Performing MOSFET Impact Ionization

The impact ionization current for MOSFETs is available for all levels. The controlling parameters are ALPHA, VCR, and IIRAT. The parameter IIRAT sets the fraction of the impact ionization current that goes to the source.

I ds = I ds _normal + IIRAT · I_impact

I db = I db _diode + (1-IIRAT) · I_impact

IIRAT defaults to zero, which sends all impact ionization current to bulk. Leave IIRAT at its default value unless data is available for both drain and bulk current.

Using Impact Ionization Model Parameters

Calculating the Impact Ionization Equations

The current I_impact due to impact ionization effect is calculated as follows:

where:

where:

and

Calculating Effective Output Conductance

The element template output allows gds to be output directly, for example:

.PRINT I(M1) gds=LX8(M1)

When using impact ionization current, it is important to note that gds is the derivative of I ds only, rather than the total drain current, which is I ds +I db . The complete drain output conductance is:

For example, to print the drain output resistance of device M1:

.PRINT rout=PAR('1.0/(LX8(M1)+LX10(M1))')

Figure 20-15: Drain, Source, and Bulk Currents for vgs=3, with IIRAT=0.5

Cascoding Example

Drain-to-bulk impact ionization current limits the use of cascoding to increase output impedance. The following cascode example shows the effect of changing IIRAT. When IIRAT is less than 1.0, the drain-to-bulk current lowers the output impedance of the cascode stage.

Figure 20-16: Low-frequency AC Analysis Measuring Output Impedance

Cascode Circuit

Example

iirat gout_ac rout

0.0 8.86E-6 113 K
0.5 4.30E-6 233 K
1.0 5.31E-8 18.8 Meg

Input File

$ cascode test

.param pvds=5.0 pvref=1.4 pvin=3.0

vdd dd 0 pvds ac 1

$ current monitor vd

vd dd d 0

vin in 0 pvin

vref ref 0 pvref

x1 d in ref cascode

.macro cascode out in ref

m1 out in 1 0 n L=1u W=10u

mref 1 ref 0 0 n L=1u W=10u

.eom

.param xiirat=0

.ac dec 2 100k 1x sweep xiirat poi 3 0, 0.5, 1.0

.print ir(vd)

.measure gout_ac avg ir(vd)

.model n nmos level=3

+ tox=200 vto=0.8 gamma=0.7 uo=600 kappa=0.05

+ alpha=1 vcr=15 iirat=xiirat

.end