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Modeling >> Semiconductor Devices >> MOS varactor cap value v.s. frequency
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Message started by nxing on Apr 5^th, 2013, 8:59am

Title: MOS varactor cap value v.s. frequency
Post by nxing on Apr 5^th, 2013, 8:59am

Hi,

I have saw people some paper mentioned that the capacitance value of a varactor (NMOS in accumulation mode) can varies with frequency, can anybody give an explanation why?

Thanks,

Title: Re: MOS varactor cap value v.s. frequency
Post by raja.cedt on Apr 5^th, 2013, 9:07am

Hello,
read some basic text book on Mosfet theory, basically carriers need sometime to reach channel hence for higher frequency of operation charge carrier will not reach the channel and lesser capacitance.

Thanks,
Raj.

Title: Re: MOS varactor cap value v.s. frequency
Post by nxing on Apr 5^th, 2013, 9:38am

Hi Raj

Thanks for your reply. Actually, that was what I thought. However, please refer to the following paper and in Fig 3 (the plot of measured AMOS varactor at different frequency), I found it's non-monotonic. that puzzled me.

Title: Re: MOS varactor cap value v.s. frequency
Post by raja.cedt on Apr 5^th, 2013, 9:55am

hello,
Please don't attach any publications here. I saw fig 3 it is monotonic, see some devices depends on their on their fabrication type there are several up and down's in the CV curve like Accumulated mosfet which is monotonic where as normal mosfet will have non-monodic due to region change.

Thanks,
Raj.

Title: Re: MOS varactor cap value v.s. frequency
Post by nxing on Apr 5^th, 2013, 10:16am

Hi Raj,

Thanks for remind for the publication.
When you looked at the figure 3b. Cap value is monotonic on bias voltage. However, for the same bias value, cap value is not monotonic on frequency, for example, when vbias=1v, it has C(55GHz)>C(5GHz)>C(1MHz)>C(20GHz).
If considering charge travel time, I would think that the higher the frequency, the smaller cap it will have, is that correct?

Thanks,

Norman