When self-consistency makes a difference

Compound semiconductor power RF and microwave device modeling requires, in many cases, the use of selfconsistent electrothermal equivalent circuits. The slow thermal dynamics and the thermal nonlinearity should be accurately included in the model; otherwise, some response features subtly related to the detailed frequency behavior of the slow thermal dynamics would be inaccurately reproduced or completely distorted. In this contribution we show two examples, concerning current collapse in HBTs and modeling of IMPs in GaN HEMTs. Accurate thermal modeling is proved to be be made compatible with circuit-oriented CAD tools through a proper choice of system-level approximations; in the discussion we exploit a Wiener approach, but of course the strategy should be tailored to the specific problem under consideratio

An Analytic model for high electron mobility transistors.

Thesis (Ph.D.)-University of Natal, Durban, 1986.The last six years has seen the emergence and rapid development of a new type of field effect transistor, the High Electron Mobility Transistor (HEMT), which offers improved performance in both digital and analogue circuits compared with circuits incorporating either MEtal Semiconductor (MES) or Metal Oxide Semiconductor (MOS) FETs. A new physically-based analytic model for HEMTs, which predicts the DC and RF electrical performance from the material and structural parameters of the device, is presented. The efficacy of the model is demonstrated with comparisons between simulated and measured device characteristics, at DC and microwave frequencies. The good agreement with experiment obtained with the model indicates that velocity overshoot effects are considerably less important in HEMTs than has been widely assumed, and that the electron transit velocity in submicron devices is approximately 10 cm/s, rather than around 2x10 cm/s. The Inverted HEMT, one of the major HEMT structural variants, is emphasized throughout this work because of its potential advantages over other variants, and practical results from 0.5 micron gate length Inverted HEMTs are presented

EFFECTS OF INTERNAL FIELDS IN QUANTUM DOTS

In this work we study the effect of built in electrostatic fields in Quantum Dots. Built-in electrostatic fields in Zincblende quantum dots originate mainly from--(1) the fundamental crystal atomicity and the interfaces between two dissimilar materials, (2) the strain relaxation, and (3) the piezoelectric polarization. We also study the geometric dependence of built in fields on 3 shapes namely Box, Dome and Pyramid. The main objectives are 3 fold they are (1) Explore the nature and the role of crystal atomicity at the interfaces and built-in fields (strain-field, and piezoelectric polarization) in determining the energy spectrum and the wave functions. (2) To identify the shift in the one-particle energy states, symmetry-lowering and non-degeneracy in the first excited state and strong band-mixing in the overall conduction band electronic states. (3) Finally geometric dependence of the above-mentioned phenomena. We discuss the importance atomistic effects and the need for 3 dimensional atomistic simulator NEMO 3D. We also discuss the effect of built in fields in HEMT (High Electron Mobility Transistor)

Increase in the random dopant induced threshold fluctuations and lowering in sub-100 nm MOSFETs due to quantum effects: a 3-D density-gradient simulation study

In this paper, we present a detailed simulation study of the influence of quantum mechanical effects in the inversion layer on random dopant induced threshold voltage fluctuations and lowering in sub-100 mn MOSFETs. The simulations have been performed using a three-dimensional (3-D) implementation of the density gradient (DG) formalism incorporated in our established 3-D atomistic simulation approach. This results in a self-consistent 3-D quantum mechanical picture, which implies not only the vertical inversion layer quantization but also the lateral confinement effects related to current filamentation in the “valleys” of the random potential fluctuations. We have shown that the net result of including quantum mechanical effects, while considering statistical dopant fluctuations, is an increase in both threshold voltage fluctuations and lowering. At the same time, the random dopant induced threshold voltage lowering partially compensates for the quantum mechanical threshold voltage shift in aggressively scaled MOSFETs with ultrathin gate oxides