Diffusion effects and "Ballistic transport"

The effects of both collisional drag and diffusion effects on the operation of GaAs n⁺-n-n⁺structures are determined using a momentum transport equation which includes velocity fluctuation effects. It is shown that diffusion effects are important in practical structures, and that previous interpretations of experimental results which claim to show "ballistic" transport can be seriously in error. The general limitations of single-particle transport models when used to try to understand the results of experiments, such as diode characteristic measurements which yield only ensemble averages, are discussed.     

Ballistic and tunneling GaAs static induction transistors:nano-devices for THz electronics

GaAs static induction transistors (SIT) with 10-nm scale channel and with a 100-nm channel were fabricated with molecular layer epitaxy (MLE). Area-selective epitaxy of GaAs/AlGaAs/GaAs was used for the gate. Temperature dependence of current-voltage (I-V) characteristics of the 100-nm SIT indicates ballistic injection of electrons. In the 10-nm scale SIT, electrons are transported ballistically in the drain-side electric field. Direct tunneling is responsible for the transport through the potential barrier. It is indicated by the temperature dependence and by the electroluminescence spectrum. Electron transport in the 10-nm scale SIT is nearly scattering-free. The plausible estimation of the electron transit time is 2·10^-14 s; the worst case estimation based on saturated drift velocity gives 1·10^-13 s. It makes the ISITs suitable for THz applications. Multiple area-selective MLE GaAs regrowth was used as a tool for automatic definition of the channel length     

Observation of ballistic transport in hot-electron vertical FET spectrometer using ultrathin planar-doped barrier launcher

Experimental evidence of ballistic transport by hot electron injection into a GaAs vertical FET channel using an ultra-thin planar-doped barrier has been obtained for the first time. The spectroscopy exhibited a narrow energy spread of less than 50 meV with an estimated 10% ballistic electrons.<>     

High-frequency effects of ballistic electron transport in semiconductors

The response of a semiconductor to a high-frequency electric field is studied using a simple method of electron transport which includes both ballistic and dissipative effects. Ballistic motion appears as an inductive reactance. The significance of ballistic transport can be quantitatively expressed by the Q of the free-carrier plasma resonance. It is shown that the condition Q > 1 should hold for high-purity GaAs at low temperatures and that this condition could be verified by measurements at frequencies well below the plasma frequency.     

Heterojunction vertical FETs revisited: potential for 225-GHzlarge-current operation

The high-speed operation of submicrometer Al_xGa_1-x As/GaAs unipolar heterojunction transistors is examined using two-dimensional time-dependent self-consistent ensemble Monte Carlo simulation. Careful device design can significantly increase ballistic injection over the heterojunction in steady state by eliminating retarding gate-induced space-charge reversal there. Design for optimal large-signal transient operation must also avoid gate-voltage-dependent ballistic injection. General design principles for optimizing high-speed operation are proposed. The resulting VFETs show cutoff frequencies of 225 GHz at large drain currents at 300 K, with frequency-independent two-port y parameters     

Electron transport in avalanching GaAs diodes

Magnetoresistance measurements on avalanching GaAs diodes lead to an estimate of 1.6 × 10^-15sec for the scattering time of avalanching electrons at 300K. This is consistent with the time recently calculated by Monte Carlo techniques. It confirms that impact ionization by electrons in GaAs is initiated by carriers that make several collisions in the process rather than by ballistic carriers which impact ionize without any interaction with phonons.     

Lateral space-charge effects on ballistic electron transport across graded heterojunctions

Hot electron transport across graded compound semiconductor heterojunctions has been explored using a two-dimensional formulation of the self-consistent ensemble Monte Carlo method. The Al_xGa_1-xAs/GaAs heterojunction imbedded into a vertical field effect transistor with two ohmic contacts (source, drain) and two lateral Schottky gates has been used as an example. Lateral space charges modulated by the gates are shown to control ballistic injection of electrons over the heterojunction under steady state conditions. The transient response to a gate pulse is found to be determined by carrier transit from the heavily doped source contact region into the channel. A conceptual one-dimensional section model is used to explain the Monte Carlo results.     

Orientation dependence of ballistic electron transport and impact ionisation

The orientation dependence of ballistic electron transport in GaAs is investigated using the pseudopotential band structure. It is shown that a single scattering event can permit electron impact-ionisation threshold energies to be reached in the ?111? and ?100? directions. This is in contrast to ballistic calculations, where simple thresholds do not exist in these directions.     

Current oscillations in semiconductor diodes under streaming instability conditions

The current oscillations with frequency of the order of plasma frequency in the submicron GaAs n+-n-p-p+ structure are obtained by Monte Carlo particle simulation. The streaming plasma instability responsible for the current oscillations at near ballistic transport conditions is shown.     

Npn型HBT准中性均匀基区的HHTM一维解析模型

从异质结构流体动力学模型方程（ＨＨＴＭ）出发，经适当近似，得到了Ｎｐｎ型ＨＢＴ准中性均匀基区饱和电流密度的解，定义了一个判别基区是否存在准弹道输运的判别因子─－弹道比ｒＢ，指出为了实现基区准弹道输运，应使ｒＢ《１．对（Ａｌ，Ｇａ）Ａｓ／ＧａＡｓＮｐｎ型ＨＢＴ而言，这一条件意味着基区宽度Ｗａ《１００ｎｍ。因此为了实现准弹道基区输运，应将（Ａｌ，Ｇａ）Ａｓ／ＧａＡｓＮｐｎ型ＨＢＴ的基区宽度从目前的５０～１００ｎｍ降低到３０ｎｍ以下。     

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.     

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.     

On the estimation of base transit time in AlGaAs/GaAs bipolar transistors

Electron diffusion across quasi-neutral p-type base regions representative of those used in n-p-n AlGaAs/GaAs/GaAs heterojunction bipolar transistors is investigated. Monte-Carlo simulation results demonstrate that for realistic base widths ≲ 1000 Å) electron transport cannot be described by Fick's Law. As a result, the conventional estimate of base transit time,tau_{B} = Wmin{B}max{2}/2D_{n}, will produce substantial errors for base widths typical of those employed in heterojunction bipolar transistors. Estimates based on the ballistic transport of electrons across the base are shown to significantly underestimate base transit time-even for base widths substantially narrower than those presently employed.     

Ballistic transport in semiconductor at low temperatures for low-power high-speed logic

At low temperatures, a mean free path of electrons in semiconductors may exceed device dimensions. Current-voltage characteristics, potentials, electrical field, and carrier distributions are calculated for a two-terminal device under such conditions when the electron transport is ballistic. Current-voltage characteristics of a "ballistic" FET are analyzed using an approach similar to the Shockley model. It is shown that very high drift velocities can be obtained at low voltages leading to high speed and low power consumption in possible applications in logic circuits. For example, GaAs logic devices with characteristic dimensions about a micrometer or less at 77 K will be comparable with or better than Josephson tunneling logic gates.     

Ballistic transport and properties of submicrometer Silicon MOSFET's from 300 to 4.2 K

The characteristics of submicrometer silicon MOSFET's have been measured from 300 to 4.2 K, and the mobility versus temperature and carrier velocity versus longitudinal field as a function of temperature have been plotted. Effective mobilities in 500-µm-square devices as high as 25 000 cm²/V . s at 4.2 K have been observed. Mobilities of this magnitude represent mean free path lengths that could lead to ballistic transport in submicrometer devices. Effective mobilities in 0.2-µm devices were only 800 cm²/V . s at 4.2 K due to high-field effects. The mobility versus effective channel length for 0.2-, 0.7-, and 1.7-µm devices operating at drain voltages of 0.1 V has been plotted, and it has been observed that the mobility is greatly reduced in short-channel devices. The mobility versus longitudinal field was studied, resulting in the observation that ballistic transport is inhibited by the high fields in devices operating at 0.1 V. Similar high-field effects should limit the effects of ballistic transport in high-mobility semiconductors such as submicrometer GaAs FET's Operating at nominal supply voltages.     

GaAs n+-p?-n+ ballistic structure

Potential distribution and I/V characteristics of short n+-p?-n+ devices are calculated assuming ballistic electron transport (no collisions). The results of the calculation are in good agreement with experimental results for submicrometre GaAs n+-p?-n+ structures.     

Limitations to ballistic transport in semiconductors

Limitations to the range of ballistic transport in semiconductors are discussed on the basis of electron correlation within an ensemble. It is shown that transport equations, correct in the fast transient regime, include these correlation effects.     

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.