Accumulation Mode Capacitor AMOS

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hrkhari

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accumulation varactors

Dear Guys:

In implementing an AMOS where the D-S diffusions (p+doped) is removed in and implementing the bulk contacts (n+) in the place left by D-S in a n-well and p substrate. However this device is not supported by silicon foundries. The question is, if an equivalent nMOS with D-S diffusions (n+ doped) in p substrate, with grounded bulk is used to replace the MOSCAP as described above, I would get the same tuning characteristic plot as described for a typical AMOS device, provided that the tuning node is at the D-S tied together . Can anyone confirm this outcome. Thanks in advance

Rgds
 

A-Mode Varactor is a NMOS in N-Well, Normal NMOS is a NMOS in P-Sub. The difference is for A-Mode varactor, the channel is already N- dopped. As we know Normal NMOS varactor, sweep Vgd from neg to pos value, it goes through A -> D -> I states. For A-Mode, it is D -> A (Since the channel is already n type), but the physical process almost same.
 

Hi:

Thnx, that explains the physical part of AMOS, but how do one model this AMOS device for spice/spectre simulation purpose. Thanks in advance

Rgds
 

In your schematic you can replace this MOS capacitor by PMOS transistor with very small S/D area and perimeter.
I simulated such kind of transistor and compared with measured CV plot for 0.5u TSMC process (I found that in one of paper). Very close to each other.
But if left S/D floating sometimes I got some problem with convergence.
 

    hrkhari

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