Setting up a MOS Characterization test bench in Cadence

5 September 2019 Link

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Overview

The MOS characterization test bench described here is the 1st thing I do when I am Getting Familiar with a Process. This setup allows me to get good idea about the device parameters as well as the matching. I use the data in my hand calculations to calculate the worst case headrooms, matching of mirrors and offsets of amplifiers.

Schematic

Create a schematic in Cadence that looks like the following:

Notice here the current and the voltage values are setup as variables.

ADE Setup

  • We will run a DC sweep of the drain current and save the operating point for the whole sweep.
  • Create a text file and add the following:
save MN0:oppoint
Note here that MN0 should be a BSIM model device already. Sometimes it may be a subcircuit in that case MN0 should be replaced by MN0.mnbsim, etc.
  • Now save this file somewhere and include it in the Setup->Simulation Files->Definition Files list so that it gets included in the netlist
  • Now setup the variables as follows:
wid = Any width you want to characterize
len = Any Length you want to characterize
vgd = Gate to drain voltage can be set to 0 or any value if you want to simulate near triode regions or on the other end hot carrier effects
vbs = can be set to the Bulk Source voltage desired
ids = di*(TMP+273)/300 - if the current is to be PTAT in biasing otherwise just set it equal to di
di = average current to characterize
IDL = current for lower extreme measurement = dil*(TMP+273)/300 
IDH = current for higher extreme measurement = dih*(TMP+273)/300 
dil = <val> here <val> is the lower tolerance of di. If not PTAT then set it simply to <val>
dih = <val> here <val> is the higher tolerance of di. If not PTAT then set it simply to <val>
TMP = temp (if PTAT operation) or 27 (If absolute values of dil and dih need to be used
  • Now setup the DC sweep to sweep di from IDL to IDH.
  • Now setup the measurements as:
VTHL = value(v("NM0:vth" ?result "dc") VAR("IDL"))
VGSL = value(VS("/GATE") VAR("IDL"))
VDSATL = value(v("NM0:vdsat" ?result "dc") VAR("IDL"))
GML = value(getData("NM0:gm" ?result "dc") VAR("IDL"))
VTHH = value(v("NM0:vth" ?result "dc") VAR("IDH"))
VGSH = value(VS("/GATE") VAR("IDH"))
VDSATH = value(v("NM0:vdsat" ?result "dc") VAR("IDH"))
GMH = value(getData("NM0:gm" ?result "dc") VAR("IDH"))
ROL = (1 / value(getData("NM0:gds" ?result "dc") VAR("IDL")))
ROH = (1 / value(getData("NM0:gds" ?result "dc") VAR("IDH")))
IDSL = value(IS("/V4/PLUS") VAR("IDL"))
IDSH = value(IS("/V4/PLUS") VAR("IDH"))
  • Now all the data can be generated for corners like typical and all skew with temperature to be imported in a chart like:

{
	{	
		corners = "tttt",	
		temps = 27,
		vars = "wid=4u len=10u di=1u dil=0.7u dih=1.3u TMP=temp",
	},
	{
		corners = {"ffff", "ssss"},
		temps = {27,125,-40},
		vars = "wid=4u len=10u di=1u dil=0.7u dih=1.3u TMP=temp",
	}
}

See Also