We address experimental and theoretical study of a two-dimensional electron gas transport at low and moderate electric fields. The devices under study are group-III nitride-based ͑AlGaN/GaN͒ gateless heterostructures grown on sapphire. The transmission line model patterns of different channel lengths, L, and of the same channel width are used. A strong dependence of the device I-V characteristics on the channel length has been found. We have developed a simple theoretical model to adequately describe the observed peculiarities in the I-V characteristics measured in steady-state and pulsed (10 Ϫ6 s) regimes. The effect of the Joule heating of a heterostructure is clearly distinguished. The thermal impedance and the channel temperature rise caused by the Joule self-heating have been extracted for the devices of different L at different values of dissipated power. The current reduction due to both self-heating and hot-electron effects is determined quantitatively as a function of the electric field.
The low-frequency 1/f noise characteristics of AlGaN/GaN high-electron-mobility transistors with gate length scaled down to 150 nm grown on sapphire by metalorganic chemical vapor deposition have been studied. Certain features of the 1/f noise have been revealed in these short-gate transistors. The low-frequency noise spectra show drastically different behavior depending on the gate voltage V G in the range of low (V Gt рV G р0) and high (V G ϽV Gt ) biases. The noise spectra-gate bias dependences allow one to distinguish a spatial redistribution of effective noise sources in the transistor channel. The group III-nitride-based electronic devices are very attractive for power applications at high frequencies. Nitride material systems offer great potential for providing much higher output power, operational voltage, and temperature. Due to a unique combination of high current density, high breakdown electric field, and good thermal conductivity, these materials are in the focus of state-of-the-art semiconductor research.1-3 The low-frequency 1/f noise is one of the important factors that limits the device performance and determines its capability for power microwave applications. The level of such noise and corresponding value of the Hooge parameter may often be significant in AlGaN/GaN structures depending on growth technique and different type of substrates ͑mainly sapphire and silicon carbide are used as most suitable͒. In previous studies, a low-frequency noise was measured in high-electron-mobility transistor ͑HEMT͒ devices with gate length of a micrometer range scale.4 -11 The devices with a shorter gate length have smaller characteristic transit times and provide higher operating frequencies. Moreover, certain features of carrier transport and noise can manifest themselves with scaling down the device feature lengths, which will impact the device performance. In this letter, we present the study of low-frequency noise of AlGaN/GaN-based HEMTs with gate length scaled down to deep submicrometer range of 150 nm.AlGaN/GaN HEMTs under investigation were grown by metalorganic chemical vapor deposition on sapphire substrates. A 40 nm AlGaN ͑16% Al͒ nucleation layer grown on the substrate was followed by the deposition of a 1.1 m nominally undoped GaN buffer layer and a 23 nm n-AlGaN ͑33% Al͒ undoped barrier layer. Transistors were fabricated using Ti/Al/Ti/Au metallization annealed for 40 s at 800°C for source and drain ohmic contacts and Ni/Au for Schottky gate contacts. The surface was covered with 320 nm Si 3 N 4 . The devices had gate length of 0.15, 0.25, 0.30, 0.35 m, gate width of 100-400 m, and drain-gate spacing of 1 m. A source-drain spacing was of 3 m. A complete direct current characterization has been carried out in HEMT devices with variety of channel widths and lengths. Hall mobility and sheet carrier concentration in ungated structures were extracted from Hall measurements. A room temperature electron mobility of 1250 cm 2 /V s at a sheet carrier density of 1.05ϫ10 13 cm Ϫ2 was measured in the chan...
The theory of Bloch electron dynamics for carriers in homogeneous electric and magnetic fields of arbitrary time dependence is developed in the framework of the Liouville equation. The Wigner distribution function (WDF) is determined from the single particle density matrix in the ballistic regime, i.e., collision effects are excluded. The single particle transport equation is established with the electric field described in the vector potential gauge, and the magnetic field is treated in the symmetric gauge.The general approach is to employ the accelerated Bloch state representation (ABR) as a basis so that the dependence upon the electric field, including multiband Zener tunneling, is treated exactly. In the formulation of the WDF, we transform to a new set of variables so that the final WDF is gauge invariant and is expressed explicitly in terms of the position, kinetic momentum, and time.The methodology for developing the WDF is illustrated by deriving the exact WDF equation for free electrons in homogeneous electric and magnetic fields. The methodology is then extended to the case of electrons described by an effective Hamiltonian corresponding to an arbitrary energy band function. In treating the problem of Bloch electrons in a periodic potential, the methodology for deriving the WDF reveals a multiband character due to the inherent nature of the Bloch states. In examining the single-band WDF, it is found that the collisionless WDF equation matches the equivalent Boltzmann transport equation to first order in the magnetic field. These results are necessarily extended to second order in the magnetic field by employing a unitary transformation that diagonalizes the Hamiltonian using the ABR to second order. The work includes a discussion of the multiband WDF transport analysis and the identification of the combined Zener-magnetic field induced tunneling.
In this study, we explored the possibility of enhanced spontaneous emission of radiation beyond the free space value by analyzing a semiconductor superlattice structure placed in a microcavity whose resonant modes were tuned to the Bloch frequency. In particular, we considered the spontaneous emission of Bloch radiation into the dominant mode of a rectangular waveguide. In the REPORT DOCUMENTATION PAGE Form Approved OMB NO. 0704-0188 Public Reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comment regarding this burden estimates or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for information Operations and Reports,
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