Takayuki Hisaka

Degradation Uniformity of RF-Power GaAs PHEMTs Under Electrical Stress

We have studied the electrical degradation of RF-power PHEMTs by means of in situ 2-D light-emission measurements. Electroluminescence originates in the recombination of holes that have been generated by impact ionization. The local light intensity, thus, maps the electric-field distribution at the drain side of the device. This allows us to probe the uniformity of electrical degradation due to electric-field-driven mechanisms. We find that electrical degradation proceeds in a highly nonuniform manner across the width of the device. In an initial phase, degradation takes place preferentially toward the center of the gate finger. In advanced stages of degradation, the edges of the device degrade at a preferential rate. We identify the origin of this behavior as a small systematic nonuniformity in the recess geometry that impacts the magnitude of the electric field on the drain of the device. Our research suggests that a close examination of the width distribution of electric field in RF-power PHEMTs (and FETs in general) is essential to enhance their long-term reliability.

Corrosion-induced degradation of GaAs PHEMTs under operation in high humidity conditions

Abstract We have comprehensively investigated the degradation mechanism of AlGaAs/InGaAs pseudomorphic high-electron-mobility transistors (PHEMTs) under operation in high humidity conditions. PHEMTs degradation under high humidity with bias consists of a decrease in maximum drain current (Imax) caused by a corrosion reaction at the semiconductor surface at the drain side. The decrease in Imax is markedly accelerated by the external gate–drain bias (Vdg). This originates from a reduction in the actual activation energy (Ea0) by Vdg. The degradation depends on the surface treatment prior to deposition of the SiNx passivation film. The reduction of As-oxide at the SiNx/semiconductor interface suppresses the corrosion reaction.

Degradation Mechanism of AlGaAs/InGaAs Power Pseudomorphic High-Electron-Mobility Transistors under Large-Signal Operation

We have comprehensively investigated the degradation mechanism of output power in AlGaAs/InGaAs pseudomorphic high-electron-mobility transistors (PHEMTs) under large-signal operation. The degradation of output power is caused by the decrease in maximum drain current (Imax) around the knee voltage (Vk) with an increase in drain resistance (Rd). The decrease in Imax originates from a degradation layer containing a significant amount of oxygen formed at the drain recess surface region. This layer causes a reduced carrier density in the drain region underneath, resulting in a decreased Imax and an increased Rd. The decrease in Imax is accelerated with increasing drain voltage, temperature, and humidity. These results suggest that the degradation of output power in the PHEMTs is mainly determined by the growth of a corrosion layer owing to an electrochemical reaction between O2 and/or H2O and the semiconductor surface. We successfully suppress the output power degradation due to this electrochemical corrosion reaction with the help of a special surface treatment prior to the deposition of a passivation film on the recess surface. We demonstrate a highly reliable RF operation of PHEMTs in air even without a hermetic package.

Drain Corrosion in RF Power GaAs PHEMTs

We have investigated drain degradation in a set of experimental RF power GaAs PHEMTs. Drain degradation was observed in the form of an increase in RD and a reduction in Imax in a variety of conditions. We found that both forms of degradation arise from surface corrosion that takes place on different locations on the drain and dominate in different regimes of operation. Specifically, the increase in RDwas prominent in the ON-state and was found to be associated with corrosion on the drain n+GaAs ledge. The reduction in Imax, in contrast, was prominent in the OFF-state and was associated with corrosion on the exposed AlGaAs barrier close to the gate. A significant finding is that in both cases, the drain-to-gate voltage emerges as a significant accelerating factor of drain corrosion.

Non-uniform degradation behavior across device width in RF power GaAs PHEMTs

We have studied the electrical degradation of RF power PHEMTs by means of light-emission measurements performed during bias-stress experiments. We show that electrical degradation can proceed in a highly non-uniform manner across the width of the device. We identify the origin of this as a small systematic non-uniformity in the recess geometry that impacts the electric field and the impact ionization rate on the drain of the device. Our research suggests that a close examination of the width distribution of electric field in RF power PHEMTs (and FETs in general) is essential to enhance their long-term reliability

Degradation mechanisms of GaAs PHEMTs under operation in high humidity conditions

We have comprehensively investigated the degradation mechanism of AlGaAs/InGaAs pseudomorphic highelectron- mobility transistors (PHEMTs) under operation in high humidity conditions. The degraded samples under high humidity condition with bias show a decrease in maximum drain current (Imax).The decrease of Imax is accelerated with increasing drain voltage, temperature and humidity. The PHEMT degradation is caused by corrosion reaction at the semiconductor surface at drain side. The rate of corrosion degradation is increased by RH3. Higher Vgd decreases the actual activation energy for corrosion. The degradation depends on surface treatment prior to deposition of a SiNx passivation film.The reduction of As-oxide at SiNx/semiconductor interface might suppress the corrosion reaction.

Characteristics of SiN/GaAs interface under exposure to high-temperature and high-humidity conditions measured by photoreflectance spectroscopy

We investigated the changes in the electrical properties of a SiN/GaAs interface under high-temperature and high-humidity conditions, using photoreflectance (PR) spectroscopy and the electrical device characteristics. The PR spectra show the Franz-Keldysh oscillation (FKO); these spectra show that the period decreases after the sample is exposed to humidity. The electric field strength obtained from the FKO period indicates that the initial high electric field decreases with humidity exposure. Decomposed water molecules are supposed to diffuse into the SiN layer and react with the SiN/GaAs interface, causing a decrease in the interface states.

Simultaneous achievement of high performance and high reliability in a 38/77GHz InGaAs/AlGaAs PHEMT MMIC

In order to meet the demand for mass production of 77GHz automotive radar systems, a low cost and high performance 38/77GHz AlGaAs/InGaAs PHEMT MMIC transmit amplifier with a multiplier has been realized. The chip is packaged in an inexpensive conventional non-hermetic package. Excellent power performance is demonstrated with a 15dBm output power and 7dB maximum conversion gain from 38 to 76.5GHz. Also, highly reliable RF operation of a bare MMIC chip is obtained with less than 0.7dB reduction in output power during 106 hr at Vd=4V and Ta=25°C in air.

Effect of Oxidation using Ultraviolet Light and Ozone and Subsequent Nitridation using Electron Cyclotron Resonance Plasma on Gate Portion of GaAs Field-Effect Transistors

A recess-gate-type n-channel depletion mode metal–semiconductor field-effect transistor (MESFET), a metal–oxide–semiconductor field-effect transistor (MOSFET) prepared by UV and ozone oxidation, and metal–insulator–semiconductor field-effect transistors (MISFETs) prepared by oxidation followed by nitrogen plasma treatment for different time durations with an electron cyclotron resonance system (oxinitridation) were fabricated using an ex-situ process and equally current controlled recessed wafers. The MISFETs, especially the longest-nitrided one, showed the highest pinch-off gate voltage (-1.5 V) and the highest peak transconductance (170 mS/mm) at a 0.9 V gate voltage. The MOSFET showed the largest drain current, whereas the longest-nitrided MISFET showed the smallest drain current, owing to the difference in applicable forward gate voltage. These were clearly confirmed with statistical data. The transconductance obtained reproduces that of our previous experiment, and its increase is due to the improvement of crystallographic order in the vicinity of the insulator/semiconductor interface. The different applicable gate voltage implies that the subsequent nitridation weakens the barrier effect of the oxidized layer. An AlGaAs layer, as the mother material for oxinitridation, improves it.