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Design >> Analog Design >> how channel leakage generated?
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Message started by lhlbluesky_lhl on Nov 30^th, 2012, 5:53am

Title: how channel leakage generated?
Post by lhlbluesky_lhl on Nov 30^th, 2012, 5:53am

in the following picture, the schematic is a part of an ir receiver, here is a twin-t notch filter and a non-inverting amplifier.
in the notch filter, R is programmable (s[2:0]), and switch here is nmos transistor (having body effect), when simulation(assuming s[2:0]=100), in some corner, the leakage current of s[2] is 25nA also, causing a voltage drop of 10mv between VI and non-inverting input of opamp of notch filter; while in other corner, the leakage current of s[2] is only 20pA (what i expected).
i checked the operating point, the leakage current of s[2] is not substrate leakage, it is channel leakage between source and drain. and if i disconnect the output of notch filter with input of the non-inverting amplifier (VR is reference voltage from bandgap), the above leakage current of s[2] can be 20pA~50pA for all corners, as i expected. this is very strange. i wonder why.
how is the channel leakage current of s[2] generated? or is it gate leakage of input pair of notch filter opamp?
please give me some suggestions, thanks all.

Title: Re: how channel leakage generated?
Post by loose-electron on Nov 30^th, 2012, 1:36pm

A MOS device is not an ideal switch. Never has been.

Beyond that simple statement you need to have more information on the particular CMOS
process you are on and the leakage characteristics (not just gate leakage!) of the particular
transistors being used.

Get information from the foundry.

Title: Re: how channel leakage generated?
Post by lhlbluesky_lhl on Dec 1^st, 2012, 8:24pm

i have checked the CMOS process what i used, the leakage is in pA order (smaller than 100pA for my above case), but the max channel leakage is above 20nA here, so i wonder why.
and i'm also confused why leakage current is related to the loadind of notch filter, the loading is connected with inverting-end of notch filter opamp, while channel leakage occurs in non-inverting-end of notch filter opamp.