The shifted-rectangle approximation for simplifying the analysis of ion-implanted MOSFETs and MESFETs

Abstract: In this paper we present a new theoretical approach in MOS modeling to derive analytical, physics-based model equations for the geometry and voltage dependence of threshold voltage and for the subthreshold behavior of short-channel MOSFETs. Our approach uses conformal mapping techniques to analytically solve the two-dimensional Poisson equation, whereby inhomogeneous substrate doping is taken into account. The presented model consists of analytical equations in closed form and uses only physically meaningful parameters. Therefore, the results are not only useful in circuit simulators but also in calculations of scaling behavior, where planned processes can be investigated. Comparison with numerical device simulation results and measurements confirm the high accuracy of the presented model.

Abstract: In circuit design and device scaling investigations, there is still a demand for improved analytical models of MOSFETs with less fitting parameters and a good scalability. In this paper, PREDICTMOS-a predictive compact model for structure oriented simulation of MOS devices is presented which has been developed by use of strongly physics-based model equations. For threshold voltage, surface potential in weak inversion, and currents in strong inversion including the saturation regime, the equations have been derived using our recently published conformal mapping techniques for solving the two-dimensional Poisson equation, and a new way to solve the transistor current differential equation. They make use of real structural parameters without any need of physically meaningless fitting parameters. This results in a strong link between electrical parameters and the process and layout data of the device and an excellent scalability while keeping physical insight. PREDICTMOS has been implemented in the ELDO circuit simulator. Its results in comparison with numerical device simulations and measurements show good agreement down to dimensions of 0.1 μm .

Abstract: It is shown that, from the point of view of the behavior of the charge and position of the Two-Dimensional Electron Gas (2-DEG) as a function of gate-source and drain-source voltages, the complex High Electron Mobility Transistor (HEMT) can be regarded as a simple Buried-Channel (BC) MOSFET. Thus, the characteristics of a HEMT, namely channel charge and capacitance/transconductance as a function of gate voltage below and above threshold are akin those of a BC MOSFET. Hence, there are discrepancies in the conventional Surface Channel MOSFET-like approach to HEMT modeling. Existing simple BC MOSFET dc and ac models can be used for on-paper analysis and computer aided simulation of HEMT devices and circuits, if the HEMT is represented by an equivalent BC MOSFET as derived in this paper. The new representation can be useful for modeling of short-channel HEMT phenomena.

Abstract: In this paper we present a physics-based, compact model for the threshold voltage shift in short-channel MOSFET’s, which is based upon a new theoretical approach in MOS modeling. This method uses conformai mapping techniques to solve the 2D Poisson equation in the space charge region underneath the gate and considers inhomogeneous doping profiles therein. The derived model equations appear in closed form and require only two physical fitting parameters related to a geometry and a doping approximation. A comparison with numerical device simulations reveals a high degree of accurateness down to channel lengths of 0.2µm.

Abstract: Silicon carbide is a wide bandgap semiconductor that is well suited for high power, high temperature electronic devices due to its remarkable electronic and thermal properties Photosensitive devices in the 6H polytype of Sic have also been demonstrated, showing high sensitivity in ultraviolet wavelengths near 270 nm Furthermore, the native oxide on Sic is silicon dioxide, meaning that SIC can be thermally oxidized to form a high quality gate dielectric, making metal-oxide-semiconductor (MOS) devices possible These qualities make silicon carbide ideal for constructing UV sensitive CCD imagers 'This work investigates the feasibility for developing imagers in Sic through the fabrication and demonstration of a buried channel CCD linear shift array Several elements of the MOS field effect family were studied With careful surface preparation and device processing techniques, SiC/silicondioxide interfaces have been ameliorated to achieve surface state densities below 2e l1 per-centimeters-squared and electron surface mobilities above 40 centimeters-squared-per-volt-second Buried channel MOSFETs were fabricated with ion implantation of nitrogen at elevated temperatures and have functioned with electron mobilities in excess of 180 centimeters-squared-pervolt-second, which shows an advantage of using the buried channel structure Studies of capacitance characteristics of the buried channel devices hold good agreement with a general one-dimensional depletion model A double polysilicon level, overlapping gate process was adapted to the SiC/MOS system A four phase buried channel CCD shift register was built and operated in ,the pseudo-two phase configuration at room temperature Device clocking frequencies were limited to 30 kHz by slow charge readout techniques, but higher speeds have been estimated In this frequency range, charge transfer efficiencies were probably dominated by carrier trapping in bulk states, which may be present due to ion implantation Recommendations for improvement of device performance and methods of integrating the CCD with UV photodetectors are given

Abstract: A new short-channel threshold voltage model based on an analytic solution of the two-dimensional Poisson equation in the depletion region under the gate of an MOS transistor (MOSTs) is presented. A simple closed-form expression for the variation of threshold voltage as a function of drain voltage, substrate bias, channel length, oxide thickness, and channel doping is derived. An exponential dependence on channel length and a linear dependence on drain and substrate biases is prediced for the reduction in the short-channel threshold voltage. These results are in qualitative and quantitative agreement with simulated and experimental results reported in literature. The predictions for the threshold voltage and subthreshold drain current are in close agreement with measured characteristics of MOS transistors down to submicron dimensions. The closed-form expressions for the threshold voltage and subthreshold drain current are well suited for circuit simulation and for determining performance limits of MOSTs.

Abstract: A model is developed for an ion-implanted MESFET device in terms of the range and straggle parameters of the Gaussian profile. Reasonable approximations are introduced to obtain a closed-form solution for the linear and triode regions of operation. Simplified square-law approximations are obtained under certain conditions and the validity of these forms is discussed. The simplified expressions are compared with those of the MOSFET and parallels are drawn. Data are presented to confirm the dc results of the theory over a wide range of implant parameters and demonstrate the value of the model for computer-aided design applications.

Abstract: This paper describes analytical models for the calculation of the current-voltage characteristics of ion-implanted GaAs FET's. The models, which take into account backgating, capping, the source and drain series resistances, and the output conductance, provide simple analytical expressions for the current-voltage characteristics and are quite suitable for the parameter acquisition and computer-aided design of GaAs FET's and IC's. In particular, the effective implanted charge and, hence, the activation efficiency may be deduced from the measured pinchoff voltage. The theory may be also used for optimization of doping profiles of GaAs FET's. The results of the calculation are in good agreement with experimental data.

Abstract: Using simple charge-voltage relationships, a four-terminal model is developed for the depletion-mode IGFET. Various conditions which can coexist at the surface, such as accumulation, depletion, and inversion, are taken into account. The implanted channel is approximated by a box profile. The basic model elements, namely, the source-drain transport current and the various charging currents, are explicitly given in terms of known processing data and implanted channel parameters. Device threshold voltage, drain saturation voltage, and conditions for surface inversion are explicitly given as a function of these parameters.

Abstract: A simple and accurate semi-empirical model for the threshold voltage of a small geometry double implanted enhancement type MOSFET, especially useful in a circuit simulation program like SPICE, has been developed. The effect of short channel length and narrow width on the threshold voltage has been taken into account through a geometrical approximation, which involves parameters whose values can be determined from the curve fitting experimental data. A model for the temperature dependence of the threshold voltage for the implanted devices has also been presented. The temperature coefficient of the threshold voltage was found to change with decreasing channel length and width. Experimental results from various device sizes, both short and narrow, show very good agreement with the model. The model has been implemented in SPICE as a part of the complete d.c. model.