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https://designers-guide.org/forum/YaBB.pl Modeling >> Semiconductor Devices >> Device fmax and ft https://designers-guide.org/forum/YaBB.pl?num=1130286100 Message started by crhu on Oct 25th, 2005, 5:21pm |
Title: Device fmax and ft Post by crhu on Oct 25th, 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 14th, 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). |
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