Using JFET and MESFET Model Statements

Syntax

.MODEL mname NJF <LEVEL = val> <pname1 = val1> ...

.MODEL mname PJF <LEVEL = val> <pname1 = val1> ...

JFET and MESFET Model Parameters

DC characteristics are defined by the model parameters VTO and BETA. These parameters determine the variation of drain current with gate voltage. LAMBDA determines the output conductance, and IS, the saturation current, of the two gate junctions. Two ohmic resistances, RD and RS, are included. The charge storage is modeled by nonlinear depletion-layer capacitances for both gate junctions that vary as the -M power of junction voltage, and are defined by the parameters CGS, CGD, and PB.

Use parameters KF and AF to model noise, which is also a function of the series source and drain resistances (RS and RD), in addition to temperature. Use the parameters ALPHA and A to model MESFETs.

The AREA model parameter is common to both the element and model parameters. The AREA element parameter always overrides the AREA model parameter.

The following subsections provide information about:

• Gate Diode DC Parameters
• DC Model LEVEL 1 Parameters
• DC Model LEVEL 2 Parameters
• DC Model LEVEL 3 Parameters
• ACM (Area Calculation Method) Parameter Equations

Gate Diode DC Parameters

Gate Capacitance LEVEL 1, 2, and 3 Parameters

DC Model LEVEL 1 Parameters

DC Model LEVEL 2 Parameters

DC Model LEVEL 3 Parameters

ACM (Area Calculation Method) Parameter Equations

The JFET model parameter ACM lets you select between the SPICE unitless gate area calculations and the Star-Hspice area calculations. The ACM=0 method (SPICE) uses the ratio of W/L to keep AREA unitless. The ACM=1 model (Star-Hspice) requires parameters such as IS, CGS, CGD, and BETA to have proper physics-based units.

In the following equations, lower case "m" indicates the element multiplier.

ACM=0

ACM=1

Or if RD=0,

or if RG=0,

or if RS=0,

Resulting calculations

Example

j1 10 20 0 40 nj_acm0 w=10u l=1u

j2a 10 20 0 41 nj_acm1 w=10u l=1u

.model nj_acm0 njf LEVEL=3 capop=1 sat=3 acm=0

+ is=1e-14 cgs=1e-15 cgd=.3e-15
$$$ note different units for is,cgs,cgd

+ rs=100 rd=100 rg=5 beta=5e-4

+ vto=.3 n=1 ng=1.4 nd=1

+ k1=.2 vgexp=2 alpha=4 ucrit=1e-4 lambda=.1
+ satexp=2

+ eg=1.5 gap1=5e-4 gap2=200 d=13

.model nj_acm1 njf LEVEL=3 capop=1 sat=3 acm=1

+ is=1e-2 cgs=1e-3 cgd=.3e-3
$$$ note different units for is,cgs,cgd

+ rs=100 rd=100 rg=5 beta=5e-4

+ vto=.3 n=1 ng=1.4 nd=1

+ k1=.2 vgexp=2 alpha=4 ucrit=1e-4 lambda=.1
+ satexp=2

+ eg=1.5 gap1=5e-4 gap2=200 d=13

JFET and MESFET Capacitances

Gate Capacitance CAPOP=0

The DCAP option switch selects the diode forward bias capacitance equation:

DCAP=1

Reverse Bias:

vgd < FC · PB

vgs < FC · PB

Forward Bias:

DCAP=2 (Star-Hspice Default)

Reverse Bias:

vgd < 0

vgs < 0

Forward Bias:

DCAP=3

Limits peak depletion capacitance to FC · CGDeff or FC · CGSeff, with proper fall-off when forward bias exceeds PB (FC > 1).

Gate Capacitance CAPOP=1

Gate capacitance CAPOP=1 is a charge conserving symmetric capacitor model most often used for MESFET model LEVEL 3.

where:

and:

CGD = High -vds Cgd at vgs = 0

CGS = High -vds Cgs at vgs = 0

CGD - CGDeff

CGS - CGSeff

Gate Capacitance CAPOP=2

The Statz capacitance equations (See H. Statz, P.Newman, I.W.Smith, R.A. Pucel, and H.A. Haus, GaAs FET Device and Circuit Simulation in Spice) (CAPOP=1) contain mathematical behavior that has been found to be problematic when trying to fit data.

• For vgs below the threshold voltage and Vds>0 (normal bias condition), Cgd is greater than Cgs and rises with Vds, while Cgs drops with Vds.
• Although Cgd properly goes to a small constant representing a sidewall capacitance, Cgs drops asymptotically to zero with decreasing Vgs.
• (For the behavior for Vds<0, interchange Cgs and Cgd and replace Vds with -Vds in the above descriptions.)
• It can be difficult to simultaneously fit the DC characteristics and the gate capacitances (measured by S-parameters) with the parameters that are shared between the DC model and the capacitance model.
• The capacitance model in the CAPOP=1 implementation also lacks a junction grading coefficient and an adjustable width for the Vgs transition to the threshold voltage. The width is fixed at 0.2.
• Finally, an internal parameter for limiting forward gate voltage is set to 0.8 · PB in the CAPOP=1 implementation. This is not always consistent with a good fit.

The CAPOP=2 capacitance equations help to solve the problems described above.

CAPOP=2 Parameters

Capacitance Comparison (CAPOP=1 and CAPOP=2)

CAPOP=1 vs. CAPOP=2. Cgs, Cgd vs. Vgs for Vds=0, 1, 2, 3, 4 and CAPOP=1 vs. CAPOP=2. Cgs, Cgd vs. Vds for Vgs=-1.5, -1.0, -0.5, 0 show comparisons of CAPOP=1 and CAPOP=2. Note in CAPOP=1 vs. CAPOP=2. Cgs, Cgd vs. Vgs for Vds=0, 1, 2, 3, 4 that below threshold (-0.6 v) Cgs for CAPOP=2 drops towards the same value as Cgd, while for CAPOP=1, CGS --> 0.

Note in CAPOP=1 vs. CAPOP=2. Cgs, Cgd vs. Vds for Vgs=-1.5, -1.0, -0.5, 0 how the Cgs-Cgd characteristic curve "flips over" below threshold for CAPOP=1, whereas for CAPOP=2, it is well-behaved.

Figure 17-5: CAPOP=1 vs. CAPOP=2. Cgs, Cgd vs. Vgs for Vds=0, 1, 2, 3, 4

Figure 17-6: CAPOP=1 vs. CAPOP=2. Cgs, Cgd vs. Vds for Vgs=-1.5, -1.0, -0.5, 0

JFET and MESFET DC Equations

DC Model Level 1

JFET DC characteristics are represented by the nonlinear current source, ids. The value of ids is determined by the equations:

vgst< 0 Channel pinched off

0<vgst<vds Saturated region

0<vds<vgst Linear region

The drain current at zero vgs bias (ids) is related to VTO and BETA by the equation:

At a given vgs, LAMBDA can be determined from a pair of drain current and drain voltage points measured in the saturation region where vgst<vds:

DC Model LEVEL 2

The DC characteristics of the JFET LEVEL 2 model are represented by the nonlinear current source (ids). The value of ids is determined by the equations:

vgst<0 Channel pinched off

Saturated region, forward bias

0<vgst<vds, vgs<0 Saturated region, reverse bias

0<vds<vgst Linear region

DC Model LEVEL 3

The DC characteristics of the MESFET LEVEL 3 model are represented by the nonlinear hyperbolic tangent current source (ids). The value of ids is determined by the equations:

vds>0 Forward region

If model parameters VP and VTO are not specified they are calculated as:

then,

vgst<0 Channel pinched off

vgst>0, SAT=0 On region

vgst>0, SAT=1 On region

vgst>0, SAT=2, vds<3/ALPHA On region

vgst>0, SAT=2, vds>3/ALPHA On region

If vgst >0, SAT=3 is the same as SAT=2, except exponent 3 and denominator 3 are parameterized as SATEXP, and exponent 2 of vgst is parameterized as VGEXP.