Hydrodynamic carrier transport in semiconductors with multiple bandminima

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     

Charge storage in InAlAs/InGaAs/InP floating gate heterostructures

Charge retention in floating gate InAlAs/InGaAs/InP field effect transistors is limited by lateral electron motion along the storage channel, a different direction for motion than found for AlAs/GaAs devices. Storage times as a function of temperature for the InP based alloy devices are reported and compared with similar AlAs/GaAs devices by using Poisson equation models.<>     

Compensation-circuit to improve radiation-hardness in GaAsdigital-circuits, based on X-ray studies

Earlier results have shown that GaAs devices do not exhibit appreciable degradation up to a radiation dose of nearly 10⁸ rad (GaAs). The results of this work suggest that GaAs devices and circuits are sensitive to radiation exposure at dose levels below 10⁸ rad(GaAs). Degradation was observed in E-MESFET and D-MESFET parameters and in circuit performance for devices which were designed and fabricated in a 1.2 μm GaAs process, when exposed to varying doses of 1.49 keV X-rays in the range 40-65 Mrad (GaAs). The degradation is attributed to the change in the properties of the MESFET channel region, caused by the transport of the atomic hydrogen from the passivation layer to the channel. A compensation circuit, based on the observed behavior of radiation effects on GaAs devices, has been designed to improve the radiation insensitivity of GaAs (E/D) based circuits under SPICE (Simulation Program with IC Emphasis) simulated conditions. Its usefulness is demonstrated through a DCFL inverter circuit up to nearly 10⁸ rad (GaAs) dose level. The results of this work can be used in the design of complex-function radiation-insensitive DCFL based circuits     

异质谷间转移电子器件的计算机模拟

使用弛豫时间近似求出了描述两能谷间电子交换的方程。在此基础上建立了双能谷电子的连续性方程和油松方程。对具体的ＧａＡｓ／ＡｌｘＧａ１－ｘＡｓ异质谷间转移电子器件结构求解了这些方程组，得到了直流工作时器件内的电场分布、双能谷中载流子的布居以及两个能谷的电流分布。这些结果正好和常规Ｇｕｎｎ器件相反，说明了两种器件的不同工作机理。最后通过对低Ａｌ组份势垒结构的计算说明了Ｘ谷电子注入对器件工作的重要作用。     

A new decoupled algorithm for nonstationary, transient simulationsof GaAs MESFETs

Simulation of devices for which nonlocal, hot carrier transport cannot be ignored requires solution of the Poisson equation and at least the first three moments of the Boltzmann transport equation or the use of Monte Carlo techniques. These equations form nonlinear, coupled, time-dependent partial differential equations. In conventional decoupled solvers, decoupling of the equations puts a limit on the maximum allowable time step Δt, which should be kept smaller than the dielectric relaxation time τ_d of the material. This constraint makes these solvers very inefficient, especially for obtaining steady-state solutions. A highly efficient decoupled numerical algorithm which allows Δt as large as 20-50 times τ _d is presented. Results of simulations of GaAs MESFETs using both a conventional and the new decoupled solver and CPU time taken on a CRAY Y-MP by the two solvers are discussed     

Scaled performance for submicron GaAs MESFET's

Scaling schemes for GaAs MESFET's below the submicron gate length are proposed. The corresponding switching times are calculated accurately down to 0.25µm gate length devices using an ensemble Monte Carlo simulation program. It is demonstrated that the proposed scaled devices offer ultrashort switching time due to the nonstationary carrier transport effects.     

A complementary heterostructure field effect transistor technologybased on InAs/AlSb/GaSb

A complementary heterojunction field effect transistor technology based on the InAs/AlSb/GaSb system is proposed. The structure is formed by the vertical integration of InAs n-channel and GaSb p-channel HFET devices. The superior transport properties of electrons in InAs and holes in GaSb and their band offsets to AlSb or AlSbAs yield devices with transconductances much greater than AlGaAs/GaAs n- and p-channel HFETs. It is shown that a complementary circuit fabricated from these devices could provide room-temperature performance up to six times greater than that predicted for AlGaAs/GaAs complementary circuits     

Near ballistic electron transport in GaAs devices at 77°K

The effect of collisions on near ballistic electron transport in short GaAs terminal devices at 77°K is analyzed in the frame of the model based on the equations of momentum and energy balance where momentum and energy relaxation times are fit to agree with the results of Monte Carlo calculation for static constant electric fields. Solving the equations by the iteration technique yields the criterion of the ballistic transport $\text{(L}{}_{\mathrm{(\mu m}}\text{) ? 0.44 μ (m}{}^2\text{/V.s) V}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(v)}$ for GaAs at low voltages). The computer solution is used to obtain current-voltage characteristics, field, energy, velocity and voltage distributions for GaAs devices at 77°K. The results of the calculation show that the ballistic effects are dominant at 77°K even at relatively high doping levels (such as 10¹⁶ cm^?3) for short devices (～ 0.2 μm long). These effects lead to a higher electron velocity at low voltages and could be utilized in building high speed GaAs integrated circuits.     

Ballistic transport in GaAs

Ballistic transport in GaAs has been studied using an ensemble Monte Carlo simulation. Duration and spatial extent of ballistic transport for a hot electron distribution can be defined from such studies. Mean displacement of the ensemble increases quadratically with time for a specified interval. This observation provides a phenomenological definition of ensemble ballistic transport. This phenomenological definition is compared with a theoretical definition based on time at which a significant fraction of an ensemble have experienced at least one collision. From these studies, times and distances are given for which a single-particle ballistic equation and a Langevin equation accurately describe ensemble transport in GaAs.     

Temperature dependence study of two-dimensional electron gas effecton the noise performance of high frequency field effect transistors

We present experimental evidence that the noise figure (NF) and associated gain equal to those achieved with GaAs pseudomorphic high electron mobility transistors (GaAs p-HEMT's) can also be accomplished by ion implanted GaAs metal-semiconductor field-effect transistors (GaAs MESFET's). These measured noise figure results as a function of low temperature for GaAs MESFET's and p-HEMT's clearly suggest that the transport properties of the two-dimensional electron gas in HEMT's and p-HEMT's do not make a significant contribution to the noise reduction at high frequency operation of these devices     

Accurate modeling for submicrometer-gate Si and GaAs MESFET's using two-dimensional particle simulation

Device characteristics, including nonstationary carrier-transport effects such as velocity overshoot phenomena in submicrometergate Si and GaAs MESFET's, are presented in detail by two-dimensional full Monte Carlo particle simulation. Accurate current-voltage characteristics and transient current response are successfully obtained without relaxation time approximation. Moreover, the carrier dynamics influence on device operation is clarified in a realistic device model, compared with the conventional simulation. It can be pointed out that such nonstationary carrier transport is acutely important for accurate modeling of submicrometer-gate GaAs MESFET's, but is not as important for that of Si MESFET's.     

GaAs/Si/AlAs异质结的DLTS实验研究

利用深能级瞬态谱(DLTS)技术研究了Si夹层和GaAs层不同生长温度对GaAs/AlAs异质结晶体品质的影响.发现Si夹层的引入并没有引起明显深能级缺陷,而不同温度下生长的GaAs/Si/AlAs异质结随着温度的降低,深能级缺陷明显增加,并进行了分析,得到深能级是由Ga空位引起的,在600℃时生长的晶体质量最佳.     

An assessment of approximate nonstationary charge transport modelsused for GaAs device modeling

A study of the utility of deterministic nonstationary models of charge transport in GaAs is presented. The models considered include energy and momentum conserving, energy conserving, and electron-temperature formulations. Predictions of the models are compared with results calculated using a more detailed Monte Carlo-based scattering-process-level simulation. The basis of the comparison is calculated trajectories in velocity-energy-field space for a range of time-dependent electric field forcing functions. All the nonstationary transport models considered are found to be in reasonable agreement with Monte Carlo results for all but the most extreme circumstances considered and to be greatly superior to the drift-diffusion approximation. Strengths, weaknesses, and applicability of individual models are discussed     

Quasi-two-dimensional modeling of GaAs MESFET's

A numerical simulation of GaAs MESFET structures is presented. The approach taken in this paper combines an analytical solution with a full simulation. Poisson's equation, the current continuity equation, and an electron-temperature equation are formulated in terms of a geometry factor that defines the shape of the conducting channel in the MESFET. The transport equations are then solved in one dimension and the channel geometry factor is found analytically. This method was found to be considerably faster than full two-dimensional simulations. The model has been compared to full two-dimensional drift-diffusion and energy-momentum results to determine its validity.     

Band-Structure Effects on the Performance of III–V Ultrathin-Body SOI MOSFETs

This paper examines the impact of band structure on deeply scaled III-V devices by using a self-consistent 20-band -SO semiempirical atomistic tight-binding model. The density of states and the ballistic transport for both GaAs and InAs ultrathin-body n-MOSFETs are calculated and compared with the commonly used bulk effective mass approximation, including all the valleys (, , and ). Our results show that for III-V semiconductors under strong quantum confinement, the conduction band nonparabolicity affects the confinement effective masses and, therefore, changes the relative importance of different valleys. A parabolic effective mass model with bulk effective masses fails to capture these effects and leads to significant errors, and therefore, a rigorous treatment of the full band structure is required.     

Response characteristics of trapping loss in acoustic chargetransport devices

A model of the response characteristics of trapping loss in acoustic charge transport (ACT) devices on GaAs is presented. Through the use of a computer simulation of the interactions of signal electrons with traps, the model can be used to predict the time or frequency domain responses of the device for arbitrary signal waveforms and various trap distributions. The analysis procedures are discussed in detail for two cases of interest in normal ACT operation, namely, the cases of periodic pulse and single-frequency sinusoidal input signals. For these cases, the predicted dependences of the pulse degradation and frequency response, respectively, on the trap characteristics are presented. The predicted responses are compared to experimental results and to predictions from a previous analytical model     

Barrier-limited transport in semiconductor devices

Current transport over a simple parabolic barrier is studied using the diffusion theory (dissipative transport) and the kinetic theory (ballistic transport). The solution of the diffusion equation for the barrier-limited case shows that the current density is inversely proportional to the width of the barrier. The solution of the kinetic theory, described by Euler's equation of classical fluid mechanics, gives a current density that is independent of the width of the energy barrier, and which is equal to 1.52 times the current density obtained from the thermionic emission theory. Euler's equation and the diffusion equation may be naturally combined into a single transport equation, and the current density derived from this equation shows the effects of both the ballistic and dissipative limits. The ballistic limits are of significance for the operation of the permeable base transistor and similar devices.