# Cgs,Cgd capacitor in Mosfet/ problem with measurement unit

I am a little confused with this problem.

I would like to ask if I have a Mosfet(in saturation)and I have find that Cgs=16fF and Coverlap = 0.5fF/um(this is given in the description of the exercise), how I can calculate the Cgstotal = Cgs + Coverlap and the Cgd = 0 + Coveralp?

The measurement units for the first equation shouldn't have been the same?Also in the second equation for Cgd, units is F/m but I think should be only Farad(F).

Can anyone help I have no idea how I can solve this problem.

• You already know Cgs, you do not know Coverlap but it is given in x fF/um so that means you need some length or width. When would Coverlap double ? If I doubled the L or W of the MOS ?? – Bimpelrekkie Jun 7 '17 at 8:02
• Τhank you Bimpelrekkie for your response.But I know the value of Cov=0.5fF/um.The problem is only with the measurement units and not with the numerical values as they are known – elecV1 Jun 7 '17 at 8:07
• Read my comment again and expecially: you need some length or width – Bimpelrekkie Jun 7 '17 at 9:58
• Sorry Bimpelrekkie but I can understand your thought.You wrote me that "you do not know Coverlap" but I wrote you in the description that Coverlap = 0.5fF/um. – elecV1 Jun 7 '17 at 11:12
• Coverlap cannot be 0.5 fF/um as that is Capacitance per length. So you need the width of the MOSFET to know Coverlap. If the MOSFET has a W/L of 10um/1um then that Coverlap is 10 um x 0.5 fF/um = 5 fF. If the assignment shows no length then it is useless to specify Coverlap in fF/um. – Bimpelrekkie Jun 7 '17 at 11:21