J. Liberis

Ultrafast Removal of LO-Mode Heat From a GaN-Based Two-Dimensional Channel

Dissipation of the Joule heat, accumulated in non-equilibrium longitudinal optical (LO) phonon modes, is considered in terms of LO-phonon lifetime. The dependence of the lifetime on electron density, hot-electron temperature, and supplied electric power are presented for a voltage-biased GaN-based channel with a two-dimensional electron gas (2DEG). An improved understanding of conversion of LO phonons into acoustic and other phonons is reached. A nonmonotonous dependence of the lifetime on the electron density is observed. The optimal 2DEG density for ultrafast decay of the LO-mode heat is estimated and explained in terms of LO-phonon-plasmon resonance. A new limitation of the frequency performance is predicted for heterostructure field effect transistors under the off-resonance conditions of operation. The shortest hot-phonon lifetime of ~60± 20 fs is found, at a high level of supplied power, in nearly lattice matched InAlN/AlN/GaN.

Experiments on hot electron noise in semiconductor materials for high-speed devices

Experimental results on noise temperature and spectral density of current fluctuations (electron diffusion) at high electric fields in silicon, gallium arsenide, and indium phosphide are presented. The dominant noise sources are discussed in their relation to electron scattering mechanisms. Physical backgrounds of high speed-low noise performance (noise-speed tradeoff) are considered. Suppression in short samples of the fluctuations having long correlation time constant and (or) high threshold energy is discussed. >

Window for better reliability of nitride heterostructure field effect transistors

Abstract Phase-noise technique was applied to monitor channel degradation of nitride heterostructure field effect transistors (HFETs) subjected to a fixed drain voltage stress at different fixed gate voltages. The slowest degradation and the lowest noise were found for the electron-density window centered at ∼1xa0×xa010 13 xa0cm −2 where ultrafast decay of hot phonons took place. A possibility to shift the window towards higher sheet densities was demonstrated experimentally and accounted by plasmon-assisted dissipation of Joule heat.

Hot-electron energy relaxation, noise, and lattice strain in InGaAs quantum well channels

Energy loss by hot electrons in lattice-matched and strained InGaAs layers is estimated from experimental data on microwave noise obtained for InP-based quantum well channels containing two-dimensional electron gas. A strong correlation of the energy relaxation time and the lattice strain is observed.

Hot-electron transport in 4H–SiC

Nanosecond-pulsed technique is used to study hot-electron transport in donor-doped 4H–SiC (n=2×1017cm−3) biased parallel to the basal plane. The measurements of current with 1ns voltage pulses are carried out at average electric fields up to 570kV∕cm. A region with a negative differential conductance is observed for the range of fields exceeding 280kV∕cm, followed by a sharp increase in the current at fields over 345kV∕cm. The dependence of drift velocity on electric field is deduced for the field range below the onset of the negative differential conductance to appear: the value of the saturation velocity is estimated as 1.4×107cm∕s at room temperature.

Hot-electron noise and diffusion in AlGaAs/GaAs

The electric field dependence of the microwave noise temperature and the diffusion coefficient are found to be significantly different for two sets of AlGaAs/GaAs structures with different spacer thickness and aluminium mole fraction. In the samples with a wide quantum well (QW), one maximum of the diffusion coefficient is observed at fields below the threshold for the intervalley diffusion, but two maxima are characteristic of a narrow QW with an intersubband gap wider than the KBTO. A microscopic explanation of the observed noise spectra of AlGaAs/GaAs is given and used to determine kinetic parameters of the heterostructures.

Comparative analysis of microwave noise in GaAs and AlGaAs/GaAs channels

Abstract Hot electron noise at 10 GHz microwave frequency is investigated in an AlGaAs/GaAs heterostructure channel with two-dimensional electron gas (2DEG), and the results are compared to those for 3DEG channels of GaAs. An experimental evidence of noise due to 2DEG⇆3DEG transfer at 80 K is presented. The related longitudinal diffusion of hot electrons in long 2DEG channels is found to dominate at the intermediate fields which are well below the threshold field for intervalley diffusion in GaAs. A tendency to suppress the intervalley noise and diffusionby the 2DEG⇆DEG transfer is observed at the high fields. Possible suppression of the dominant sources of microwave noise in modulation-doped field-effect transistor channels is discussed.

Microwave Noise In Biased AlGaN/GaN And AlGaN/AlN/GaN Channels

Noise temperature is measured at 10 GHz at room temperature for biased AlGaN/GaN and AlGaN/AlN/GaN two‐dimensional channels. Interpretation of the experimental results, through Monte Carlo simulation, takes into account interaction of hot electrons with phonons. The calculated longitudinal noise temperature exceeds the transverse one, but the resultant anisotropy of noise becomes substantially weaker when accumulation of nonequilibrium (hot) optical phonons is taken into account. The considered intense interaction of hot electrons with hot phonons and weak coupling with the thermal bath shifts the hot‐electron‐hot‐phonon subsystem closer to the equilibrium at the elevated temperature as compared with the hot‐electron subsystem interacting with the equilibrium phonons.

Length dependent hot electron noise in doped GaAs

Abstract Hot electron microwave noise is studied in 0.2 … 7.5 micron-long samples of GaAs at strong electric fields at 80 K and 293 K ambient temperatures. A new noise source is observed in doped (3·10 17 cm −3 samples. This source is related to the impurity levels lying 0.2 eV above the principle edge of the conduction band. At a given electric field the doped short samples show a higher noise temperature as compared to pure samples of the same length. Onset of the resonance impurity scattering gives rise to efficient transformation of the drift energy into the chaotic one and stimulates the intervalley noise sources. The threshold voltages of these sources are determined. The enhanced hot electron diffusion is observed in the vicinity of the threshold voltages. The critical length is less than one micron in doped GaAs.

Velocity Overshoot and Suppression of Diffusivity and Microwave Noise in Short n+-n-n+ Structures of GaAs

Fast devices usually require semiconductors with high drift velocity of electrons and prefer the conditions of operation favourable for low noise and diffusivity.