Seiya Kasai

Nearly Temperature-Independent Saturation Drain Current in a Multi-Mesa-Channel AlGaN/GaN High Electron Mobility Transistor

We fabricated a multi-mesa-channel (MMC) structure by forming a periodic trench just under a gate electrode to improve the uniformity of effective electric field in the channel in an AlGaN/GaN high electron mobility transistor (HEMT). A unique performance, i.e., a nearly temperature-independent saturation drain current, was observed in the MMC device in a wide temperature range. A two-dimensional (2D) potential calculation indicates that the mesa-side gate effectively modulates the potential, resulting in a field surrounding 2D electron gas. Such a surrounding-field effect and a relatively lower source access resistance may be related to a unique current behavior in the MMC HEMT.

Fabrication and characterization of a GaAs-based three-terminal nanowire junction device controlled by double Schottky wrap gates

A three-terminal nanowire junction device controlled by double nanometer-sized Schottky wrap gates (WPGs), which control left and right branches independently, are fabricated utilizing AlGaAs∕GaAs etched nanowires and characterized experimentally. Fabricated device exhibits clear nonlinear characteristics of output voltage at the center terminal by applying voltages to left and right terminals in push-pull fashion. Applying asymmetric gate voltages to left and right WPGs provides clear asymmetry in the output voltage. The nonlinearity in the low voltage regions is greatly enhanced by squeezing both left and right branches using WPGs.

Lateral tunneling injection and peripheral dynamic charging in nanometer-scale Schottky gates on AlGaN/GaN hetrosturucture transistors

The gate leakage and gate control characteristics of AlGaN/GaN heterostructure field effect transistors (HFETs) were systematically investigated in an attempt to clarify possible effects of surface states. The experiments were compared to rigorous computer simulations. We observed large amounts of leakage currents in the Schottky diodes fabricated on the AlGaN epitaxial layers. By the calculation based on a thin surface barrier model in which the effects of surface defect donor were taken into account, this large leakage was well explained by enhancement of tunneling transport processes due to the barrier thinning associated with ionization of surface-defect donor. On the other hand, the analysis on the current-voltage characteristics for the nanometer-scale Schottky contacts on AlGaN/GaN HFETs, indicated additional lateral leakage components. The comparison of the gate control characteristics between experiment and calculation clearly showed that the effective lateral expansion of gate length significant...

Effect of Size Reduction on Switching Characteristics in GaAs-Based Schottky-Wrap-Gate Quantum Wire Transistors

The effect of size reduction on switching characteristics is investigated experimentally for the low-switching-power operation of GaAs-based quantum wire transistors (QWRTrs) utilizing etched AlGaAs/GaAs nanowires controlled by Schottky wrap gates (WPGs). WPG QWRTrs in which the wire width, W, and gate length, LG, are systematically changed are fabricated and characterized with respect to operation temperature, switching voltage, ΔVG, gate voltage to Fermi energy scaling factor, α, and power-delay product, PDP. When W is less than 200 nm, more than 80% of the fabricated devices exhibit quantized conductance at 30 K. The device with W=40 nm shows a large α of 0.7. Decreasing LG into the sub-100-nm range is found to be effective for improving power consumption, since the short channel effect is suppressed by tight potential control in the WPG structure.

Study on Nonlinear Electrical Characteristics in GaAs-based Three-branch Nanowire Junctions Controlled by Schottky Wrap Gates

Three-branch nanowire junctions (TBJs) have unique nonlinear electrical characteristics based on ballistic transport, which are useful for various logic and analog circuits. However, TBJ devices having rather large dimensions compared with electron mean free path, IF, also show clear nonlinear curves even at room temperature (RT) and the origin has not been clarified yet. In this paper, we investigate a size and temperature dependence of nonlinear electrical characteristics of GaAs-based TBJ devices which have nanometer-sized Schottky wrap gates (WPGs). A simple model was introduced, which could explain experimental data reasonably well.

Device interference in GaAs quantum wire transistors and its suppression by surface passivation using Si interface control layer

In order to establish feasibility of high density integration of gate-controlled GaAs nanodevices, this article investigates device interference in GaAs-based quantum wire transistors (QWRTrs) by using a side-gating test structure and attempts to suppress the observed anomalously large side-gating with surface passivation using a silicon interface control layer (Si ICL). QWRTrs were formed on AlGaAs∕GaAs etched quantum wires (QWRs) and were controlled by nanometer sized Schottky wrap gates. A Schottky side gate was formed at a distance dsg from the QWR. When dsg was large, the QWRTr showed weak side gating which can be explained by the electrostatic side gating. However, when the side gates was placed close to the nanowire with dsg<500nm, anomalously large side gating started to take place which cannot be explained by the electrostatic side gating. On the basis of detailed measurements of side-gating behavior and side-gate leakage currents at various temperatures, the anomalous side gating was explained b...

Novel structure of GaAs-based interdigital-gated HEMT plasma devices for solid-state THz wave amplifier

Theoretical analysis of potential distribution in the interdigital-gated high electron mobility transistor (HEMT) plasma wave device was carried out. The dc I-V characteristics of capacitively coupled interdigital structure showed that uniformity of electric field under the interdigital gates was improved compared to the dc-connected interdigital gate structure. Admittance measurements of capacitively coupled interdigital gate structure in the microwave region of 10-40GHz showed the conductance modulation by drain-source voltage. These results indicate the existence of plasma wave interactions.

Integration of interdigital-gated plasma wave device for proximity communication system application

Interdigital-gated AlGaAs/GaAs high-electron-mobility transistor (HEMT) structure was used to investigate the interaction between the drifting carrier plasma waves and electromagnetic (EM) waves. It was shown theoretically that the interaction in the range from microwave to terahertz (THz) at room temperature should produce negative conductance characteristics when the carrier drift velocity slightly exceeds the phase velocity of EM waves. S-parameter reflection measurements were carried out at room temperature for a frequency range from 1 to 20 GHz and a drastic change in conductance was observed at 5 and 10 GHz with the increase of drain-source voltage. Large conductance change over 1000 mS/mm was obtained and it showed a peak at a certain frequency. The peak position could be controlled by changing the pitch size of the interdigital gates. These characteristics can be used for high-frequency applications such as high-speed switching devices although a feature size of our interdigital-gated HEMT device is much larger than conventional HEMT device.

Multipath-switching device utilizing a GaAs-based multiterminal nanowire junction with size-controlled dual Schottky wrap gates

A multipath-switching device using a multiterminal nanowire junction with size-controlled dual gates is proposed and demonstrated experimentally. The device switches a number of output terminals according to multiple-valued input voltages for electrons entering from a root terminal. The switching function is implemented by dual gating on multiple nanowires with different threshold voltages Vth. Systematic Vth shift is made by changing gate lengths in nanometer scale. A triple-path-switching device is fabricated using AlGaAs∕GaAs etched nanowires and nanometer-scale Schottky wrap gates. Its correct operation is confirmed at room temperature. Obtained results are explained by a simple analytical model.

Large Modulation of Conductance in Interdigital-Gated HEMT Devices Due to Surface Plasma Wave Interactions

To investigate the presence of interactions between surface plasma waves of carriers in a two-dimensional electron gas (2DEG) at AlGaAs/GaAs heterostructure and electromagnetic space harmonic slow waves, interdigital-gated high-electron-mobility transistor (HEMT) devices were fabricated, and their input admittances were measured in the microwave region of 1–15 GHz. A large modulation of conductance, more than 1000 mS/mm, was observed. The conductance modulation was controlled by a drain-source voltage and showed a peak at a certain frequency whose position could be controlled by changing the pitch of the interdigital gates. The observed conductance and capacitance characteristics were in good agreement with the transverse magnetic (TM) mode analysis taking into account a nonuniform field distribution along the 2DEG channel. The result seems to prove the existence of surface plasma wave interactions even under the strongly collision-dominant situation in the microwave region and provides great hope for increased interactions at THz frequencies under nearly collision-free conditions.