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Modeling >> Semiconductor Devices >> Device fmax and ft
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Message started by crhu on Oct 25^th, 2005, 5:21pm

Title: Device fmax and ft
Post by crhu on Oct 25^th, 2005, 5:21pm

Hi, forum,
I am doing some device characterization, and I am really puzzled by the results I got for fmax and ft

fmax ~ the freq. @ which unity unilaterizated max power gain occurs

ft ~ the freq. @ which unity current gain occurs

Based on the equation:
fmax = 1/2*sqrt(rds/Rg)*ft,
for MOSFET,
rds = 1/gds (~10k - 100k),
Rg = Rpoly + Rchannel=1/3*W/L*Rsh + 1/5*1/gm (for long device) (~ 10 - 1k),
fmax should be always larger than ft for MOSFET. However, the results I got always show that ft>fmax.

Thanks.

-ch

Title: Re: Device fmax and ft
Post by Geoffrey_Coram on Nov 14^th, 2005, 8:14am

I've always seen fmax >= ft. Are you sure you're measuring ft correctly? In some cases, the unity current gain point is subject to strange effects, and one should really use a linear extrapolation from the 3dB point (where the gain has dropped 3dB from the dc value).