R. Howes

A small-signal model for the frequency-dependent drain admittance in floating-substrate MOSFET's

The frequency-dependent drain admittance of silicon-on-sapphire (SOS) MOSFETs is examined from the perspective of the circuit designer. Measurements of small-signal drain characteristics as a function of frequency, bias conditions, and device geometry, which have major implications for analog circuit design, are presented. These are explained in terms of a small-signal circuit model. Physical explanations for the observations are given and the poles and zeros of the model identified to assist designers carrying out hand calculations with easily manipulated expressions. Frequency-dependent thermal effects are discussed. It is shown that similar effects can be expected in other SOI technologies. >

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>>

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>>

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>>