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In MOSFET, we use silicon in crystal direction [1 0 0] , while we use direction [1 1 1] in BJT. What is the reason? I expect the opposite, because we see more atoms on the surface in the direction [1 1 1] and so diffusion of atoms is very harder. Also, in BJT, we need a deeper diffusion.

UPDATE

crystal directions are specified by miller index.

mohammad rezza
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    You might need to post a picture that explains your co-ordinate system (unfathomable at the moment). – Andy aka Nov 20 '22 at 12:23
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    Probably referring to the [Miller index](https://en.wikipedia.org/wiki/Miller_index). – Nicolas Nov 20 '22 at 12:41
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    There's a lot of technology (aka know-how) goes in to what crystal direction to use for any given semiconductor application. A one-property thing like 'more atoms' sounds unlikely to capture the pros and cons of carrier mobility, any diffusion anisotropy, etc etc. The physics site might suit this question better. – Neil_UK Nov 20 '22 at 13:56
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    Physics site is a general forum. This question is specific to electronic engineering. – mohammad rezza Nov 20 '22 at 14:18
  • I guess the [001] is the standard choice due to the easier dicing. So one can check what other advantages 111 wafers have. A very quick search suggested that oxide growth was faster on 111. – tobalt Nov 20 '22 at 18:56
  • Relevant link: https://www.universitywafer.com/silicon-orientation.html – Spehro Pefhany Nov 20 '22 at 20:40

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