Using MOS Diode Equations

This section describes the MOS diode equations.

DC Current

The drain and source MOS diodes are paralleled with GMINDC conductance in the DC analysis and with GMIN in the transient analysis. The total DC current is the sum of diode current and the conductance current. The diode current is calculated as follows.

Drain and Source Diodes Forward Biased

vbs > 0,

vbd > 0,

Drain and Source Diodes Reverse Biased

For 0>vbs>VNDS,

For vbs < VNDS,

For 0 > vbd > VNDS,

For vbd < VNDS,

where

, and

Using MOS Diode Capacitance Equations

Each MOS diode capacitance is the sum of diffusion and depletion capacitance. The diffusion capacitance is evaluated in terms of the small signal conductance of the diode and a model parameter TT, representing the transit time of the diode. The depletion capacitance depends on the choice of ACM, and is discussed below.

Calculate the bias-dependent depletion capacitance by defining the intermediate quantities: C0BS, C0BD, C0BS_SW, and C0BD_SW, which depend on geometric parameters, such as ASeff and PSeff calculated under various ACM specifications.

When ACM=3, the intermediate quantities C0BS_SW, and C0BD_SW include an extra term to account for CJGATE.

For ACM=2, the parameter CJGATE has been added in a backward compatible manner. Therefore, the default behavior of CJGATE makes the intermediate quantities C0BS_SW and C0BD_SW the same as for previous versions. The default patterns are:

• If neither CJSW nor CJGATE is specified, both default to zero.
• If CJGATE is not specified, it defaults to CJSW, which in turn defaults to zero.
• If CJGATE is specified, and CJSW is not specified, then CJSW defaults to zero.

The intermediate quantities C0BS, C0BS_SW, C0BD, and C0BD_SW are calculated as follows.

C0BS = CJscaled*ASeff

C0BD = CJscaled*ADeff

If (ACM= 0 or 1), then:

C0BS_SW = CJSWscaled*PSeff

C0BD_SW = CJSWscaled*PDeff

If (ACM=2):

If (PSeff < Weff), then:

C0BS_SW = CJGATEscaled*PSeff

Otherwise:

C0BS_SW = CJSWscaled*(PSeff-Weff) + CJGATEscaled*Weff

If (PDeff < Weff), then:

C0BD_SW = CJGATEscaled*PDeff

Otherwise:

C0BD_SW = CJSWscaled*(PDeff-Weff) + CJGATEscaled*Weff

If (ACM=3), then:

C0BS_SW = CJSWscaled*PSeff + CJGATEscaled*Weff

C0BD_SW = CJSWscaled*PDeff + CJGATEscaled*Weff

Source Diode Capacitance

If (C0BS + C0BS_SW) > 0, then:

For vbs < 0,

For vbs > 0,

Otherwise, if (C0BS + C0BS_SW) <= 0, then:

For vbs < 0,

For vbs > 0,

Drain Diode Capacitance

If (C0BD + C0BD_SW) > 0, then:

For vbd < 0,

For vbd > 0,

Otherwise, if (ADeff · CJscaled + PDeff · CJSWscaled) <= 0, then:

For vbd < 0,

For vbd > 0,