Average Electron Drift Velocity Research Articles

Studying average electron drift velocity in pHEMT structures

Small-signal characteristics of pseudomorphic high-electron-mobility transistors based on donor–acceptor doped heterostructures (DA-pHEMTs) are compared to those of analogous transistors (pHEMTs) based on traditional heterostructures without acceptor doping. It is established that DA-pHEMTs, under otherwise equal conditions, exhibit (despite lower values of the low-field mobility of electrons) a much higher gain compared to that of usual pHEMTs. This behavior is related to the fact that the average electron drift velocity under the gate in DA-pHEMTs is significantly (1.4–1.6 times) higher than that in pHEMTs. This increase in the electron drift velocity is explained by two main factors of comparable influence: (i) decreasing role of transverse spatial transfer, which is caused by enhanced localization of hot electrons in the channel, and (ii) reduced scattering of hot electrons, which is caused by their strong confinement (dimensional quantization) in the potential well of DA-pHEMT heterostructures.

Measurement of the electron saturation velocity in an AlGaAs/InGaAs quantum well

The dependence of the average electron-drift velocity on the electric field strength is measured for a twodimensional electron gas in a quantum well of an AlGaAs/InGaAs heterostructure in the temperature range 200–400 K. It is shown that the saturation velocity varies from 1.55·107 to 1.3·107 cm/s in this temperature range and substantially exceeds the velocity in bulk gallium arsenide.

Electron transport property in wurtzite GaN at high electric field with Monte Carlo simulation

We present the results of the electron transport property in wurtize GaN using an ensemble full band Monte Carlo simulation. The data of wurtzite GaN band structure calculated with the first-principles total-energy pseudopotential method is used in the simulations. The impact ionization scattering rate is calculated based on Cartier’s method. The average electron drift velocity and the average electron energy each as a function of electric field are computed. The electron impact ionization coefficient is calculated as a function of applied electric field. The analysis of the impact ionization coefficient shows that when the applied electric field is greater than 1 MV/cm, the obvious impact ionization events occur. The analysis of the quantum yield shows that when the electron energy is greater than 7 eV, the quantum yield increases rapidly with electron energy increasing. We study the occupancy of the electrons in the eight conduction bands at the applied electric field ranging from 0 to 4 MV / cm. For the case of the low applied electric field all of the electrons are located in the 1st conduction band. With the increase of the applied electric field, some of the electrons move to high index conduction bands. For the whole range of the applied electric field, most of the electrons are located in the 1st conduction band and 2nd conduction band, a small number of the electrons are located in the 3rd, 4th and 5th conduction band, and very few electrons are located in the 7th and 8th conduction band.

Dynamic Analysis of High-Power and High-Speed Near-Ballistic Unitraveling Carrier Photodiodes at $W$-Band

In this letter, we demonstrate and analyze the high-speed and high-power performance of a back-illuminated near-ballistic unitraveling-carrier photodiode (PD) at TV-band. We utilize a three-port equivalent-circuit-modeling technique to show that the extracted average electron drift velocity in the whole epi-structure is around 5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s, which corresponds to an ultrahigh transit time limited bandwidth (~400 GHz). Such high internal bandwidth means that the demonstrated device can thus release the burden imposed on downscaling the device active area and epi-layer thickness for achieving ultrahigh-speed performance of PDs. With a collector thickness of 410 nm and an active area of 64 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , we can achieve a wide 3-dB bandwidth (120 GHz), and high saturation current bandwidth product performance (120 GHz, 24.6 mA, 2952 mAGHz) under 25 Omega loading at W-band.

Kinetic properties of surface electrons over liquid helium under strong electron-electron interaction

The mobility μ and lifetime of ground-level electrons τ⊥ are studied experimentally in a two-dimensional electron system at the surface of liquid helium at temperatures of0.4 to1.4 K and charge concentrations of (2.8–12.0) × 108 cm−2. It is shown that for fairly low temperature and high concentration where the frequency of interelectronic collisions is much higher than that of electron-ripplon ones, the so-called complete control condition is realized in the electron system, i.e., when the average drift velocity of electrons and the effective electron temperature can be introduced. This model is found to describe well the kinetic properties of surface electrons in the range of charge concentrations up to 4×108 cm−2, where the one-particle approach is no longer applicable. For the value of parameter Γ=e2(πns)1/2/T=47 that corresponds to a strong electron-electron interaction, the lifetime τ⊥ is found to increase sharply and the short-range order typical of a liquid state appears in the two-dimensional electron layer.

Small-signal characteristics of n/sup +/-Ge gate AlGaAs/GaAs MISFET's

The small-signal characteristics have been clarified by S-parameter measurements and equivalent circuit modeling. A large intrinsic transconductance of 630 mS/mm and a maximum cutoff frequence f/sub T/ of 70 GHz have been achieved for a MISFET with a gate length of 0.4 mu m. The average electron drift velocity in the channel, evaluated from the f/sub T/, was as high as 1.7*10/sup 7/ cm/s. In obtaining an equivalent-circuit model, a gate conductance parallel to the gate-source capacitance is introduced to take into account the gate forward current of normally-off FETs The gate conductance does not cause the f/sub T/ of the MISFET to deteriorate due to a small gate forward current at a large gate bias, in contrast to GaAs MESFETs. >

Monte Carlo simulations of repeated velocity overshoot structures

We have performed self-consistent Monte Carlo simulations on repeated velocity overshoot structures. From our calculations, we may assess the potential of such structures to enhance the average drift velocity of electrons. We find that there is no enhancement for uniformly doped structures. For spike-doped structures there is a small enhancement, but this is gained at the expense of a drop in current density. In both structures the velocity enhancement is limited by intervalley scattering.

Microscopic mobility

An explicit formal expression is obtained for the energy-dependent microscopic mobility of a disordered system of independent electrons. It is this mobility which gives the average drift velocity of electrons of a given initial energy in response to an electric field. Both this microscopic mobility and the more familiar microscopic conductivity are local in energy. The microscopic mobility is proportional to the energy derivative of the microscopic conductivity, not to the microscopic conductivity itself, which leads to the scaling relation $s=\ensuremath{\tau}+1$, where $s$ is the conductivity exponent and $\ensuremath{\tau}$ the mobility exponent. The absurdity of a mobility divergent at the mobility edge leads to the restriction $s\ensuremath{\ge}1$. In the presence of interactions, values of $s&lt;1$ can occur. If a quasiparticle representation is valid, $0\ensuremath{\le}\ensuremath{\tau}&lt;s$ and the density of states diverges as the mobility edge is approached from the extended-state side with an exponent $u=\ensuremath{-}[\ensuremath{\tau}+(1\ensuremath{-}s)]$. The microscopic mobility provides a dynamical basis for the introduction of the hole concept into the theory of strongly disordered materials. Comparison with the thermopower then leads to a deeper understanding of the nature of holes in disordered materials. A carrier representation can then be derived for transport in disordered semiconductors.

Monte Carlo simulation of reflecting contact behavior on ballistic device speed

We present a Monte Carlo simulation of the behavior of a reflecting n-n+contact in InP. Electrons are injected at high energy into the InP and are accelerated by an applied electric field over a length of ∼ 1000 A. At the collecting contact they encounter a possible reflection back into the device. The reflection coefficient at the contact is chosen to vary between 0 and 0.70. The results show that the average electron drift velocity is greatly lowered at low fields throughout the entire device by the reflection at the contact.

Modulation-doped MBE GaAs/n-AlxGa1-xAs MESFETs

Modulation-doped GaAs/n-Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</inf> As MESFETs have been fabricated. At 77 K, DC transconductance of 160 mS mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> was observed, which is the highest transconductance value ever reported in this type of structure. The intrinsic transconductance was calculated to be 350 mSmm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , and the corresponding average electron drift velocity is 1.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , which demonstrates the real advantage of this type of device in high-speed applications.

Anomalous resistivity of a turbulent plasma in a strong electric field

The enhanced effective collision frequency in a turbulent plasma produced by a strong electric field is studied using a small toroidal device. The collision frequency is presented as a function of the electric field E and the average electron drift velocity. Two collision frequency regimes are identified. In the regime I where v ∝ E½, the current driven ion-acoustic turbulence is considered responsible.

Slide-away distributions and relevant collective modes in high-temperature plasmas

The evolution of the electron distribution function, when an electric field that is not too small in comparison with the critical electron runaway field is applied along the confining magnetic field of a high temperature plasma, is analysed. In the regimes considered, a finite fraction of the electron population has magnetically trapped orbits, and is not appreciably affected by the applied electric field, while the distribution of circulating electrons tends to “slide away” as a whole. Then the Spitzer-Härm model for the current-carrying electron distribution is inadequate, and the role that collective modes, in particular current-driven microinstabilities, and collisions can play in producing a stationary electron distribution is analysed. Modes at the ion plasma frequency, ωpi, that are driven by the positive slope of the current-carrying electron distribution, can be excited, when the average electron drift velocity is a finite fraction of the electron thermal velocity, and transfer transverse energy to the main body of the electron distribution. These features are consistent with the experimental observations performed on the Alcator device. Modes at the “reduced” electron plasma frequency (k‖/k)ωpe can also be excited both in connection with the modes at ωpi and independently. Modes at the electron gyrofrequency Ωe associated with the loss-cone feature that the electron distribution tends to develop are considered, among others, as a factor for the strongly enhanced electron cyclotron emission experimentally observed in regimes where non-thermal electron distributions have been realized.

Nonsaturating velocity-field characteristic of gallium arsenide experimentally determined from domain measurements

A new method is presented for the measurement of the average electron drift velocity as a function of the electric field (velocity-field characteristic) for semiconductor materials which exhibit bulk negative differential conductance. The characteristic is determined from capacitive probe measurements made on high-field Gunn-effect domains. This method is most applicable for values of E &amp;gt; 20 kV/cm, where very little information exists. The method was applied to the measurement of the velocity-field characteristic of GaAs. The characteristic found is nonsaturating and slowly decreasing up to 100 kV/cm, the maximum measured electric field. The results correlate well with the velocity-field characteristic theoretically calculated by Heinle.

ERROR IN MICROWAVE MEASUREMENTS OF THE VELOCITY-FIELD CHARACTERISTIC OF n-TYPE GaAs DUE TO ENERGY RELAXATION EFFECTS

It is shown theoretically that the average drift velocity of electrons in GaAs as measured by microwave techniques differs substantially from the dc characteristic. This error, arising from electron-energy relaxation effects, has always been neglected in past measurements. The simple model predicts an error of 16% in measured velocity at 35 GHz

A natural form for large amplitude time-periodic acoustic travelling waves in piezoelectric semiconductors

The propagation of time-periodic acoustic waves in piezoelectric semiconductors is treated in the limit of weak electromechanical coupling. The perturbed acoustic mode of the linear regime is shown to have a natural extension into the nonlinear regime which is defined uniquely without introducing ad hoc boundary conditions. The properties of this natural solution are investigated by means of a perturbation expansion in powers of the fundamental amplitude. When the average electron drift velocity is just above the velocity of sound, the nonlinear gain at all frequencies is intermediate between the two values obtained from the linear gain formula by inserting the drift velocity and the dc field times the mobility.