SPICE MOSFET Declaration The MOSFET is a 4-terminal device that is specified in the netlist as: Mname ND NG NS NB ModName <Optional para> The optional para are: L= value W= value AD=value AS=value PD=value PS=value NRD=value where: L,W: gate length and width (in m). Default: meter AD,AS: source and drain diffusion (in sq. m). Default: PD, PS: perimeter of the source and drain (in m). Default: meter NRD: number of squares for drain diffusion, for resistance calculations. Default: sq. NG ND NS NB If an optional parameter is omitted the default values are used. Here is an example declaration for the devices at right: 3 M 3 4 MyFet L=u W=u 4 4 3 Additional device para, including whether the device is NMOS or PMOS, are defined in the MODEL declaration for MyFet (next slide!)
SPICE MOS Model The SPICE MOSFET Model is defined in the netlist as.model ModName xmos (para) (xmos is either NMOS or PMOS) Some key para are: Parameter Description Units Default VTO Zero-bias threshold voltage KP Transconductance parameter, μ C ox Amps/ E-5 GAMMA Bulk threshold parameter / PHI Surface potential.6 LAMBDA Channel-length modulation - CGSO Gate-source overlap capacitance/channel width CGDO Gate-drain overlap capacitance/channel width CGBO Gate-bulk overlap capacitance/channel width U Mobility μ cm /V-s 6 TOX Oxide thickness In terms of these para some of the basic device characteristics are: I KP W V VTO LAMBDA V L d gs ds t V VTO GAMMA PHI V PHI sb A simple NMOS device with no capacitances or body effects might be specified as.model MyFet NMOS (VTO= KP=.5) If the user specifies U and TOX, then KP is computed as: KP C Key Device Capacitances: Cgs CoxWLCGSOW Cdg CGDO W 3 ox C ox 3.9 TOX
Level MOS SPICE Para Parameter VTO KP GAMMA PHI LAMBDA LD WD RD RS RG RB RDS RSH IS JS PB CBD CBS CJ CJSW MJ MJSW FC CGSO CGDO CGBO NSUB NSS NFS U TOX TPG KF AF Description Zero-bias threshold voltage Transconductance parameter, μ Cox Bulk threshold parameter Surface potential Channel-length modulation Lateral diffusion length Lateral diffusion width Drain ohmic resistance Source ohmic resistance Gate ohmic resistance Bulk ohmic resistance Drain-source shunt resistance Drain-source diffusion sheet resistance Bulk p-n saturation current Bulk p-n saturation/current area Bulk p-n potential Bulk-drain zero-bias p-n capacitance Bulk-source zero-bias p-n capacitance Bulk p-n zero-bias bottom capacitance/length Bulk p-n zero-bias perimeter capacitance/length Bulk p-n bottom grading coefficient Bulk p-n sidewall grading coefficient Bulk p-n forward-bias capacitance coefficient Gate-source overlap capacitance/channel width Gate-drain overlap capacitance/channel width Gate-bulk overlap capacitance/channel width Substate doping density Surface-state density Fast surface-state density Mobility Oxide thickness Gate material type: + =opposite of substrate, -=same, =Aluminum Flicker noise coefficient Flicker noise exponent Units Amps/ / - /square Amps Amps/ Farads Farads /centimeter 3 /centimeter /centimeter cm /V-s Default E-5.6 E-4.8.5.33.5 6 +
MOS Devices in MultiSim Select Place Component This will appear in the circuit schematic Q6 um um Double-click on the component: This is where we modify the SPICE model para In MultiSim A number of virtual devices to choose from, here we select the 3- terminal enhancement-mode MOSFET Click Edit Model to see SPICE Model declaration
Simple NMOS/PMOS Devices in MultiSim In ECE we will often solve problems assuming a very basic device described by a threshold voltage, transconductance, and output resistance. We can create simple MOS models with these three para as follows: NMOS PMOS I KP W V VTO LAMBDA V L d gs ds Remember: PMOS enhancement devices have a negative VTO! After making desired changes in the model, select Change Part Model. Only use Change All Models if you want the same model to apply to all the FETs in your circuit!
Typical Para (Long and Short).MODEL N7 NMOS (LEVEL=3 RS=.5 NSUB=.E5 DELTA=. +KAPPA=.56 TPG= CGDO=3.76E-9 RD=.39 VTO=. VMAX=.E7 +ETA=.389 NFS=6.6E TOX=.E-7 LD=.698E-9 UO=86.45 +XJ=6.4666E-7 THETA=.E-5 CGSO=9.9E-9 L=.5E-6 W=.8E-)