Characterization of Surface Topography

A method for extracting values of oxide and interface charge from the current-voltage (I-V) characteristics of long-channel MOSFETs is described. The one-dimensional charge-sheet model developed by Brews provides the basis for the I-V characteristics. The I-V characteristics given by this model are optimized with respect to a set of experimental data for an irradiated devrice with the flatband voltage and the mobility the only free parameters. Simple relationships between these parameters and the radiation-induced interface and oxide charge are assumed.

Charge-Sheet Model Fitting to Extract Radiation-Induced Oxide and Interface Charge

Carrier transport equations for analysis of semiconductor devices fabricated in materials with multiple band minima, such as GaAs, are presented. An approach is taken in which the carrier density is conserved and an approximation to the distribution function in terms of quasi-Fermi potentials, carrier temperatures, and other fixed parameters is used that satisfies the particle energy and temperature distributions for each valley in the material. A model of a GaAs MESFET, which illustrates the importance of the physical effects and achieves reasonable agreement with experiment without use of adjustable parameters, is presented as an example. >

Hydrodynamic carrier transport in semiconductors with multiple band minima

An approximate analytical solution of Poisson's equation for the short-channel MOSFET operating in the subthreshold regime is presented. The analysis, developed by using the weighted-residual method, enables us to achieve a surface potential distribution along the channel of the device very close to that obtained with two-dimensional numerical simulation. It predicts a dependence of the threshold voltage on channel length, oxide thickness, substrate doping, junction depth and drain and substrate voltages in very good agreement with 2-D analysis and with available experimental data. Also the methods of this work, which yield a gain, for threshold predictions, of a factor of about 103 in CPU time with respect to numerical modeling, seems particularly suited for statistical modeling. Finally, as an example, probability distributions of the threshold voltage in short-channel MOSFETs induced from a gaussian distribution of device and process parameters are presented.

A new analytical and statistical-oriented approach for the two-dimensional threshold analysis of short-channel MOSFETs

Earlier models have successfully modeled currents associated with device degradation caused by channel hot electrons. In this work, a high accuracy two-dimensional model of a silicon MOSFET is combined with a model of the SiO 2 - Si interface which includes both the energy dependence of the interface traps within the silicon bandgap and the dependence of the oxide charge and the interface traps along the channel of the transistor. This model allows modeling of the effects of channel hot electrons on the subthreshold, linear, and saturation region after injection of the device to be modeled without introducing free parameters.

Two-dimensional modeling of channel hot-electron effects in silicon MOSFETS

When short-channel MOSFET transistor models are compared to experimental data, the uncertainty in some of the physical input variables often requires that some of the input variables be adjusted to fit the data. This uncertainty is increased by a lack of knowledge of process sensitivity information on critical parameters. These uncertainties have been eliminated using a two-dimensional finite-element model of a MOSFET with no free parameters. The model is compared to four self-aligned silicon-gate n-channel MOSFET's with channel lengths of 0.80, 1.83, 2.19, and 8.17 µm. The 0.80, 1.83, and 8.17-µm devices have phosphorus sources and drains. The 2.19-µm device has an arsenic source and drain. These devices span the range of channel lengths from a short-channel device, totally dominated by velocity saturation and source-drain profile shape, to a long-channel device, well characterized by a long-channel model. Using the data obtained from the measurements described in this work, it is possible to model the drain current for all of the transistors studied without adjustable parameters. Transistors with 0.80-µm channel length differ in model input from those with 8.17-µm channel length only in the length of the polysilicon gate. If sufficiently accurate parameters are available, these methods allow the characteristics of submicrometer transistors to be predicted with ±5-percent accuracy. These simulations show that the observed short-channel effects can be accounted for by existing mobility data and a simple empirical model of these data. Triode and saturation effects are dominated by two-dimensional drain field penetration of the channel region. Subthreshold effects are caused by distortion of fields in the entire channel region by the drain field.

J.L. Blue

Papers

High-accuracy physical modeling of submicrometer MOSFET's