hello
I know the differences between ohmic and schottky contacts but I want to know how to do a shottky contact in my device?? what is it related to??I got confused when I saw the articles . I want to make a device in both contacts and compare them with each other
Hello,
if you apply a fresh metal layer on your semiconductor you'll have a Shottky, i.e. diode-like, contact. It is the way they made the first semiconductor diodes, the Germanium rectifier (a Ge-tip pressed on an metal, the tip enhances the electrical field and makes the device more efficient). The metal layer, mostly gold is used, can then be annealed at increased temeratures to transform in a more ohmic contact. This happens also if you operate a device with such a contact, as it happens during the burn-in phase in semiconductor lasers. If you like to have a nice ohmic contact right from the start you can use, e.g. gold germanium mixture to coat your semiconductor, the germanium will form crytal 'spikes' penetrating the metal semiconductor junction and lower the resitance. Be aware that this affects the semiconductor up to several microns in depth, that is why AuGe is only used on the substrate side of semiconductor laser diodes, otherwise it would affact the epitaxial structure.
You could have a look on the metal-semiconductor junction chapter in a solid state text book. However, annealing at sufficient temperatures will give you a more ohmic contact.
A schottky contact is typically formed using a metal with workfunction higher than the semiconductor conduction band. It depends on which semiconductor you are using. For n-type Si, you can use Ti for ohmic contact and Ni for schottky contact.
Hello Geetak
thanks alot for your anwser. Excuse me, I dont know the meaning of this part" the semiconductor conduction band'.? do you mean Eg?!!!
A contact metal-semiconductor depends on the work function of the metal, denoted here as Fm. The needed barrier-surmounting energy Fb is the sum of the built-in potential and the offset between the Fermi level and the conduction band of the semiconductor. Equivalently for n-type semiconductors Fb=Fm-EAs, where EAs is the electron affinity of the semiconductor. For p-type semiconductor Fb=Eg+EAs-Fm, where Eg is the semiconductor band gap. For ohmic contact you want to minimize Fb. So naively, metals whose work functions are close to the semiconductor's electron affinity should most easily form ohmic contacts to n-type semiconductors. Metals with high work functions (Fm=Eg+EAs) form the best contacts to p-type semiconductors. Unfortunately experiments have shown that the predictive power of theory doesn't extend much beyond this statement. Under realistic conditions, contact metals may react with semiconductor surfaces to form a compound with new electronic properties.
Data about FM, EAs and Eg can easy be found in the literature/web.
This is a hard question. The theory does not adequately describe experimental results. Basically, for a p-type semiconductor, use a high work function metal. For n-type, use a low work function metal. This rule of thumb is adequate, but is not absolute.
Note that Ohmic contacts are often obtained by heavily doping the surface or by creating a tunneling interface (like SiO2). More often than not, Ohmic contacts cannot be made by simply evaporating metal on it. Annealing can cause diffusion and alloying, which helps the contact behave more "Ohmically," but usually it is not complete, and the behavior is somewhere between the two.
Note for detectors you need at least 1 Ohmic contact. The other can be of either type and depends on the design. I suggest the text by Lowe and Sareen, Semiconductor X-ray Detectors. It is an excellent reference with respect to silicon. For compounds SC detectors, check out the text by Owens, Compound Semiconductor Radiation Detectors. For general information on electronic properties of contacts, see the text by SZe and NG, Physics of Semiconductor Devices.
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