imelt and imax

imelt and imax are used on devices containing p-n junctions. These devices include

• JFETs
• GaAs FETs
• Physical resistors (phy_res)
• All MOSFET models
• Diodes
• All BJTs (G-P, VBIC, and HBT)

imelt is used to help convergence and to prevent numerical problems. The Spectre^® circuit simulator uses imelt to linearize the junction current. When the junction current is larger than imelt (a model parameter), the junction current is calculated as a linear function instead of an exponential function of the junction voltage. Therefore, if the junction current is larger than imelt, the current calculated by the Spectre circuit simulator is not what the original junction current model predicts, and the Spectre circuit simulator issues a warning.

imax is a warning control parameter. Normally, imax does not affect the simulation results. Whenever the junction current is larger than imax, the Spectre circuit simulator issues a warning. The default value of imax is 1A, and the default value of imelt is imax. Therefore, imax can affect the simulation results only when imelt is not specified and takes its value from imax.

Typically, the junction current is much less than 1 A. When imelt is 1A, the conductance of the junction at room temperature is about 40 siemens, which is much larger than those of typical semiconductor devices. The Spectre circuit simulator issues a warning message telling you the junction current is linearized when

• The imelt value is set too small
• The connection to devices is incorrect
• Temporary voltage overshoot due to capacitive coupling at fast transient transition
  If this happens, the warnings can be ignored in most cases unless the voltage overshoot is purposely designed to trigger some circuit functions.

imelt and imax have current density counterparts, jmelt and jmax, which can be specified if you want the absolute current values to depend on the device area.

If the is parameter is not specified, its value is calculated as area*js, and imax and imelt values will also be calculated as area*jmax and area*jmelt. In this condition, values of imax and imelt given in the model card will have no effect.

In MOSFETs, you can use jmelt instead of imelt. The function of jmelt is identical to that of imelt, except that the junction current is linearized based on the current density specified by jmelt. If both imelt and jmelt are specified, imelt takes precedence.

For BJTs, imelt and imax are used to control the base-emitter and the substrate junctions. imelt1 and imax1 are used to control the base-collector junction.

Related Topics

Scaling Factors (scale and scalem)

Multiplication Factor (m)

SPICE Compatibility Flag (compatible)

Parameters Common to All Devices

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