Improved MOSFET Model Achieves Higher Accuracy

Jan 1, 2007 12:00 PM
By Scott Pearson, Modeling Engineer, Sylvie Tran, Product Modeling Engineer, and Steven Sapp, Produc

An improved SPICE model has been developed by Fairchild engineers for the simulation of trench power devices using the BSIM3 MOSFET model. The new model architecture seeks to eliminate shortcomings in the level 1 and level 3 subcircuit methods used extensively for modeling MOSFETs in power circuits. The new model offers excellent correlation to product data, transistor scaling not possible with other power MOSFET models, robust simulation and reduced simulation time.

Level 1 Subcircuit Model

The level 1 SPICE MOSFET model is the simplest and most basic of all models. Semiconductor manufacturers have used it widely for the simulation of power MOSFET devices. But because of its simplicity, it does have limitations. Among them is a lack of voltage-dependent capacitance. This will limit the model's ability to accurately simulate switching events.

Poor simulation of conduction in the subthreshold region is another limitation of the level 1 model, so linear-mode circuits may have discontinuous transitions from one gate-drive condition to another. To overcome these limitations, an elaborate subcircuit model was developed providing for voltage-dependant capacitance, subthreshold conduction, breakdown voltage and body-diode forward conduction (Fig. 1).

While the circuit shown can accurately model a power MOSFET, it occasionally has convergence errors and the simulation time is relatively long. This model is implemented with no process or physical properties related to the device; thus, it is not easily scaled for new die sizes.

A typical example of a level 3 subcircuit model was published by Motorola in 1989.[1] This approach has a single MOSFET for dc characterization and two controlled switches to model the variation of capacitance with applied voltage. Although it's an improvement over the level 1 MOSFET model, the level 3 model also has weaknesses in the subthreshold region and lacks voltage-dependent capacitances. The controlled switches enable the subcircuit model to simulate the voltage-dependent capacitances.

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