Transport and fluctuations in nonlinear-dissipative systems: role of interparticle collisions

The goal of the paper is to overview contemporary theoretical and experimental research of the microwave electric noise and fluctuations of hot carriers in semiconductors, revealing sensitivity of the noise spectra to non-linearity in the applied electric field strength and, especially, in the carrier density. During the last years, investigation of electronic noise and electron diffusion phenomena in doped semiconductors was in a rapid progress. By combining analytic and Monte Carlo methods as well as the available experimental results on noise, it became possible to obtain the electron diffusion coefficients in the range of electric fields where inter-electron collisions are important and Price’s relation is not necessarily valid. Correspondingly, a special attention to the role of inter-electron collisions and of the non-linearity in the carrier density while shaping electric noise and diffusion phenomena in the non-equilibrium states will be paid. The basic and up-to-date information will be presented on methods and advances in this contemporary field - the field in which methods of non-linear analytic and computational analysis are indispensable while seeking coherent understanding and interpretation of experimental results

High-field electron transport in doped ZnO

Current-voltage characteristics have been measured for ZnO:Ga and Zn:Sb epitaxial layers with electron densities ranging from 1.4x10(17) cm(-3) to 1.1 x 10(20) cm(-3). Two-terminal samples with coplanar electrodes demonstrate virtually ohmic behavior until thermal effects come into play. Soft damage of the samples takes place at high currents. The threshold power (per electron) for the damage is nearly inversely proportional to the electron density over a wide range of electron densities. Pulsed voltage is applied in order to minimize the thermal effects, and thus an average electric field of 150 kV cm(-1) is reached in some samples subjected to 2 ns voltage pulses. The results are treated in terms of electron drift velocity estimated from the data on current and electron density under the assumption of uniform electric field. The highest velocity of similar to 1.5 x 10(7) cm s(-1) is found at an electric field of similar to 100 kV cm(-1) for the sample with an electron density of 1.4 x 10(17) cm(-3). The nonohmic behavior due to hot-electron effects is weak, and the dependence of the electron drift velocity on the doping resembles the variation of mobility

Ultrafast decay of hot phonons in an AlGaN/AlN/AlGaN/GaN camelback channel

A bottleneck for heat dissipation from the channel of a GaN-based heterostructure field-effect transistor is treated in terms of the lifetime of nonequilibrium (hot) longitudinal optical phonons, which are responsible for additional scattering of electrons in the voltage-biased quasi-two-dimensional channel. The hot-phonon lifetime is measured for an Al0.33Ga0.67N/AlN/Al0.1Ga0.9N/GaN heterostructure where the mobile electrons are spread in a composite Al0.1Ga0.9N/GaN channel and form a camelback electron density profile at high electric fields. In accordance with plasmon-assisted hot-phonon decay, the parameter of importance for the lifetime is not the total charge in the channel (the electron sheet density) but rather the electron density profile. This is demonstrated by comparing two structures with equal sheet densities (1 × 1013 cm−2), but with different density profiles. The camelback channel profile exhibits a shorter hot-phonon lifetime of ∼270 fs as compared with ∼500 fs reported for a standard Al0.33Ga0.67N/AlN/GaN channel at low supplied power levels. When supplied power is sufficient to heat the electrons \u3e 600 K, ultrafast decay of hot phonons is observed in the case of the composite channel structure. In this case, the electron density profile spreads to form a camelback profile, and hot-phonon lifetime reduces to ∼50 fs

Hot electron effects on efficiency degradation in InGaN light emitting diodes and designs to mitigate them

Hot electrons and the associated ballistic and quasiballistic transport, heretofore neglected endemically, across the active regions of InGaN light emitting diodes (LEDs) have been incorporated into a first order simple model which explains the experimental observations of electron spillover and the efficiency degradation at high injection levels. The model is in good agreement with experiments wherein an adjustable barrier hot electron stopper, commonly called the electron blocking layer (EBL), is incorporated. The model is also in agreement with experiments wherein the electrons are cooled, eliminating hot electrons, inside a staircase electron injector (SEI) prior to their injection into the active region. Thermionic emission from the active region, even if one uses an uncharacteristically high junction temperature of 1000 K, fails to account for the carrier spillover and the experimental observations in our laboratory in samples with varying EBL barrier heights. The model has been successfully applied to both m-plane (lacking polarization induced electric field) and c-plane (with polarization induced field) InGaN double heterostructure (DH) LEDs with a 6 nm active region featuring a variable barrier hot electron stopper, and a SEI, and the various combinations thereof. The choice of DH LEDs stems from our desire to keep the sample structure simple as well as the model calculations. In this paper, the theoretical and experimental data along with their comparison followed by an insightful discussion are given. The model and the approaches to eliminate carrier spillover proposed here for InGaN LEDs are also applicable to GaN-based laser diodes

The effect of stair case electron injector design on electron overflow in InGaN light emitting diodes

Effect of two-layer (In0.04Ga0.96N and In0.08Ga0.92N) staircase electron injector (SEI) on quantum efficiency of light-emitting-diodes (LEDs) in the context of active regions composed of single and quad 3 nm double heterostructures (DHs) is reported. The experiments were augmented with the first order model calculations of electron overflow percentile. Increasing the two-layer SEI thickness from 4 + 4 nm up to 20 + 20 nm substantially reduced, if not totally eliminated, the electron overflow in single DH LEDs at low injections without degrading the material quality evidenced by the high optical efficiency observed at 15K and room temperature. The improvement in quad 3 nm DH LEDs with increasing SEI thickness is not so pronounced as the influence of SEI is less for thicker active regions, which in and of themselves necessarily thermalize the carriers. (C) 2013 AIP Publishing LLC

Degradation and phase noise of InAlN/AlN/GaN heterojunction field effect transistors: Implications for hot electron/phonon effects

In15.7%Al84.3%N/AlN/GaN heterojunctionfield effect transistors have been electrically stressed under four different bias conditions: on-state-low-field stress, reverse-gate-bias stress, off-state-high-field stress, and on-state-high-field stress, in an effort to elaborate on hot electron/phonon and thermal effects. DC current and phase noise have been measured before and after the stress. The possible locations of the failures as well as their influence on the electrical properties have been identified. The reverse-gate-bias stress causes trap generation around the gate area near the surface which has indirect influence on the channel. The off-state-high-field stress and the on-state-high-field stress induce deterioration of the channel, reduce drain current and increase phase noise. The channel degradation is ascribed to the hot-electron and hot-phonon effects

Electron drift velocity in lattice-matched AlInN/AlN/GaN channel at high electric fields

Hot-electron transport was probed by nanosecond-pulsed measurements for a nominally undoped two-dimensional channel confined in a nearly lattice-matched Al0.82In0.18N/AlN/GaN structure at room temperature. The electric field was applied parallel to the interface, the pulsed technique enabled minimization of Joule heating. No current saturation was reached at fields up to 180 kV/cm. The effect of the channel length on the current is considered. The electron drift velocity is deduced under the assumption of uniform electric field and field-independent electron density. The highest estimated drift velocity reaches ∼3.2×107 cm/s when the AlN spacer thickness is 1 nm. At high fields, a weak (if any) dependence of the drift velocity on the spacer thickness is found in the range from 1 to 2 nm. The measured drift velocity is low for heterostructures with thinner spacers (0.3 nm)

Plasmon-enhanced heat dissipation in GaN-based two-dimensional channels

Decay of nonequilibrium longitudinal optical (LO) phonons is investigated at room temperature in two-dimensional electron gas channels confined in nearly lattice-matched InAlN/AlN/GaN structures. A nonmonotonous dependence of the LO-phonon lifetime on the supplied electric power is reported for the first time and explained in terms of plasmon–LO-phonon resonance tuned by applied bias at a fixed sheet density (8×1012 cm−2). The shortest lifetime of 30±15 fs is found at the power of 20±10 nW/electron

InGaN staircase electron injector for reduction of electron overflow in InGaN light emitting diodes

Ballistic and quasiballistic electron transport across the active InGaN layer are shown to be responsible for electron overflow and electroluminescence efficiency droop at high current levels in InGaN light emitting diodes both experimentally and by first-order calculations. An InGaN staircase electron injector with step-like increased In composition, an “electron cooler,” is proposed for an enhanced thermalization of the injected hot electrons to reduce the overflow and mitigate the efficiency droop. The experimental data show that the staircase electron injector results in essentially the same electroluminescence performance for the diodes with and without an electron blocking layer, confirming substantial electron thermalization. On the other hand, if no InGaN staircase electron injector is employed, the diodes without the electron blocking layer have shown significantly lower (three to five times) electroluminescence intensity than the diodes with the blocking layer. These results demonstrate a feasible method for the elimination of electron overflow across the active region, and therefore, the efficiency droop in InGaN light emitting diodes