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hi,
the gate of MOSFET use poly instead of METAL so that the MOS structure is self aligned. To explain what is self aligned structure assume that we use metal for making gate. For this we need to first do the source/drain diffusion and then the metal layer. Now on doing so the METAL gate may not be perfectly aligned to the source and drain diffusions and in worse case it can so happen that the metal gate may not be overlapping with either source/drain due to misalignment. This degrades the performance of MOS and I think it may not work as a MOS either. So inorder to workaround this problem what ppl used to do was to increase the gate length thereby ensuring that the gate will have overlap with source and drain diffusions. Now this method had an issue that the gate/source overlap capacitance was huge here affecting the design. Also very small channel lengths was not possible here.
If we use poly then what we do is we will first grow poly gates on the silicon oxide and on top of that we will do diffusion . This ensures that the poly is perfectly alligned with the source and drain diffusion and also shoter channel lengths with lesser gate/source overlap capacitance is made possible. For a more detailed explanation refer Razavi ...
if u use metal as gate,u can't do selfaligned s,d implant,coz if u doing so,the s,d drivein will melt the metal gate(al gate),so u have to do the s,d implant and drivein(anneal) first then gate deposition.
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