The shifted-rectangle approximation for simplifying the analysis of ion-implanted MOSFETs and MESFETs
TL;DR: In this article, a shifted-rectangle approximation (SRA) was proposed to obtain a quadratic function of the gate controlled depletion charge for the V d -Q d characteristics of implanted FETs.
Abstract: The ability to express the depletion layer voltage drop V d in terms of a quadratic function of the gate controlled depletion charge Q d is shown to be a key to the success of analytical modelling of ion-implanted MOSFETs and MESFETs. It is shown that such a quadratic function for the V d - Q d characteristics of implanted FETs can be obtained by approximating the implanted doping profile by a “shifted-rectangle” profile whose parameters can be derived directly from implantation parameters. It is also shown that the shifted-rectangle approximation (SRA) is not just an artifice for simplicity but accurately conserves the actual Q d , V d and depletion width conditions of both shallow and deep implanted Gaussian shaped doping profiles. The SRA simplifies the analysis of multiple-implanted devices and can be considered to be a basic approximation to be used along with the depletion and gradual channel approximations for a simple and accurate analysis of the non-uniformly doped FETs.
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TL;DR: In this paper, a conformal mapping technique is used to analytically solve the two-dimensional Poisson equation, whereby inhomogeneous substrate doping is taken into account, for the geometry and voltage dependence of threshold voltage and for the subthreshold behavior of short-channel MOSFETs.
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.
71 citations
TL;DR: 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.
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 .
14 citations
TL;DR: In this article, it was shown that the behavior of the charge and position of the 2-DEG as a function of gate-source and drain-source voltages can be regarded as a simple buried-channel (BC) MOSFET.
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.
12 citations
01 Jan 1995
TL;DR: In this paper, a physics-based, compact model for the threshold voltage shift in short-channel MOSFETs is presented, which is based upon a new theoretical approach in MOS modeling.
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.
2 citations
01 Jan 1996
TL;DR: In this article, the feasibility for developing imagers in Sic through the fabrication and demonstration of a buried channel CCD linear shift array was investigated, and the MOS field effect family was studied.
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
2 citations
Cites methods from "The shifted-rectangle approximation..."
...In order to better utilize the simpler model, device researchers have even tailored multiple implants to make the resulting profile as rectangular as possible [83], [84]....
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TL;DR: In this article, the authors defined the threshold shift due to ion implantation as the shift in curves of N ǫ versus V G, where N Ã is the inversion layer carrier density per unit area and V G is the gate bias.
Abstract: The threshold shift due to ion implantation may be defined as the shift in curves of N_{\inv} versus V G , where N_{\inv} is the inversion layer carrier density per unit area and V G is the gate bias. This definition corresponds to the experimental shift of current versus gate bias curves, because current is proportional to N_{\inv} in the linear regime of operation of the MOSFET (metal-oxide-semiconductor field-effect transistor). In addition, the shift in N_{\inv} versus V G curves is not sensitive to the value chosen for N_{\inv} , provided this value is not so low as to fall within the weak-inversion, subthreshold regime. Therefore, this definition of threshold shift avoids the use of arbitrary criteria for threshold, such as 2φ B (φ B = bulk Fermi level measured from midgap), which have uncertain meaning in implanted structures. Here, the use of N_{\inv} versus V G curves is made practical by introduction of a simplified method of calculation. It is shown how to evaluate N_{\inv} and V G without using a numerical solution of Poisson's equation, by invoking a charge-sheet approximation for the inversion layer and a modified depletion approximation for majority carriers. The resulting formulation applies provided the profile does not vary rapidly within a Debye length of the depletion edge, and provided most of the implant extends beyond the inversion layer. Threshold shift is shown to depend primarily upon the zero-order and first-order moments of the excess surface charge the implant has introduced into the depletion region of the device (the dose and centroid of this portion of the implanted charge). For fully depleted implants, a simple equivalent delta-function implant with the same dose and centroid as the real implant can be used to find threshold, and calculations can be made on a programmable pocket calculator. For a partially depleted implant, the moments of the depleted portion of the implant are needed. In addition, built-in junction effects can become significant. Comparison of the delta-function approach with a slightly more complex calculation for Gaussian implants is made. For fully depleted implants, agreement is complete. For partially depleted implants, a good estimate is obtained by introducing the ideas of "effective dose" and of "clamping of the depletion edge."
35 citations
TL;DR: In this paper, the effects of the substrate resistivity are considered for low, moderate and high doping concentrations, and it is shown that only when the channel thickness is much smaller than the oxide thickness the device saturation characteristics can be approximated by a square function of the gate voltage.
Abstract: This paper analyses the d.c. and small-signal a.c. characteristics of MOS-FETs with a built-in conducting channel that can either be completely depleted or enhanced in conductivity by carrier accumulation. The effects of the substrate resistivity are considered for low, moderate and high doping concentrations. It is shown that only when the channel thickness is much smaller than the oxide thickness the device saturation characteristics can be approximated by a square function of the gate voltage. Results of small-signal calculations are presented for both generalized and specific device structures which depict the gate voltage dependence of the gate and substrate transconductance and the input capacitance parameters for both depletion and accumulation mode of operations. A small signal equivalent network model is proposed which can be used for the calculation of high-frequency device limitations.
24 citations
TL;DR: In this paper, an accurate model for an n-channel, silicon gate, depletion mode insulated gate field effect transitor (IGFET), in configuration most often used in LSI design (VGS=OV), is derived from basic semiconductor charge analysis.
Abstract: An accurate model has been described for an n-channel, silicon gate, depletion mode insulated gate field effect transitor (IGFET), in configuration most often used in LSI design (VGS=OV). The model is derived from basic semiconductor charge analysis, approximating the profile of the redistributed implanted impurities in the channel. Excellent agreement with experimental results is shown. This model has the potential to accurately predict the charge capacity behavior in storage cells, in very high density random access memories, where “pseudo depletion mode” devices are used.
23 citations
TL;DR: In this article, a new doping transformation procedure for the modeling of arbitrarily doped enhancement-mode MOSFET's is presented based on conservation of charge and electrostatic energy in the depletion region along with the conservation of surface potential and depletion width.
Abstract: A new doping transformation procedure for the modeling of arbitrarily doped enhancement-mode MOSFET's is presented. The procedure is based on conservation of charge and electrostatic energy in the depletion region along with the conservation of surface potential and depletion width. The transformation can be extended to short-channel MOSFET's using a charge sharing approximation. Experimental results obtained on n-well CMOS devices with effective channel lengths down to 1.5 µm are used to verify the validity of the models for threshold voltage, drain conductance, and drain current.
23 citations
TL;DR: In this paper, a novel analytical model for the threshold voltage of long-channel MOSFETs with implanted channels is presented, where the analytical difficulty posed by the gaussian function to solve Poisson's equation has been removed by the use of a new integrable function that closely fits with the Gaussian and a doping transformation based on a piecewise averaging approximation.
Abstract: A novel analytical model for the threshold voltage of long-channel MOSFETs with implanted channels is presented. The model is simple from a computational point of view and yields accurate values of the threshold voltage, if the implant profile is gaussian. The analytical difficulty posed by the gaussian function to solve Poisson's equation has been removed by the use of a new integrable function that closely fits with the gaussian and a doping transformation based on a piecewise averaging approximation. The use of this function opens the way to a new technique of modelling semiconductor devices with gaussian doping. The threshold voltage and the profiles of potential and electric field inside the channel are calculated from their closed-form expressions for different doping and biasing conditions. The accuracy of the model is established by comparison with the results of the numerical solution for a gaussian doped channel and some experimental results. The possibilities of extending the model to short-cha...
21 citations