I'm trying to make a level 2 simulation of a voltage buffer made by a CMOS circuit:

Buffer http://www.ele.uri.edu/images/Gif/buff_tran.gif

For this, I have been told to use level 2 CNM25 technology Spice models for the transistors, and I have been given the models:

.model NEWPMOS PMOS (LEVEL=2 TOX = 380E-10    VTO = -1.139  NSUB = 1.355E16 UO = 212.7 UCRIT = 1E4      UEXP = .1159  NFS = 662.4E9   WD = .4268E-6 DELTA = 1.824    RS = 134.9    LD = .8101E-6   XJ = 2.783E-9 VMAX = 120.3E3   NEFF = .06665 CJ = 3.816E-4   MJ = .3499 CJSW = 7.384E-10 MJSW = .3916  PB = .56)

.model NEWNMOS NMOS (LEVEL=2 TOX = 380E-10    VTO = .9418    NSUB = 2.639E16 UO = 648.2 UCRIT = 1E4      UEXP = .06857  NFS = 710.8E9   WD = .4086E-6 DELTA = 2.2      RS = 93.77     LD = .9132E-6   XJ = .0824E-6 VMAX = 59.62E3   NEFF = 1.479   CJ = 3.495E-4   MJ = .3950 CJSW = 5.952E-10 MJSW = .2941   PB = .65)

When I simulate the circuit, the output signal has rising peaks at the edges when the input signal goes from high to low and from low to high:


When I zoom in into the peaks, they are really weird and have a 400mV amplitude which I don't know where it comes from.

Is this suppose to happen or is there something wrong with the models? or, if it is correct, why does these peaks occur?


1 Answer 1


These look like charge injection from the overlap capacitance of gate to S/D. If you overlay the intermediate node output (the output from the first inverter) you will find that these edges are related to the the signal swing of this internal node.

As the Output is swinging from low to high the internal node must first swing from high to low and some of the edge will couple into the output signal. That 's why it looks odd, it appears to be happening before the the signal switches.

I grabbed this pictures from ITU website in which there is a discussion of capacitances of a MOSFET.

enter image description here

This picture is for a MOSFET when it is turned off, when on, there will be a Gate to channel cap and a channel to bulk cap.

  • Your simulation is running fine.

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