Kotaro Tsubaki

High-temperature electron transport properties in AlGaN/GaN heterostructures

Electron transport properties in the Al0.15Ga0.85N/GaN heterostructure field effect transistors (HFETs) have been examined from room temperature up to 400 °C. The temperature dependencies of the two-dimensional electron gas (2DEG) mobility have been systematically measured for the samples with different 2DEG densities. The 2DEG mobility has been shown to decrease with increasing the temperature, with the lower decrease ratio at higher temperatures, and moreover, shown to be less dependent on the 2DEG density at higher temperatures. These features well agree with those of the longitudinal optical phonon-limited mobility theoretically predicted, although the effect of alloy and interface scattering should further be examined and analyzed. The observed 2DEG mobilities at 400 °C were as high as from 100 to 120 cm2/V s, directly providing the evidence for suitability of the HFET of this material system for high-temperature applications. Moreover, Si-doped Al0.15Ga0.85N single layer has been shown to exhibit a ...

Superior Pinch-Off Characteristics at 400°C in AlGaN/GaN Heterostructure Field Effect Transistors

AlGaN/GaN field effect transistors have been fabricated on SiC(0001) substrates, and the I–V characteristics in the devices have been examined from room temperature to 400°C. In addition to excellent current saturation characteristics, sufficient pinch-off characteristics have been obtained up to a temperature of 400°C for the first time, as the result of reduced crystal defects and reduced etching damage in the devices. The temperature dependence of the transconductance has been also examined. The degradation rate in the transconductance has been proved to be low above 300°C: the transconductance degraded by only 8% for a temperature increase from 350 to 400°C. Sufficient pinch-off characteristics and a relatively low degradation rate in the transconductance ensure the practical use of the devices at high temperatures.

Conductivity oscillation due to quantum interference in a proposed wash-board transistor

Conductivity oscillation due to quantum interference is theoretically studied in a proposed wash‐board transistor. The transistor has a two‐dimensional electron gas modulated by a weak periodic potential, and its gate voltage can change the Fermi energy. The periodic potential is approximated by a square well considering the screening effect. Conductivity oscillation is found to originate in the electron velocity modulation caused by a quantum interference effect, not in the density of states modulation. This result is confirmed by comparison with experimental results, which give some clues to the real potential formed on a two‐dimensional interface.

Differential negative resistance caused by inter-subband scattering in a 2-dimensional electron gas

Abstract Differential negative resistance has been observed in the range 0.5∼1.2 kV cm at 77 K in a two-dimensional electron gas formed at a modulation-doped AlGaAs/GaAs heterojuction interface. This differential negative resistance is attributed to the sudden onset of inter-subband scattering when electrons achieve sufficient energy from the electric field.

Spatially modulated photoconductivity at N‐AlGaAs/GaAs heterojunctions and formation of persistent charge patterns with submicron dimensions

The electron concentration at an N‐AlGaAs/GaAs interface is shown to be spatially modulated at low temperatures by the selective photoionization of deep donors in AlGaAs using two interfering laser beams. The charge pattern thus formed is persistent and detected by measuring the anisotropy of the channel conductivity. Grating patterns with a period as small as 0.6 μm were successfully recorded in accordance with the prediction that spatial resolution comparable with the AlGaAs layer thickness (∼0.1 μm) will be achieved.

Coherence length in quantum interference devices having periodic potential

A measurement of coherence length in the quantum interference device using drain current oscillation is proposed. The coherence length for a quantum interference device, called the washboard transistor, is measured by the proposed method. The merit of this method is that the quantitative value of the coherence length is measured using only quantum interference phenomena.

High field electron transport in n‐InP/GaInAs two‐dimensional electron gas

High field electron transport in n‐InP/GaInAs two‐dimensional electron gas (2DEG) is studied at liquid helium temperature. In this experiment two kinds of samples are used: the first has a thick enough GaInAs layer to contain a three‐dimensional gas; the other has a thin GaInAs layer containing only a 2DEG on the GaInAs side of the heterojunction. In the sample with the thick GaInAs layer, Gunn oscillations are observed above 2.2 kV/cm. In the sample with the thin GaInAs layer, Gunn oscillations do not occur and an irreversible decrease of the sheet concentration of 2DEG is obtained after applying a high electric field. The highest measured drift velocity of this sample is 4.4×107 cm/s at 3.8 kV/cm.

Two-dimensional electron gas transport properties in AlGaN/GaN single- and double-heterostructure field effect transistors

Two-dimensional electron gas transport properties have been investigated in both AlGaN/GaN single- and AlGaN/GaN/AlGaN double-heterostructure field effect transistors, by means of the Hall effect measurement under the gate-voltage application. In AlGaN/GaN single-heterostructures, the dependencies of the electron mobility on the electron density have systematically been examined from 30 to 400 K. Below room temperature, the mobility has been shown to assume a maximum value at a critical electron density where electrons begin to overflow into the AlGaN barrier layer. Above room temperature, even at high electron densities that exceed the channel electron capacity, the degradation in the mobility has been found to be small. This is a feature favorable for high-power and high-temperature device operation. In AlGaN/GaN/AlGaN double-heterostructures, a striking effect has been observed that the mobility is drastically enhanced compared with that in the AlGaN/GaN single-heterostructure at low temperatures. The dependencies of the mobility on the electron density measured at 4.2 K in both heterostructures have been analyzed from the viewpoint of the electron distribution in the channel. The observed mobility enhancement in the double-heterostructure has been shown to be mainly due to the enhanced polarization-induced electron confinement in the double-heterostructure, and additionally due to the improvement of the interface roughness in the structure. Transport properties specific to AlGaN/GaN single- and double-heterostructures have thus successfully been revealed through the mobility-density relations that have been determined by the Hall effect measurement under the gate-voltage application.

Device characterization of p-channel AlGaAs/GaAs MIS-like heterostructure FET's

p-channel AlGaAs/GaAs MIS-like heterostructure FETs (p-MIS HFETs) are characterized concerning their gate-source leakage current. Device performance is confirmed to improve approximately inversely to layer thickness d<inf>t</inf>between the channel and metal gate, at low gate voltages. A high transconductance g<inf>m</inf>of 110 ms. mm<inf>-1</inf>is obtained at 77 K by reducing d<inf>t</inf>to 20 nm. Maximum transconductance is limited by gate-source leakage current I<inf>gs</inf>. I<inf>gs</inf>is governed mainly by the leakage current through the ion-implanted gate edge and is reduced by decreasing the dose level of ion-implantation at the gate edge to 2 × 10¹³cm^-2. The contact resistance is reduced to about 0.1 ω. mm by ion implantation into the ohmic contact region to a dose of 2 × 10¹⁴cm^-2. Calculations indicate that, by reducing I<inf>gs</inf>and the gate-source resistance to 1 ω. mm with the lightly doped drain (LDD) structure, g<inf>m</inf>around 200 mS. mm^-1at 300 K and 300 mS. mm^-1at 77 k are achievable with a 1-µm gate structure.

Electron wave interference device with fractional layer superlattices

Modulation‐doped Al0.3Ga0.7As/GaAs heterostructure electron wave interference devices with fractional layer superlattices are fabricated. The periods of the fractional layer superlattice in the electron wave interference devices are 16, 12, and 8 nm, respectively. These devices show drain current oscillation due to electron wave interference at 4.2 K. The oscillation period is determined by the period of the fractional layer superlattice. From the analysis of the drain current oscillation, the peaks of the structure function agree with the multiples of the periods of the fractional layer superlattice.