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| Effective mobility in MOSFET |
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| bonzer:
Hello everyone! Please help me to understand how to find the mobility that we use in current equation of a MOSFET when dealing in detail with microelectronics for example: I have an nMOS with the p type "well" (I'm not sure about the term) that has Boron atoms in a concentration NB and Gate, Drain terminals drugged by Phosphorus atoms. As the free carriers (electrons in this case) inside the channel move through Boron, then I have to look at the mobility as function of Boron concentration right? (so it's useless for me to know the type of dopant used for gate and drain terminals, I could care less about the phosphorus ?) . I mean, what I try to do is to understand how properly use this graph to find the mobility (look at the attachment) |
| bson:
I would assume that majority carrier (electrons out of the n-doped terminals) mobility in the channel (p-doped with boron in your example) is the mobility that matters. But there may well be a marginal minority carrier lack of mobility (holes being less mobile than electrons) that might slightly degrade overall small-signal mobility around the quiescent/operating point (assuming saturation mode). I'm just guessing though. |
| bonzer:
The problem is that if we assume I'm right and what is rappresented in the graph is the mean through which carriers flow, then it looks kinda strange because the mobility inside Boron (and therefore of electrons) is from the graph slower than that inside Phosphorus (of holes) which would not be true because electrons are usually faster than holes. ??? |
| rfeecs:
Your plot is not the mobility "inside Boron". It is a plot of mobility inside Silicon, doped with either Arsenic, Boron or Phosphorus. Here is a similar plot: from here: https://ecee.colorado.edu/~bart/book/book/chapter2/ch2_7.htm For an NMOS FET, use the electron mobility. |
| bonzer:
Thanks for the answer. Well I knew that we were talking about dopants I just wanted to focus on my real doubt. I anyway almost got it with your help. Now what I understand is that it's the mobility of electrons within the phosphorus-doped silicon, or holes within the boron-doped silicon. And that makes sense with the values. But in an nMOS I have electrons from phosphorus moving through p-type because the n-channel is built inside the p-type body (made for example of boron-doped silicon) and this confuses me. How to deal with that? How to find the mobility? Inside current formula they indicate for nMOS mu_n , and I suppose because they want electrons mobility through the channel, but what is the concentration? Is it of the dopant of the body? If that's the case then where do I find the electrons mobility inside Boron - it's an acceptor so it has to do with holes mobility and not electrons. |
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