift time by reducing the distance L between the two electrodes.

So, to increase the sensitivities of the photo detector  and its speed it is required to reduce the distance between any faced adjacent electrodes.

As the area of the photo detector is such that its dimentions is many times the the required distance then the solution is to configure the electrodes in a digitated form.

For more information please consult the famous reference: Physics of semiconductor devices, by S.M.Sze.

wish you success

Thank you sir for your kind response but my question is slightly different that why in papers people have use different finger electrodes e.g in this paper thay have used two electrodes Ti/Au(10nm/100nm)  on the same detector.

Hello Kanika,

I think Abdelhalim has answered a more complicated question, so I have the easy job of answering what I believe you were trying to ask.

Assuming you understand that interdigitated electrodes (IDEs) are comprised of two separate entities, then I believe you are asking why two metals are used? i.e. Ti/Au or Cr/Au.

If that is the case then the answer is that before evaporating or sputtering the main electrode layer (Au, Pt, etc.) it is common to deposit an adhesion layer to promote adhesion to the substrate (glass, Si, etc.). Both Ti and Cr are common adhesion layers. For my electrochemical sensors I prefer to use Ti as Cr can diffuse into Au during a latter process with elevated temperature.

The different 'dimensions' (in this case thickness) are then down to the fact that the adhesion layer can be relatively thin compared to the main electrode material (I've used adhesion layer thicknesses from 4nm to 20nm). For my application (nanotrench inside microfluidic trench) I have tried to minimise the adhesion layer thickness to reduce any electrochemical and electrostatic contributions.

I hope this helps.

Jules

Yes Jules i was thinking in that way only ,thank you for clearing my doubt :)

Dear Kanika,

This is the first time that i take care of your comment on my answer on your question. Jules gave a satisfactory answer to your question.

However, my advice is that it is always good to precise the words to become easy to understand.

You ask why one uses multilayer metallization to build metal electrodes. There is requirement on the electrode construction which must :

Have good adhesion to the substrate or the sensor

High conductivity to have as small resistance as possible

Suitable work function to build ohmic contacts

bond ability or solder ability for external electrode accessing

in many cases one metal layer can not satisfy these functions. Then the solution is in multilayer electrodes.

Best wishes