MOSFETs

None of Spice's standard MOSFET models fit the characteristics of trench or vertical MOSFETs too well. Consequently, the MOSFET models supplied have been made using subcircuits that include additional components to improve simulation accuracy. A typical less-complex MOSFET model is shown as follows:

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*ZETEX ZXMN3A14F Spice Model v1.0 Last revision 31/5/06
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.SUBCKT ZXMN3A14F 30 40 50
*------connections-------D-G-S
M1 6 2 5 5 Nmod L=1.16E-6 W=0.76
M2 5 2 5 6 Pmod L=1.3E-6 W=0.35
RG 4 2 4.5
RIN 2 5 1E12
RD 3 6 Rmod 0.04
RS 5 55 Rmod 0.015
RL 3 5 3E9
C1 2 5 8.5E-12
C2 3 4 3E-12
D1 5 3 Dbodymod
LD 3 30 0.5E-9
LG 4 40 1.0E-9
LS 55 50 1.0E-9
.MODEL Nmod NMOS (LEVEL=3 TOX=5.5E-8 NSUB=5E16 VTO=2.13
+KP=2.5E-5 NFS=2E11 KAPPA=0.06 UO=650 IS=1E-15 N=10)
.MODEL Pmod PMOS (LEVEL=3 TOX=5.5E-8 NSUB=1.5E16
+TPG=-1 IS=1E-15 N=10)
.MODEL Dbodymod D (IS=6E-13 RS=.025 IKF=0.1 TRS1=1.5e-3
+CJO=150e-12 BV=33 TT=12e-9)
.MODEL Rmod RES (TC1=2.8e-3 TC2=0.8E-5)
.ENDS ZXMN3A14F
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*$
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In the MOSFET model:

• L relates to a process parameter.
• W relates to a process parameter.
• TOX relates to a process parameter.
• NSUB relates to a process parameter.
• VTO defines Vgs(th).
• KP controls Gm.
• NFS fast surface state density.
• KAPPA saturation field factor.
• UO mobility.
• RS and RD add series terminal resistance with temperature characteristic modeled.
• IS and N suppress the behavior of the MOSFET model's default body diode.
• CGDO, derived from process related parameters, controls Crss.
• CGSO, derived from process related parameters, controls Ciss.
• CBD, derived from process related parameters, controls Coss.

In this trench MOSFET the NMOS models the walls of the trench and the PMOS models the bottom of the trench. Added to the Spice standard MOSFET models are a gate resistor to control switching speeds, gate source and drain-source resistors to control leakage, drain and source series resistance, a drain-source diode to accurately reflect the performance of the MOSFET's body diode and inductors to model inductance inside the package.

Recent MOSFET models mirror the performance of the real devices reasonably well in most areas. One area not covered well by the older less complex models is the way that Crss and Coss vary with drain-source voltage. Thus if the less complex models are used at a drain-source voltage well away from datasheet capacitance definition voltages and capacitance is critical, then the values used for CGSO and CGDO may need adjustment.

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