# A New Surface-Potentials Based MOSFET Model : HiSIM

HiSIM stands for **H**iroshima-university **S**TARC**
I**GFET** M**odel. 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,

**provides all the services commonly proposed for MOSFET models. Among them are:**

*SmartSpice*- 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

TOX |
Oxide thickness |

XLD |
Gate-overlap length |

XWD |
Gate-overlap width |

XPOLYD |
Difference between gate-poly and design length |

TPOLY |
Height of the gate poly-Si |

RS |
Source contact resistance |

RD |
Gate contact resistance |

NSUBC |
Substrate-impurity concentration |

NSUBP |
Maximum pocket concentration |

VFBC |
Flat-band voltage |

LP |
Pocket penetration length |

XJ |
Junction depth |

XQY* |
Distance from D junction to maximum electric field |

Temperature dependence

BGTMP1 |
Bandgap narrowing |

BGTMP2 |
Bandgap narrowing |

Quantum effect

QME1 |
Coefficient 1 for quantum mecanical effect |

QME2 |
Coefficient 2 for quantum mecanical effect |

QME3 |
Coefficient 3 for quantum mecanical effect |

Poly depletion

PGD1 |
Strength of poly depletion |

PGD2 |
Threshold voltage of poly depletion |

PGD3 |
VDS dependence of poly depletion |

Short channel

PARL1 |
Strength of later-electric-field gradient |

PARL2 |
Depletion width of channel/contact junction |

SC1 |
Short-channel coefficient 1 |

SC2 |
Short-channel coefficient 2 |

SC3 |
Short-channel coefficient 3 |

SCP1 |
Short-channel coefficient 1 for pocket |

SCP2 |
Short-channel coefficient 2 for pocket |

SCP3 |
Short-channel coefficient 3 for pocket |

Narrow channel

WFC |
Threshold voltage reduction |

MUEPH2 |
Mobility reduction |

W0 |
Minimum gate width |

WVTHSC* |
SHort-channel effect at the STI edge |

NSTI* |
Substrate impurity concentration at the STI edge |

WSTI* |
Width of the high-field region at STI |

Mobility

VDS0 |
Drain voltage for extracting the low-field mobility |

MUECB0 |
Coulomb scattering |

MUECB1 |
Coulomb scattering |

MUEPH0 |
Phonon scattering |

MUEPH1 |
Phonon scattering |

MUETMP |
Temperature dependence of phonon scattering |

MUESR0 |
Surface-roughness scattering |

MUESR1 |
Surface-roughness scattering |

NDEP |
Coefficient 1 of effective-electric |

NINV |
Coefficient 2 of effective-electric |

NINVD |
Modification of NINV |

BB |
High-field-mobility degradation |

VMAX |
Maximum saturation velocity |

VOVER |
Velocity overshoot effect |

VOVERP |
LGATE dependence of velocity overshoot |

RPOCK1 |
Resistance coefficient 1 caused by the potential barrier |

RPOCK2 |
Resistance coefficient 2 caused by the potential barrier |

RPOCP1* |
Resistance coefficient 3 caused by the potential barrier |

RPOCP2* |
Resistance coefficient 4 caused by the potential barrier |

Channel-length modulation

CLM1 |
Hardness coefficient of channel/contact junction |

CLM2 |
Coefficient for QB contribution |

CLM3 |
Coefficient for QI contribution |

Substrate current

SUB1 |
Substrate current coefficient 1 |

SUB2 |
Substrate current coefficient 2 |

SUB3 |
Substrate current coefficient 3 |

Gate current

GLEAK1 |
Gate current coefficient 1 |

GLEAK2 |
Gate current coefficient 2 |

GLEAK3 |
Gate current coefficient 3 |

GIDL current

GIDL1 |
GIDL current coefficient 1 |

GIDL2 |
GIDL current coefficient 2 |

GIDL3 |
GIDL current coefficient 3 |

Flicker noise

NFALP |
Contribution of the mobility fluctuation |

NFTRP |
Ratio of trap density to attenuation coefficient |

CIT |
Capacitance caused by the interface trapped carriers |

Symmetry at VDS=0

VZADD0 |
Symmetry conservation coefficient 1 |

PZADD0 |
Symmetry conservation coefficient 2 |

* available only in version 1.1.0