gm/gds versus gate bias (lv_nmos)

[bmurmann]

The simulated gm/gds versus gate bias (for various L) for an lv_nmos looks quite odd for short channels (see plot below). Does the measured data support this? I found only plots of gds versus drain bias in the documentation. The netlist to duplicate this plot is below.

---

** sch_path: /foss/designs/gds_n.sch
**.subckt gds_n
Xm1 d g GND b sg13_lv_nmos W={Wx} L={Lx} ng=1 m=1
Vgs g GND 0.6
Vds d GND 1.2
Vsb GND b 0
**** begin user architecture code
.lib /foss/pdks/ihp-sg13g2/libs.tech/ngspice/models/cornerMOSlv.lib mos_tt

.param temp=27
.param Wx=5u
.param Lx=0.13u
.dc Vgs 0 1.2 0.01
.save @n.xm1.nsg13_lv_nmos[gm]
.save @n.xm1.nsg13_lv_nmos[gds]

.control
*pre_osdi ./psp103_nqs.osdi

foreach L_val 0.13u 0.14u 0.15u 0.16u 0.17u 0.18u 0.19u 0.2u 0.3u 0.4u 0.5u 1u 2u 3u
alterparam Lx = $L_val
reset
run
let av=@n.xm1.nsg13_lv_nmos[gm]/@n.xm1.nsg13_lv_nmos[gds]
end

plot all.av

.endc

**** end user architecture code
**.ends
.GLOBAL GND
.end

[bmurmann]

Also, I am surprised to see the sign of gm flip negative when VDS=0. I know it's a small value, but I've never seen a MOS model that does this. The model expression should give exactly zero in this condition.

PSPNQS103VA: A simulator independent device loaded with OSDI
device n.xm1.nsg13_lv_nmos
model xm1:sg13g2_lv_nmos_ps
_mfactor 1
m 1
l 1.3e-07
w 5e-06
sa 0
sb 0
sd 0
sca 0
scb 0
scc 0
sc 0
nf 1
ngcon 2
xgw 1e-07
nrs 0
nrd 0
jw 1e-06
delvto 0
factuo 1
delvtoedge 0
factuoedge 1
absource 1e-12
lssource 1e-06
lgsource 1e-06
abdrain 1e-12
lsdrain 1e-06
lgdrain 1e-06
as 1.7e-12
ps 1.068e-05
ad 1.7e-12
pd 1.068e-05
mult 1
dta 0
ctype 1
sdint 1
ise -1.41647e-12
ige 2.83294e-12
ide -1.41647e-12
ibe -3.38614e-22
ids 0
idb 0
isb 0
igs 1.41647e-12
igd 1.41647e-12
igb 3.38614e-22
idedge 0
igcs 1.19916e-12
igcd 1.19916e-12
iavl 0
igisl 0
igidl 0
ijs 0
ijsbot 0
ijsgat 0
ijssti 0
ijd 0
ijdbot 0
ijdgat 0
ijdsti 0
vds 0
vgs 0.6
vsb 0
vto 0.436045
vts 0.436045
vth 0.436045
vgt 0.163955
vdss 0.193087
vsat -0.193087
gm -1.38899e-11
gmb -3.15928e-13
gds 0.00127697