Koichi Maezawa

A study on current collapse in AlGaN/GaN HEMTs induced by bias stress

Drain current collapse in AlGaN/GaN HEMTs has been studied systematically by applying bias stress to the device. The collapse was suppressed by light illumination with energy smaller than the bandgap. The position dependence of the light illumination and the measurement of series source and drain resistances revealed that the collapse was caused by the surface states between the gate and drain electrodes, which captured electrons injected from the gate. The current collapse was eliminated by the passivation of the device surface with Si/sub 3/N/sub 4/ film.

A New Resonant Tunneling Logic Gate Employing Monostable-Bistable Transition

This letter describes a new resonant tunneling logic gate. The concept of the proposed gate has two features: 1) to employ the monostable-to-bistable transition of a circuit consisting of two N-type negative differential resistance (NDR) devices connected serially, and 2) to drive the logic gate by oscillating the bias voltage to produce the transition. This mode of operation has a significant advantage in that a large number of fanouts is possible without sacrificing the high-speed operation. Serially connected resonant tunneling field effect transistors having p+-junction gates were fabricated to test the above operation principle. The inverter operation of the proposed logic gate has been successfully achieved at room temperature.

High-performance InP-based enhancement-mode HEMTs using non-alloyed ohmic contacts and Pt-based buried-gate technologies

High performance InP-based InAlAs/InGaAs enhancement-mode HEMTs are demonstrated using two improved approaches to device structure design and fabrication, i.e., nonalloyed ohmic contacts and Pt-based buried-gate technologies, to reduce the source resistance (R/sub S/). With specially designed cap layer structures, nonalloyed ohmic contacts to the device channel were obtained providing contact resistance as low as 0.067 /spl Omega//spl middot/mm. Furthermore, in device fabrication, a Pt-based buried-gate approach is used in which depletion-mode HEMTs are first intentionally fabricated, and then, the Pt-based gate metal is annealed at 250/spl deg/C, causing the Pt-InAlAs reaction to take place under the gate electrode so that Pt sinks into InAlAs and depletes the channel. As a result, the depletion-mode HEMTs are changed to enhancement-mode, while the channel region between the source and gate electrodes remain undepleted, and therefore, the small R/sub S/ of 0.2 /spl Omega//spl middot/mm can be maintained. Excellent maximum transconductance of 1170 mS/mm was obtained for a 0.5-/spl mu/m-gate device. A maximum current-gain cutoff frequency f/sub T/ of 41.2 GHz and maximum unilateral power-gain cutoff frequency f/sub max/ of 61 GHz were demonstrated for a 0.6-/spl mu/m-gate enhancement-mode HEMT.

InP-based high-performance monostable-bistable transition logic elements (MOBILEs) using integrated multiple-input resonant-tunneling devices

MOBILEs (monostable-bistable transition logic elements), which have the advantages of multiple-input and multiple-function, are demonstrated in InP-based material system using monolithic integration of resonant-tunneling diodes and high electron mobility transistors. The high peak current density, high peak-to-valley ratio, and high transconductance, which are required for high-performance MOBILEs, are demonstrated in this InP-based material system. A fabricated MOBILE with three-input gates having 1:2:4 width ratio can perform weighted-sum threshold logic operation, and has a wide range of applications in new computing architectures, such as neural networks.

High-speed and low-power operation of a resonant tunneling logic gate MOBILE

High-speed operations up to 35 Gb/s were demonstrated for a resonant tunneling (RT) logic gate monostable-bistable transition logic element (MOBILE). The test circuit consisted of a MOBILE and a DCFL-type output buffer, and it was fabricated using InP-based resonant tunneling diode/HEMT integration technology. This operation bit rate is close to the cutoff frequency of the 0.7-/spl mu/m gate HEMTs used in the circuit, and was obtained after improvement of the output buffer design. This result indicates the high-speed potential of the MOBILE, though the speed is still limited by the buffer. The power dissipation of the MOBILE was also discussed based on a simple equivalent circuit model of RTDs. This revealed that the power dissipation is as small as 2 mW/gate over a wide range of operation bit rates.

Effects of surface passivation on breakdown of AlGaN/GaN high-electron-mobility transistors

The effect of Si3N4 surface passivation on breakdown of AlGaN/GaN high-electron-mobility transistors was studied in detail by investigating dependences of the off-state breakdown voltage on temperature and gate reverse current, and by measuring electroluminescence distribution. Impact ionization in the channel which was triggered by the gate reverse current was responsible for the off-state breakdown. Surface passivation by Si3N4 film was effective to improve the off-state breakdown voltage. This has been explained by a change in the potential distribution due to suppression of electron trapping at the surface states, based on results of electroluminescence measurements.

Functions and applications of monostable-bistable transition logic elements (MOBILE's) having multiple-input terminals

The MOBILE is a logic gate exploiting the monostable-bistable transition of a circuit that consists of two resonant tunneling transistors connected in series. It has several advantages including multiple inputs and multiple functions. This paper describes the output characteristics of multiple-input MOBILEs and discusses their applications. For a two-input MOBILE, it is demonstrated that both NAND and NOR operations are possible with the appropriate control voltage. This implies the possibility of a variable function logic gate. Furthermore, the threshold logic operations for a weighted sum of input signals are demonstrated for a three-input MOBILE with a weight ratio of 4:2:1. The applications of MOBILEs in parallel processing architectures such as cellular automata and cellular neural networks are discussed based on the above results. Circuit simulations using a simple model of resonant tunneling transistors successfully reproduce the basic characteristics of MOBILEs, and demonstrate the usefulness of MOBILEs in such applications. >

Weighted sum threshold logic operation of MOBILE (monostable-bistable transition logic element) using resonant-tunneling transistors

The functional operation of the MOBILE (monostable-bistable transition logic element) has been studied using multiple-input logic gates. The MOBILE uses two resonant-tunneling transistors (RTTs) connected in series and driven by an oscillating bias voltage to produce a mono-to-bistable transition of the circuit. A MOBILE having three input gates with a 1:2:4 width ratio can distinguish all 8 (2/sup 3/) input patterns corresponding to each weighted sum, depending on the threshold value selected by the control gate. The results confirm the realization of the weighted sum threshold logic operation of input signals.<<ETX>>

Monolithic integration of resonant tunneling diodes and FET's for monostable-bistable transition logic elements (MOBILE's)

A MOBILE (monostable-bistable transition logic element), employing two n-type negative differential resistance devices connected in series, is a functional logic gate with the advantages of multiple inputs and multiple functions. In this paper, a novel approach to achieve MOBILE operation is demonstrated using monolithic integration of resonant tunneling diodes (RTD) and FETs. In our new integration structure, an RTD and FET are connected in parallel. This structure offers several advantages including separate optimization of RTDs and FETs, and flexible circuit design abilities. For a single-input MOBILE gate, inverter operation at room temperature is demonstrated as the evidence of monostable-to-bistable transition.<<ETX>>

Large gate leakage current in AlGaN/GaN high electron mobility transistors

A large gate leakage current in AlGaN/GaN high electron mobility transistors (HEMTs) was observed. Temperature dependence of Ig-Vg characteristics revealed that tunneling current is dominant in the leakage current. By introducing ECR plasma treatment before the gate metal deposition, the gate leakage current was reduced by two to three orders of magnitude.