K.G. Nichols

An SOS MOSFET model based on calculation of the surface potential

A circuit simulation model is presented suitable for the design of analogue and digital SOS MOSFET integrated circuits. Both the drift and diffusion components of channel current are modeled, which are computed from the surface potentials at the drain and source ends of the channel. The surface potential function varies continuously from subthreshold to strong inversion allowing a smooth transition of device conductances and capacitances at the threshold voltage. Charge is conserved in the model formulation yielding reliable simulation results in transient analysis. The model has been implemented in the SPICE program, together with important extrinsic elements such as impact ionization current, pn-junction current and capacitances, and substrate resistance. The pn-junction current expression includes a physical formulation for the drain leakage current. The influence of temperature on device characteristics is included, making the model valid from /spl minus/55 to 125/spl deg/C. Simulation results are compared with measured dc device characteristics showing considerable improvement over bulk MOS models in predicting the drain conductance. In subthreshold, the model predicts the observed increase in inverse subthreshold slope with drain bias for n-channel devices. Transient simulations show that capacitive coupling from drain, gate and source nodes can strongly influence the floating substrate potential. The model has been successfully applied to the design of analogue SOS circuits. >

A charge-conserving silicon-on-sapphire SPICE MOSFET model for analogue design

A CAD model for silicon-on-sapphire (SOS) MOSFETs that is suitable for analogue circuit design is presented. The model accounts for drift and diffusion components of channel current and is therefore continuous from subthreshold to strong inversion. The channel current components are specified in terms of the surface potential drain and source, which is calculated without the need for an iterative solution. The kink effect is included in both the drift and diffusion components and predicts the observed increase in subthreshold slope with drain voltage. An equivalent circuit is presented which includes substrate resistance. Implementation of the nine-node model in SPICE2 has been achieved and sample simulation results are presented and compared with device and circuit measurements.<<ETX>>

Confidence in mixed-mode circuit simulation

Abstract Mixed-mode simulation is a compromise between an accurate and expensive modelling technique (that solves circuit equations), and a less accurate, but cheaper, technique (that manipulates a finite algebra). The combination of these modelling regimes requires a mapping between two ‘state vectors’ that is neither unique nor context-independent. To address this, a duple, {state, confidence} , is associated with each node at each timepoint. Confidence is a set of flags that is used to indicate that the simulator has made a decision with which the user may be unhappy. Graphical postprocessing allows the user to scan for combinations of flags. This creates a ‘flag-browsing’ capability that allows rapid access to areas of uncertainty in the time histories.

A SOS MOSFET SPICE model for confident analogue circuit design

The authors present details of a charge-based circuit simulation model which is continuous from subthreshold to strong inversion and is therefore suitable for analog design. The model has been implemented in the SPICE2 program. The model equivalent circuit is shown, and a set of simulated subthreshold characteristics for an n-channel device is presented. The current is smooth and continuous from cutoff through subthreshold to strong inversion, and the influence of the kink effect can be seen in increasing the inverse subthreshold slope with increasing drain bias. Sample capacitance characteristics are illustrated, and these are also smooth and continuous into subthreshold, allowing accurate small-signal simulation and enhancing convergence in transient simulation. Good modeling of the drain conductance is achieved throughout the kink region, ensuring correct prediction of circuit small-signal gain. In the frequency domain, the model predicts the correct behavior of drain admittance, which is particularly important in analog design. The typical measured threshold shift dependence on total radiation dose is also illustrated.<<ETX>>

New concepts of SOI modelling for use in circuit simulator

In this paper some new concepts, which allow us to solve surface potentials for the two surfaces of the double gate controlled SOI device and to carry out general modelling of the device, are presented. Some simulation results from SPICE3e2 are also demonstrated.<<ETX>>

A charge-conserving SOS MOSFET model including radiation effects for circuit simulation

A circuit simulation model for SOS MOSFETs is presented which is valid from cutoff to strong inversion. The unique floating substrate effects are accounted for by modelling the substrate potential, making the model suitable for both digital as well as analogue circuit design. The model has been implemented in the SPICE2 program and sample simulation results are presented. Total dose radiation effects on threshold voltage and leakage current have also been included within the simulation environment.<<ETX>>

Simulation of large nonlinear circuits

Abstract This paper discusses some of the techniques currently being researched to resolve the problem of time-domain simulation of large nonlinear systems such as those that may be realized on VLSI circuits.