We analyzed the operation speed of the resonant tunneling logic gate, MOBILE, using a simple equivalent circuit model and varying parameters of I-V characteristics and capacitance of RTTs(resonant tunneling transistors). The switching time for large peak-to-valley(P/V)current ratios is smaller at small Vbmax(maximum bias voltage), but larger at large Vbmax than that for small P/V ratios in the case of present I-V characteristics with flat valley current. It is also demonstrated that the MOBILE operation fails if the bias voltage rises too fast, when the capacitance of the load and the driver is different due to the displacement current through the capacitance. These behaviors can be explained by considering the potential diagrams of the circuit.

We have fabricated enhancement-mode n-channel GaN MOSFETs with overlap gate structure on a p-GaN using thick HfO2 as a gate insulator. The maximum transconductance of 23 mS/mm which is 4 times larger, to our knowledge, than the best-reported value of the normally-off GaN MOSFETs with SiO2 gate oxide has been obtained.

The heterogeneous integration of GaAs HEMTs on a polyimide-covered AlN ceramic substrate was demonstrated using a fluidic self-assembly (FSA) technique. We used thin device blocks for the FSA, which have various advantages. In particular, they can reduce the drain-source capacitance Cds of the assembled HEMTs if the substrate has a low dielectric constant. This is a novel kind of semiconductor-on-insulator (SOI) technology. The dc and RF properties of the GaAs HEMTs on the polyimide/AlN substrate were studied and the reduction of Cds was confirmed. This technique was successfully applied to the SPDT switch, where a low Cds is essential for good isolation.

A novel resonant tunneling diode (RTD) oscillator is proposed, which overcomes the problems of the conventional RTD oscillators, such as the low-frequency spurious oscillation and the bias instability. Our proposal consists of two RTDs connected serially, and the resonator connected to the node between two RTDs. This circuit separates the oscillation node from the bias nodes, and suppresses the above mentioned problems. This relaxes the severe restriction on the RTD area, and makes it possible to supply higher power to a load. Circuit simulation shows that with this circuit more than 2 mW power can be supplied to the 50 Ω resistive load at 100 GHz using RTDs having 105 A/cm2-peak current density and 20 µm2-area. It also shows that the dc-to-RF conversion efficiency is as good as that of conventional ones. Furthermore, we have studied the extension of this oscillator having 4 RTDs connected serially. Circuit simulations revealed that using this circuit the power can be doubled with a good conversion efficiency.

We performed the (NH4)2S surface treatments before Al2O3 deposition to improve the Al2O3/III-Nitride interface quality in Al2O3/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs). Interface state density at the Al2O3/GaN interface was decreased by the (NH4)2S treatment. The hysteresis width in ID-VGS and gm-VGS characteristics of the Al2O3/AlGaN MOSHFETs with the (NH4)2S treatment was smaller than that without the (NH4)2S treatment. In addition, transconductance (gm) decrease at a large gate voltage was relaxed by the (NH4)2S treatment. We also performed ultraviolet (UV) illumination during the (NH4)2S treatment for further improvement of the Al2O3/III-Nitride interface quality. Interface state density of the Al2O3/GaN MOS diodes with the UV illumination was smaller than that without the UV illumination.

A new gate current model for the AlGaAs/GaAs heterostructure MISFETs is proposed. This model is derived by taking account of the relationship between the surface potential at the hetero-interface and the gate voltage. The new gate current model is shown to be suitable for analyzing logic circuits with heterostructure MISFETs.

The low-frequency noise of InGaAs pseudomorphic HEMTs fabricated on GaAs substrate was studied. The dependence of the noise spectral density on the gate voltage indicates that the channel of the device dominates the low-frequency noise. Generation-recombination (G-R) noise was observed in the form of bulges superimposed on a background of 1/f. The activation energyof the G-R noise was 0.32-0.39 eV which is close to that of the DX center, suggesting that the origin of the G-R noise is the DX center in the AlGaAs barrier layer. Little bulge was observed in the gate current noise of the HEMTs with large InAs mole fractions of 0.4 and 0.5. Generation of the traps with different time constant can explain this behavior.

A high-speed potential of an n+-Ge gate AlGaAs/GaAs MISFET has been confirmed by a ring oscillator and a frequency divider performances. The circuit was based on SCFL with 1.0 µm gate-length MISFET's. The delay time was 42.4 ps with a power dissipation of 8.9 mW/gate. A maximum toggle frequency of 11.3 GHz with a power dissipation of 219 mW per T-F/F has been achieved at room temperature.

We have fabricated AlGaN/GaN HEMTs with a thin InGaN cap layer to implement normally-off HEMTs with a small extrinsic source resistance. The key idea is to employ the polarization-induced field in the InGaN cap layer, by which the conduction band is raised leading to the normally-off operation. Fabricated HEMT with an In0.2Ga0.8N cap layer with a thickness of 5 nm showed normally-off operation with a threshold voltage of 0.4 V and a maximum transconductance of 85 mS/mm for the device with a 1.9-µm-long gate. By etching-off the In0.2Ga0.8N cap layer at the region except under the gate using gate and ohmic electrodes as etching masks, the sheet resistance has decreased from 2.7 to 0.75 kΩ/, and the maximum transconductance has increased from 85 to 130 mS/mm due to a reduction of the extrinsic source resistance. The transconductance was increased from 130 to 145 mS/mm by annealing the devices at 250 for 20 minutes in a N2 atmosphere.