Using a MOSFET Diode Model

The Area Calculation Method (ACM) parameter allows for the precise control of modeling bulk-to-source and bulk-to-drain diodes within MOSFET models. The ACM model parameter is used to select one of three different modeling schemes for the MOSFET bulk diodes. This section discusses the model parameters and model equations used for the different MOSFET diode models.

Selecting MOSFET Diode Models

To select a MOSFET diode model, set the ACM parameter within the MOSFET model statements. If ACM=0, the pn bulk junctions of the MOSFET are modeled in the SPICE-style. The ACM=1 diode model is the original ASPEC model. The ACM=2 model parameter specifies the Star-Hspice improved diode model, which is based on a model similar to the ASPEC MOSFET diode model. The ACM=3 diode model is a further Star-Hspice improvement that deals with capacitances of shared sources and drains and gate edge source/drain-to-bulk periphery capacitance. If the ACM model parameter is not set, the diode model defaults to the ACM=0 SPICE model. ACM=0 and ACM=1 models do not permit the specification of HDIF. ACM=0 does not permit specification of LDIF. Furthermore, the geometric element parameters AD, AS, PD, and PS are not used for the ACM=1 model.

Enhancing Convergence

The GMIN and GMINDC options parallel a conductance across the bulk diodes and drain-source for transient and DC analysis, respectively. Use these options to enhance the convergence properties of the diode model, especially when the model has a high off resistance. Use the parameters RSH, RS, and RD to keep the diode from being overdriven in either a DC or transient forward bias condition. These parameters also enhance the convergence properties of the diode model.

Using MOSFET Diode Model Parameters

This section describes the diode model parameters for MOSFET.

DC Model Parameters

Using Capacitance Model Parameters

Using Drain and Source Resistance Model Parameters

Using MOS Geometry Model Parameters

Using an ACM=0 MOS Diode

The following example shows the parameter value settings for a MOSFET diode designed with a MOSFET that has a channel length of 3 µm and a channel width of 10 µm.

Figure 20-11: ACM=0 MOS Diode

Example

Consider a transistor with:
LD=.5µm W=10µm L=3µm

Calculating Effective Areas and Peripheries

For ACM=0, the effective areas and peripheries are calculated as:

Calculating Effective Saturation Current

For ACM=0, the MOS diode effective saturation currents are calculated as:

Source Diode Saturation Current

Define:

If val > 0 then,

Otherwise,

Drain Diode Saturation Current

Define:

If val > 0 then,

Otherwise,

Calculating Effective Drain and Source Resistances

For ACM=0, the effective drain and source resistances are calculated as:

Source Resistance

Define:

If val > 0 then,

Otherwise:

Drain Resistance

Define:

If val > then,

Otherwise:

Using an ACM=1 MOS Diode

Star-Hspice uses ASPEC-style diodes when the model parameter ACM=1 is specified. Parameters AD, PD, AS, and PS are not used, and the units JS and CJ differ from the SPICE style diodes (ACM=0).

Figure 20-12: ACM=1 MOS Diode

Example

The listings below are typical parameter value settings for a transistor with: LD=0.5 µm W=10 µm L=3 µm LDIF=0.5 µm

Calculating Effective Areas and Peripheries

For ACM=1, the effective areas and peripheries are calculated as follows:

where:

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NOTE: The Weff is not quite the same as the weff given in the models LEVEL 1, 2, 3, 6, and 13 sections. The term is not subtracted.

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Calculating Effective Saturation Current

For ACM=1, the MOS diode effective saturation currents are calculated as follows:

Source Diode Saturation Current

Define:

If val > 0 then,

Otherwise:

Drain Diode Saturation Current

Define:

If val > 0 then,

Otherwise,

Calculating Effective Drain and Source Resistances

For ACM=1, the effective drain and source resistances are calculated as follows:

Source Resistance

For UPDATE=0,

If UPDATE >= 1 and LDIF=0 and the ASPEC option is also specified then:

Drain Resistance

For UPDATE=0:

If UPDATE >= 1 and LDIF=0 and the ASPEC option is also specified then:

Using an ACM=2 MOS Diode

Star-Hspice uses HSPICE style MOS diodes when the model parameter ACM=2 is specified. This allows a fold-back calculation scheme similar to the ASPEC method, retaining full model-parameter compatibility with the SPICE procedure. This method also supports both lightly and heavily doped diffusions (by setting the LD, LDIF, and HDIF parameters). The units of JS, JSW, CJ, and CJSW used in SPICE are preserved, permitting full compatibility.

ACM=2 automatically generates more reasonable diode parameter values than those for ACM=1. The ACM=2 geometry can be generated one of two ways:

• Element parameters: AD, AS, PD, and PS can be used for parasitic generation when specified in the element statement. Default options values for these parameters are not applicable.
• If the diode is to be suppressed, set IS=0, AD=0, and AS=0.

The source diode is suppressed if AS=0 is set in the element and IS=0 is set in the model. This setting is useful for shared contacts.

Figure 20-13: ACM=2 MOS Diode

Example

Transistor with LD=0.07µm W=10 µm L=2 µm LDIF=1 µm HDIF= 4 µm, typical MOSFET diode parameter values are:

Calculating Effective Areas and Peripheries

For ACM=2, the effective areas and peripheries are calculated as:

If AD is not specified then,

Otherwise,

If AS is not specified then,

Otherwise,

If PD is not specified then,

Otherwise,

If PS is not specified then,

Otherwise:

where:

---

NOTE: The Weff is not quite the same as the Weff given in the model LEVEL 1, 2, 3, and 6 sections. The term is not subtracted.

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Calculating Effective Saturation Currents

For ACM=2, the MOS diode effective saturation currents are calculated as:

Source Diode Saturation Current

Define:

If val > 0 then,

Otherwise:

Drain Diode Saturation Current

Define:

If val > 0 then,

Otherwise:

Calculating Effective Drain and Source Resistances

For ACM=2, the effective drain and source resistances are calculated as:

Source Resistance

If NRS is specified then,

Otherwise:

Drain Resistance

If NRD is specified then,

Otherwise:

Using an ACM=3 MOS Diode

Use ACM=3 to model MOS diodes of the stacked devices properly. In addition, the CJGATE model parameter separately models the drain and source periphery capacitances along the gate edge. Therefore, the PD and PS calculations do not include the gate periphery length. CJGATE defaults to CJSW, which, in turn, defaults to 0.

The AD, AS, PD, PS calculations depend on the layout of the device, which is determined by the value of element parameter GEO. The GEO can be specified on the MOS element description. It can have the following values:

GEO=0: indicates the drain and source of the device are not shared by other devices (default).

GEO=1: indicates the drain is shared with another device.

GEO=2: indicates the source is shared with another device.

GEO=3: indicates the drain and source are shared with another device.

Figure 20-14: Stacked Devices and Corresponding GEO Values

Calculating Effective Areas and Peripheries

For ACM=3, the effective areas and peripheries are calculated differently, depending on the value of GEO.

If AD is not specified, then,

For GEO=0 or 2,

For GEO=1 or 3,

Otherwise:

If AS is not specified, then,

For GEO=0 or 1,

For GEO=2 or 3,

Otherwise:

If PD is not specified, then,

For GEO=0 or 2,

For GEO=1 or 3,

Otherwise:

If PS is not specified, then,

For GEO=0 or 1,

For GEO=2 or 3,

Otherwise:

The Weff and HDIFeff is calculated as follows:

---

NOTE: The Weff is not quite the same as the Weff given in the model LEVEL 1, 2, 3, and 6 sections. The term is not subtracted.

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Effective Saturation Current Calculations

The ACM=3 model calculates the MOS diode effective saturation currents the same as ACM=2.

Effective Drain and Source Resistances

The ACM=3 model calculates the effective drain and source resistances the same as ACM=2.