Designing a PCB with DEX > Projects > Circuit Simulation > The Spice Reference Manual > Circuit Description > Circuit Elements and Models > Transistors and Diodes

SPICE provides four MOSFET device models, which differ in the formulation of the I-V characteristic. The variable LEVEL specifies the model to be used:

LEVEL=1 -> Shichman-Hodges

LEVEL=2 -> MOS2 (as described in [1])

LEVEL=3 -> MOS3, a semi-empirical model(see [1])

LEVEL=4 -> BSIM (as described in [3])

LEVEL=5 -> new BSIM (BSIM2; as described in [5])

LEVEL=6 -> MOS6 (as described in [2])The dc characteristics of the level 1

through level 3 MOSFETs are defined by the device parameters VTO, KP, LAMBDA, PHI and GAMMA. These parameters are computed by SPICE if process parameters (NSUB, TOX, ...) are given, but user-specified values always override. VTO is positive (negative) for enhancement mode and negative (positive) for depletion mode N-channel (P-channel) devices. Charge storage is modeled by three constant capacitors, CGSO, CGDO, and CGBO which represent overlap capacitances, by the nonlinear thin-oxide capacitance which is distributed among the gate, source, drain, and bulk regions, and by the nonlinear depletion-layer capacitances for both substrate junctions divided into bottom and periphery, which vary as the MJ and MJSW power of junction voltage respectively, and are determined by the parameters CBD, CBS, CJ, CJSW, MJ, MJSW and PB. Charge storage effects are modeled by the piecewise linear voltages-dependent capacitance model proposed by Meyer. The thin-oxide charge-storage effects are treated slightly different for the LEVEL=1 model. These voltage-dependent capacitances are included only if TOX is specified in the input description and they are represented using Meyer's formulation.

There is some overlap among the parameters describing the junctions, e.g. the reverse current can be input either as IS (in A) or as JS (in A/m2). Whereas the first is an absolute value the second is multiplied by AD and AS to give the reverse current of the drain and source junctions respectively. This methodology has been chosen since there is no sense in relating always junction characteristics with AD and AS entered on the device line; the areas can be defaulted. The same idea applies also to the zero-bias junction capacitances CBD and CBS (in F) on one hand, and CJ (in F/m2) on the other. The parasitic drain and source series resistance can be expressed as either RD and RS (in ohms) or RSH (in ohms/sq.), the latter being multiplied by the number of squares NRD and NRS input on the device line.

A discontinuity in the MOS level 3 model with respect to the KAPPA parameter has been detected (see [10]). The supplied fix has been implemented in Spice3f2 and later. Since this fix may affect parameter fitting, the option "BADMOS3" may be set to use the old implementation (see the section on simulation variables and the ".OPTIONS" line).

SPICE level 1, 2, 3 and 6 parameters:

name	parameter	units	default	example
LEVEL	model	-	1
VTO	zero-bias threshold voltage (VT0)	V	0	1
KP	trans-conductance parameter	A/V2	2.0e-5	3.1e-5
GAMMA	bulk threshold parameter ( )	V1/2	0	0.37
PHI	surface potential ( )	V	0.6	0.65
LAMBDA	channel-length modulation (MOS1 and MOS2 only) ( )	1/V	0	0.02
RD	drain ohmic resistance	Ohm	0	1
RS	source ohmic resistance	Ohm	0	1
CBD	zero-bias B-D junction capacitance	F	0	20fF
CBS	zero-bias B-S junction capacitance	F	0	20fF
IS	bulk junction saturation current (IS)	A	1.oe-14	1.0e-15
PB	bulk junction potential	V	0.8	0.87
CGSO	gate-source overlap capacitance per meter channel width	F/m	0	4.0e-11
CGDO	gate-drain overlap capacitance per meter channel width	F/m	0	4.0e-11
CGBO	gate-bulk overlap capacitance per meter channel length	F/m	0	2e-1-
RSH	drain and source diffusion sheet resistance	Ohm/square	0	10
CJ	zero-bias bulk junction bottom cap. per sq-meter of junction area	F/m2	0	2.0e-4
MJ	bulk junction bottom grading coeff.	-	0.5	0.5
CJSW	zero-bias bulk junction sidewall cap. per meter of junction perimeter	F/m	10	1.0e-9
MJSWE	bulk junction sidewall grading coeff.	-	0.5 (level1) 0.33(level 2,3)
FS	bulk junction saturation current per sq-meter of junction area	A/m2		1.0e-8
TOX	oxide thickness	meter	1.0e-7	1.0e-7
NSUB	substrate doping	1/cm3	0	4.0e15
NSS	surface state density	1/cm2	0	1.0e10
NFS	fast surface state density	1/cm2	0	1.0e10
TPG	type of gate material: +1 opp. to substrate -1 same as substrate 0 Al gate	-	1.0
XJ	metallurgical junction depth	meter	0	1 micron
LD	lateral diffusion	meter	0	0.8 micron
UO	surface mobility	cm2/Vs	600	700
UCRIT	critical field for mobility degradation (MOS2 only	V/cm	600	1.0e-4
UEXP	critical field exponent in mobility degradation (MOS2 only)	-	1.0e04	0.1
UTRA	transverse field coeff. (mobility) (deleted for MOS2)	-	0	0.3
VMAX	maximum drift velocity of carriers	m/s	0	5.0e40.3
NEFF	total channel-charge (fixed and mobile) coefficient (MOS2 only	-	0	5.0
KF	flicker noise exponent	-	0	1.0e-26
AF	flicker noise exponent	-	1	1.2
FC	coefficient for forward-bias depletion capacitance formula	-	0.5
DELTA	width effect on threshold voltage (MOS2 and MOS3)	-	0	1.0
THETA	mobility modulation (MOS3 only	1/V	0	0.1
ETA	static feedback (MOS3 only)	-	0	1.0
KAPPA	saturation field factor (MOS3 only)	-	0.2	0.5
TNOM	parameter measurement temperature	ºC	25	50

The level 4 and level 5 (BSIM1 and BSIM2) parameters are all values obtained from process characterization, and can be generated automatically. J. Pierret [4] describes a means of generating a 'process' file, and the program Proc2Mod provided with SPICE3 converts this file into a sequence of BSIM1 ".MODEL" lines suitable for inclusion in a SPICE input file. Parameters marked below with an * in the l/w column also have corresponding parameters with a length and width dependency. For example, VFB is the basic parameter with units of Volts, and LVFB and WVFB also exist and have units of Volt- meter The formula is used to evaluate the parameter for the actual device specified.

Note that unlike the other models in SPICE, the BSIM model is designed for use with a process characterization system that provides all the parameters, thus there are no defaults for the parameters, and leaving one out is considered an error. For an example set of parameters and the format of a process file, see the SPICE2 implementation notes[3].

For more information on BSIM2, see reference [5].

SPICE BSIM (level 4) parameters:

name	parameter	units	1/w
VFB	flat-band voltage	V	*
PHI	surface inversion potential	V	*
K1	body effect coefficient	V1/2	*
K2	drain/source depletion charge-sharing coefficient	-	*
ETA	zero-bias drain-induced barrier-lowering coefficient	-	*
MUZ	zero-bias mobility	cm2/V-s
DL	shortening of channel	micron
DW	narrowing of channel	micron
U0	zero-bias transverse-field mobility degradation coefficient	V-1	*
U1	zero-bias velocity saturation coefficient	micron/V	*
X2MZ	sens. of mobility to substrate bias at vds=0	cm2/V-s	*
X2E	sens. of drain-induced barrier lowering effect to substrate bias	V-1	*
X3E	sens. of drain-induced barrier lowering effect to drain bias at Vds=Vdd	V-1	*
X2U0	sens. of transverse field mobility degradation effect to substrate bias	V-2	*
X2U1	sens. of velocity saturation effect to substrate bias	micron/V2	*
MUS	mobility at zero substrate bias and at Vds=Vdd	cm2/V-s
X2MS	sens. of mobility to substrate bias at Vds=Vdd	cm2/V-s	*
X3MS	sens. of mobility to drain bias at Vds=Vdd	cm2/V-s	*
X3U1	sens. of velocity saturation effect on drain bias at Vds=Vdd	cm2/V-s	*
TOX	TOX gate oxide thickness	micron
TEMP	temperature at which parameters were measured	ºC
VDD	measurement bias range	V
CGD0	gate-drain overlap capacitance per meter channel width	F/m
CGSO	gate-source overlap capacitance per meter channel width	F/m
CGBO	gate-bulk overlap capacitance per meter channel length	F/m
XPART	gate-oxide capacitance-charge model flag	-	*
NO	zero-bias subthreshold slope coefficient	-	*
NB	sens. of subthreshold slope to substrate bias	-	*
ND	sens. of subthreshold slope to drain bias	-
RSH	drain and source diffusion sheet resistance	Ohm/square
JS	source drain junction current density	A/m2
PB	built in potential of source drain junction	V
MJ	Grading coefficient of source drain junction	-
PBSW	built in potential of source, drain junction sidewall	V
MJSW	grading coefficient of source drain junction sidewall	-
CJ	Source drain junction capacitance per unit area	F/m2
CJSW	source drain junction sidewall capacitance per unit length	F/m
WDF	source drain junction default width	m
DELL	Source drain junction length reduction	m

XPART = 0 selects a 40/60 drain/source charge partition in saturation, while XPART=1 selects a 0/100 drain/source charge partition.

ND, NG, and NS are the drain, gate, and source nodes, respectively. MNAME is the model name, AREA is the area factor, and OFF indicates an (optional) initial condition on the device for dc analysis. If the area factor is omitted, a value of 1.0 is assumed. The (optional) initial condition specification, using IC=VDS, VGS is intended for use with the UIC option on the .TRAN control line, when a transient analysis is desired starting from other than the quiescent operating point. See the .IC control line for a better way to set initial conditions.

MOSFET Models (NMOS/PMOS)

MOSFET Models (NMOS/PMOS)