Hi.. I'm working on AlN-GaN HEMT technology and I use the TCAD sentaurus for device simulations. I want to define the source and drain contacts as ohmic and in my structure, there is no doping around source and drain regions. Given this situation, how can I define the ohmic contacts for source and drain ? How the TCAD understands that these contacts are ohmic ?
Also How can I calculate the Schottky barrier height for gate? I use the formula,
barrier height = metal work function - (electron affinity of Semiconductor)
Electron affinity for AlN= 0.25.. ?
Please do answer. Thanks in advance.
Dear Nandhakumar,
Your question in interesting. I am going to answer it from the conceptual point of view.
The whole answer lies in the behavior of the metal semiconductor contacts.
Concerning the MS contacts of the source and drain, you can make them behaving as ohmic and at the same time their resistance is low by the following methods:
By heavy doping the source and drain region such that the MS contact becomes dominated by tunneling conduction. This solution you excluded.
- In the second method,You can choose the metal for the drain such that its work function is greater than electron affinity of the semiconductor to extract the electron from the semiconductors assuming n type and at the source make the work function of the metal greater than the electron affinity of the semiconductor to ease the injection of the electrons from the metal to the semiconductor. OR generally expressed choose the metal such that potential barrier of the MS contact is sufficiently small such that with very small voltage drop across them you can pass the highest possible current of the channel.
Th barrier height to the electron flow from the conduction band to the metal is phB= phm- X, where phim is the metal work function and X is the electron affinity
This is assuming ideal interface. In case of the pretense of the traps, the barrier height will be changed.
For more information return to the book by S M Sze, physics and technology of semiconductor devices.
The third method to get an ohmic contact is to assume very high surface recombination velocity or an excess of mobile charge carriers equal to zero , delta n =delta p =zero.
Just to add some more details to the previous answer by Abdelhalim Zekry, if the barrier height at a M-S interface is less than 0.3 eV then that M-S contact can be treated as an Ohmic one. So you can choose a metal such that the barrier height for that metal contact with AlN-GaN comes out to be less than 0.3 eV. In Practical cases it has been observed that indium metal or some alloy of Au and Ti yield good Ohmic contacts for the AlN-GaN semiconductor.
Many thanks for your detailed explanation Prof. Abdelhalim.
Thanks Shammi verma..
Many thanks for your reply Prof. Shankar..
In my structure, I want to make ohmic contacts for the source and drain regions, which is on GaN. However, there is no doping on either AlN or GaN.. Also there is no doping around source and drain regions. How can I define these contacts as ohmic in TCAD sentaurus to do physical simulations ?
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