H. Tian

Analysis of delta-doped and uniformly doped AlGaAs/GaAs HEMT's by ensemble Monte Carlo simulations

Transport properties and device performance of delta-doped and uniformly doped AlGaAs/GaAs high electron mobility transistors (HEMTs) with identical threshold voltages and gate capacitors are investigated using two-dimensional self-consistent ensemble Monte Carlo simulations. The model includes the effects of real-space transfer and carrier degeneracy, as well as the influence of DX centers and surface states. A one-to-one comparison of simulation results for the two devices demonstrates superior performance for the delta-doped HEMT and provides a physical basis for the observed improvements. In particular, the delta-doped HEMT maintains its superior device performance as gate bias is increased. Reasons for these improvements are reported. >

Construction of higher‐moment terms in the hydrodynamic electron‐transport model

A critical step in the development of all hydrodynamic transport models (HTMs), derived from moments of the Boltzmann transport equation, is the introduction of accurate closure relations to terminate the resulting infinite set of macroscopic equations. In general, there are a number of resulting integral terms that are highly dependent on the form of the true electron distribution function. The so‐called heat flux term is one very important higher‐moment term that requires attention. Methods for the accurate construction of an improved heat‐flux model are presented. In this construction, a higher‐moments approach is combined with a unique definition of electron temperature (i.e., based upon an ansatz distribution) to investigate the effects of conduction‐band nonparabolicity and distributional asymmetry. The Monte Carlo method has been used to evaluate the resulting model closures and to study microscopic electron dynamics. These investigations have identified an important relationship between a particul...

Electron velocity‐field characteristics of In0.52Al0.48As

Theoretical results of electron transport in n‐type In0.52Al0.48As are presented. The transport properties of this important semiconductor were obtained using the Monte Carlo method. In particular, velocity‐electric field characteristics for different temperatures and doping concentrations in bulk In0.52Al0.48As are calculated for the first time. Physical parameters for In0.52Al0.48As (which is lattice‐matched to InP and In0.53Ga0.47As) were obtained based on interpolation of available experimental and theoretical results for InAs, AlAs, and In0.75Al0.25As. Our study suggests that In0.52Al0.48As has electron transport properties which are comparable to and complimentary with those of other materials lattice‐matched to InP.

Novel charge injection transistors with heterojunction source (launcher) and drain (blocker) configurations

The results of a theoretical study of novel charge injection transistors (CHINTs) with heterojunction source and drain are presented. The proposed device structures employ a wide band‐gap (with respect to the channel) material as the device source and/or drain regions, in contrast to the conventional, homojunction source (drain) CHINT structure. It is demonstrated that the spatial location of real‐space transfer (RST) is strongly dependent on the initial energy of injected electrons in these devices. The introduction of source and drain heterojunctions serves for enhancing the RST effect and for the blocking electrons which constitute leakage current. Results from two‐dimensional, self‐consistent ensemble Monte Carlo simulations reveal that the proposed CHINTs feature increased current drive capability, reduced drain leakage current, and faster switching speed.

Two-dimensional analysis of short-channel delta-doped GaAs MESFETs

Key design parameters for delta-doped GaAs MESFETs, such as delta doping profile, top layer background doping density, and scaling of lateral feature size, are investigated using a two-dimensional numerical simulation. A three-region (delta-doped conducting channel, top layer, and substrate) velocity-field relation is implemented in the model as appropriate for the particular device structure which is simulated. Simulation results show excellent agreement with a fabricated 0.5- mu m gate-length delta-doped GaAs MESFETs based on atomic layer epitaxy material. An extrinsic transconductance of 370 mS/mm and a drain-source current of 270 mA/mm are obtained for typical devices, and the maximum transconductance is as high as 400 mS/mm. These are the best DC results yet reported for 0.5- mu m gate-length delta-doped GaAs MESFETs. Considerations of design and optimization are discussed in terms of threshold voltage sensitivity, transconductance, current drive capability, and cutoff frequency, based on both simulation and experiment results. >

An evaluation of super-steep-retrograde channel doping for deep-submicron MOSFET applications

Performance and reliability of deep-submicron MOSFETs employing super-steep-retrograde (SSR) channel doping configurations are examined using self-consistent Monte Carlo and drift-diffusion simulations. It is found that SSR channel doped MOSFETs provide increased current drive and reduced threshold voltage shift when compared with conventional MOSFET structures. However, they also display a relatively higher substrate current and interface state generation rate. The physical mechanisms of performance enhancement/degradation and design tradeoffs for SSR channel doped MOSFETs are discussed. >

Novel heterojunction real-space transfer logic transistor structures: a model-based investigation

Ensemble Monte Carlo simulations are employed in order to explore the feasibility of a novel real-space transfer logic transistor (RSTLT) structure. The operational principles of the proposed RSTLT are based on the concept of hot electron real-space transfer (RST), including the fact that the spatial location of electron RST is determined by applied bias and heterointerface energy barrier height in a multiterminal heterojunction microstructure. The results of two-dimensional, self-consistent steady-state and transient simulations demonstrate that the proposed RSTLT features ultrafast current switching which can be used to realize NOT/EQUIVALENT logic functions in a single heterojunction device. The logic operation is easily extended to NOR/AND functions. A conservative estimate of the characteristic delay time for current switching is approximately 3 ps in the proposed RSTLT structure. >

An investigation of the effects of doping profile variations on AlGaAs/GaAs high electron mobility transistor performance

We present results from a theoretical study of the influence of doping profile variations on the performance of delta‐doped AlGaAs/GaAs high electron mobility transistors (HEMTs). An ensemble Monte Carlo simulation coupled with a self‐consistent solution of the two‐dimensional Poisson equation is used to investigate HEMTs which employ both single and double delta‐doped profiles with varying doping configurations. The calculated results reveal that single delta‐doped HEMTs designed with identical threshold voltages exhibit improved device behavior when thinner delta‐doped layers with more heavily doped concentrations are utilized. For double delta‐doped HEMTs with an identical total doping in the AlGaAs layer, improved threshold voltage control is obtained as the spacing between two delta‐doped layers increases. However, this increase in spacing also causes a degradation in transconductance, cut‐off frequency, and switching time. As gate bias increases, the dependence of device performance (or degradation)...

Effects of profile doped elevated source/drain structures on deep-submicron MOSFETs

Abstract Computer simulations have been carried out to systematically evaluate and compare device characteristics for various profile doped elevated source/drain (ESD) 0.25 μm channel-length MOSFET structures. In particular, characteristics for MOSFETs with a gradual profile doped ESD and for MOSFETs with an abrupt profile N + - N − doped ESD are examined in detail. Design considerations for key parameters related to the ESD formation (such as sidewall oxide width, elevation height and source/drain doping profile) and their influences on device characteristics are discussed. The results show the importance of ESD design parameters and structural optimization. They also indicate that the proposed gradual profile doped ESD MOSFETs can be as effective as the abrupt profile N + - N − doped ESD MOSFETs in achieving overall performance enhancement and reliability for deep-submicron device applications.

Efficient ohmic boundary conditions for the Monte Carlo simulation of electron transport

The development of macroscopic transport models, accurate for studying hot-electron transport in semiconductors, involves a direct consideration of higher-moment terms. All hydrodynamic transport models (HTMs), derived from moments of the Boltzmann transport equation, require the introduction of closure relations to terminate the resulting infinite set of macroscopic equations. These closure relations are used as analytical approximations to distributional-dependent integral coefficients. The most popular theoretical approach employed for the construction and evaluation of these higher-moment transport-parameter closures is the Monte Carlo (MC) method. Since the MC method is computationally intensive, the discovery and implementation of efficient MC modeling techniques (either numerical or physical) is of significant value. This paper reports on a relationship between device boundary conditions and the convergence range of higher-moment terms in time-independent MC simulations. Specifically, a set of ohmic BCs which offer computational advantages is presented. This particular mathematical approach, which allows for two degrees of freedom, is shown to be more efficient in generating the full electron distribution function than conventional BC methods (i.e., strictly equilibrium-based). >