A New Surface-Potentials Based MOSFET Model : HiSIM

HiSIM stands for Hiroshima-university STARC IGFET Model. It has been developped at Hiroshima University starting in 1992. It has been released as version 1.1.0 in July 2002.

HiSIM and Conventionnal MOSFET Models

HiSIM is interesting because of the way it models channel current. Conventionnal MOSFET models simplify computation of channel current by splitting calculation between a linear dependent region (due to strong inversion) and a saturation region (due to velocity saturation). Discontinuities can appear in IDS, in the transition region. Therefore, to avoid these discontinuities, extra parameters are used to smooth the transition between the different set of equations. These parameters are not physical, they are just needed to correctly fit the device’s characteristics.

Another drawback of common models is the bad modeling of short-channel effects. For deep sub-micron MOSFETs, this effect dominates the IDS-VDS characteristic. Conventional models do not use equations based on physical concepts, but add fitting parameters to each modeled effect to account for short-channel effect. This results in many unphysical fitting parameters, and makes parameter extraction difficult.

The conclusion is that dividing the IDS current into different regions and equations is not correct anymore for short-channel transistors.

HiSIM is based on a charge-sheet model. IDS current is described using only one equation, and there-fore is continuous over the whole range of operating regions. This improves MOSFETs modeling regarding at least two points :

• Equations are continuous over all operation regions, as well as their derivatives. This is a key point for today’s analog circuits, where performance is very much dependent on high order derivatives.

• Parameter number is dramatically reduced (by a factor 5) for the same level of accuracy. Parameters are not interdependent anymore, making extraction easier. Furthermore, a set of parameters is valid for all channel lengths.

Surface Potentials

HiSIM is based on charge control and charge flow through the channel. The inversion layer charge and the depletion layer charge depend on the surface potential along the channel.

To compute these charges, the surface potentials at source side _S0 and _SL at drain side are needed. These two values are directly dependent on technological parameters. They are calculated by solving the Poisson equations :

The surface potentials side _S0 and _SL are distributed in the channel according to the schematic shown in Figure 1

Figure 1.

Both Poisson equations are solved iteratively, because they are implicit. Using approximations to get explicit equations with regard to terminal voltages would not be an improvement: it would reduce accuracy, and convergence is quickly obtained when solving these two equations.The internal New-ton’s algorithm converges within one to ten iterations, depending on the circuit. This is acceptable for a circuit simulator, since simulation times are comparable to those observed using other models.

The screenshot in Figure 2 shows the surface potentials evolution when VGS increases.

Figure 1.

New in Version 1.1.0

The last improvements are: shallow-trench-isolation (STI) is accounted for in leakage current model, a lateral-field-induced capacitance has been added, and the resistance model has been improved, requiring two more model parameters. These new modeling equations make HiSIM even more accurate.

Modeled Effects

HiSIM conputes charge control using dedicated parameters to account for the following physical effects:

• Short-channel

• Reverse short-channel

• Pocket implantation

• Quantum

• Poly-depletion

• Universal mobility

• Channel-length modulation

• Velocity overshoot

• Symmetry at VDS=0

• Shallow trench isolation (version 1.1.0 only)

HiSIM and SmartSpice

HiSIM is available within SmartSpice when LEVEL=111 is specified. This model has been implemented using reference versions 1.0.0 and 1.1.1. The user can select one of these version using a selector, VERSION. Beyond this material, SmartSpice provides all the services commonly proposed for MOSFET models. Among them are:

• Advanced geometry scaling (ACM)

• Simulation performance using VZERO and BYPASS options

• Friendly diagnostics to help with convergence issues

• Extrinsic elements such as junction diodes/capacitances

The model card for HiSIM includes the following parameters:

Technological parameter

Temperature dependence

Quantum effect

Poly depletion

Short channel

Narrow channel

Mobility

Channel-length modulation

Substrate current

Gate current

GIDL current

Flicker noise

Symmetry at VDS=0

* available only in version 1.1.0