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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.

We present a new model for the the kink effect in InAlAs/InGaAs HEMTs. The model suggests that the kink is due to a threshold voltage shift which arises from a hole pile-up in the extrinsic source and an ensuing charging of the surface and/or the buffer-substrate interface. The model captures many of the observed behaviors of the kink, including the kink's dependence on bias, time, temperature, illumination, and device structure. Using the model, we have developed a simple equivalent circuit, which reproduced well the kink's dc characteristics, its time evolution in the nanosecond range, and its dependence on illumination.

We have developed a methodology to diagnose the physical mechanisms limiting the manufacturing uniformity of millimeter-wave power InAlAs/InGaAs HEMT's on InP. A statistical analysis was carried out on dc figures of merit obtained from a large number of actual devices on an experimental wafer. correlation studies and principal component analysis of the results indicated that variations in Si delta-doping concentration introduced during molecular-beam epitaxy accounted for more than half of the manufacturing variance. Variations in the gate-source distance that is determined by the electron-beam alignment in the gate formation process were found to be the second leading source of manufacturing variance. The statistical methodology used in this work is suitable for continuous process yield diagnostics and improvement in a manufacturing environment.

We have carried out a systematic study of on-state breakdown in a sample set of InAlAs/InGaAs HEMT's using a new gate current extraction technique in conjunction with sidegate and temperature-dependent measurements. We find that as the device is turned on, the breakdown voltage limiting mechanism changes from a TFE-dominated process to a multiplication-dominated process. This physical understanding allows the creation of a phenomenological physical model for breakdown which agrees well with all our experimental results, and explains the relationship between BV/sub on/ and the sheet carrier concentration. Our results suggest that depending on device design, either on-state or off-state breakdown can limit maximum power.

In spite of their outstanding transport characteristics, InP high-electron mobility transistors (HEMTs) deliver lower output power than GaAs pseudomorphic HEMTs (PHEMTs) throughout most of the millimeter-wave regime. However, the superior power-added efficiency of InP HEMTs when compared with GaAs PHEMTs makes this technology attractive for many applications. The reason for the relatively inferior power output of InP HEMTs lies in their comparatively small off-state and on-state breakdown voltages. This paper reviews the state of knowledge regarding the physics of breakdown voltage in InP HEMTs, placing it in contrast with GaAs PHEMTs. It also presents current understanding regarding burnout, a closely related phenomenon. This paper concludes by discussing strategies for improving the breakdown voltage and the power output of InP HEMTs.

Conventional techniques to extract channel mobility, /spl mu/, and sheet carrier concentration, n/sub s/, in heterostructure field-effect transistors (HFETs) do not account for the distributed nature of the device. This can result in substantial errors. To address this, we have developed a new technique that consists of measuring the gate-to-source impedance with the drain floating (Z/sub 11/) over a broad frequency range. A transmission line model (TL model) is fitted to Re[Z/sub 11/], thus obtaining the gate capacitance and channel resistance (and consequently /spl mu/(V/sub GS/) and n/sub s/(V/sub GS/)) in a single measurement. We demonstrate this technique in InAlAs-InGaAs on InP HFET's. The TL model faithfully represents Z/sub 11/ from 100 Hz to 15 MHz. Our technique can easily be automated and thus is a good tool for accurate charge control in an industrial environment.

Strained silicon-on-insulator (SSOI) is a new material system that combines the carrier transport advantages of strained Si with the reduced parasitic capacitance and improved MOSFET scalability of thin-film SOI. We demonstrate fabrication of highly uniform SiGe-free SSOI wafers with 20% Ge equivalent strain and report fully depleted n-MOSFET results. We show that enhanced mobility is maintained in strained Si films transferred directly to SiO/sub 2/ from relaxed Si/sub 0.8/Ge/sub 0.2/ virtual substrates, even after a generous MOSFET fabrication thermal budget. Further, we find the usable strained-Si thickness of SSOI significantly exceeds the critical thickness of strained Si/SiGe without deleterious leakage current effects typically associated with exceeding this limit.

We present a new technique for determining the dominant breakdown mechanism in InAlAs-InGaAs high-electron mobility transistors. By exploiting both the temperature dependence and the bias dependence of different physical mechanisms, we are able to discriminate impact ionization gate current from tunneling and thermionic field emission gate current in these devices. Our results suggest that the doping level of the supply layers plays a key role in determining the relative importance of these two effects.

We present a new simple three-terminal technique for measuring the on-state breakdown voltage in HEMTs. The gate current extraction technique involves grounding the source, and extracting a constant current from the gate. The drain current is then ramped from the off-state to the on-state, and the locus of drain voltage is measured. This locus of drain current versus drain voltage provides a simple, unambiguous definition of the on-state breakdown voltage which is consistent with the accepted definition of off-state breakdown. The technique is relatively safe and repeatable so that temperature dependent measurements of on-state breakdown can be carried out. This helps illuminate the physics of both off-state and on-state breakdown.

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

We have carried out pulsed measurements of the kink effect in InAlAs/InGaAs HEMT's on InP with nanosecond resolution. Our measurements show that the kink's characteristic time constant is strongly dependent on V/sub DS/, i.e., it drops by more than three decades, from 100 /spl mu/s down to 50 ns, between low and high values of V/sub DS/. This suggests that the kink should not be operational for frequencies in the microwave and millimeter wave regimes. We also find that the kink turn-on dynamics correlate with impact ionization. In particular, the inverse of the kink's characteristic time constant follows a classical impact ionization behavior.

We present new, unmistakable experimental evidence directly linking the kink effect with impact ionization in the channel of InAlAs-InGaAs HEMTs on InP. Through the use of a sidegate structure, we confirm that the impact ionization coefficient obeys the classic exponential dependence on the inverse electric field at the drain end of the gate, and that the onset of the kink strongly coincides with the onset of impact ionization in the devices we consider. These measurements illuminate the functional relationship between the kink and impact ionization, and therefore will allow assessment of the numerous impact-ionization related kink mechanisms that have recently been suggested in the literature.

We present a new predictive model for off-state breakdown in InAlAs/InGaAs and AlGaAs/InGaAs power high electron mobility transistors (HEMTs). The proposed model suggests that electron tunneling from the gate edge, and not impact ionization, is responsible for off-state breakdown in these devices. The model indicates that the crucial variables in determining the off-state breakdown voltage of power HEMTs are the sheet carrier concentration in the extrinsic gate-drain region, and the gate Schottky barrier height. Other design parameters have only secondary impact on the breakdown voltage for realistic device designs.

We present a new model for the dynamic behavior of the kink effect in InAlAs/InGaAs HEMTs. The model suggests that the kink is due to a threshold voltage shift which arises due to hole pileup in the extrinsic source and an ensuing charging of the surface. The model is incorporated in a simple equivalent circuit, which explains well DC characteristics of the kink, its time evolution in the nanosecond range, as well as its dependence on illumination.

In spite of their superior transport characteristics, InP HEMTs deliver lower output power than GaAs PHEMTs in the millimeter-wave regime. However, the superior power-added efficiency of InP HEMTs when compared with PHEMTs, makes this technology attractive for many applications. The reason for the lower power output of InP HEMTs is their relatively small off- and on-state breakdown voltage (BV). The authors review the state of knowledge regarding the physics of BV in InP HEMTs placing it in contrast with PHEMTs. They also discuss strategies for improving BV and the power output of InP HEMTs.

We present results of an experimental and theoretical study of the temperature dependence of the off-state breakdown voltage of InAlAs/InGaAs high electron mobility transistors (HEMTs). We find that the breakdown voltage (BV) has a negative temperature coefficient that is more prominent for lower values of the extrinsic sheet carrier concentration (n/sub s/). Structural parameters such as the insulator thickness and top-to-bottom delta doping ratio have little effect on BV if n/sub s/ is held constant. These results are consistent with an extension of a new tunneling model for breakdown in HEMTs to include thermionic-field emission.

New measurements providing direct evidence linking the kink effect and impact ionization in InAlAs/InGaAs HEMTs are reported. Current kink models are not consistent with our findings. We propose a new mechanism, barrier-induced hole pile-up at the source, to explain the kink. The new model is shown to be consistent with both room temperature and low temperature measurements. These results allow formulation of a simple equivalent circuit model of the kink.

Summary form only given. Conventional wisdom suggests that in pseudomorphic high electron mobility transistors (pHEMTs), the field between the drain and the gate determines off-state breakdown, and that the drain to gate voltage therefore sets the breakdown voltage of the device. Thus, the two terminal breakdown voltage is a widely used figure of merit, and most models for breakdown focus on the depletion region in the gate-drain gap, while altogether ignoring the source. We present new measurements and simulations that demonstrate that for power pHEMTs, the electrostatic interaction of the source seriously degrades the device's gate-drain breakdown, and must be taken into consideration in device design. As a vehicle for this study we have used a state-of-the-art L/sub G/=0.25 /spl mu/m double heterostructure pHEMT with excellent power performance (P/sub 0/= 1W, Gain= 11 dB, and PAE=60% at 10 GHz for W/sub G/=1200 /spl mu/m) and high breakdown voltage (BV/sub DG/=21 V at I/sub D/=1 mA/mm).

Any gasification facility generates three classes of waste: solid, liquid, and gaseous. All three classes were investigated in some depth and are discussed. Liquid wastes are controlled in the sense that there are none. There are no waste water streams from any of the facilities with the exception of ANG's deep well which will be used for brine disposal. All of the water is either treated for reuse or evaporated. There is little published data concerning the characteristics of water reuse treatment systems for coal gasification facilities. There is a high probability that the existing technology can be used to meet the zero discharge requirement and provide water for reuse. Gaseous emissions are under tight state standards in New Mexico and federal standards and guidelines in North Dakota. Controlled substances are NO/sub x/, non-methane hydrocarbons (HC), COS, SO/sub 2/, CO, and particulates. Hydrocarbon (HC) and carbon-monoxide emissions guidelines and standards (both federal and state) are somewhat easier to meet than the sulfur standards. Solids wastes and sludges are, at this time, uncontrolled. There is, however, significant research being conducted to establish the probable impact of solid waste disposal and to establish techniques to minimize the potential impact.

The four coal gasification plants reviewed are the ones proposed by the El Paso Natural Gas Company, the WESCO Gasification Company in New Mexico, the Natural Gas Pipeline Company of America, and the ANG Coal Gasification Company in North Dakota. This study was undertaken in order to review the nature and the amounts of the effluent emitted into the environment, to determine the cost of controlling undesirable effluents, to review water treatment facilities and pollution control technologies, to discover the projected capital and operating costs for the plants, and to identify control technology research needs. The fixed capital investment and the direct operating costs for equipment to control emission of potentially harmful effluents, such as H/sub 2/S, SO/sub 2/, hydrocarbons, coal dust, particulates, ashes, sludges, COS, CS/sub 2/, CO, and H/sub 2/ were estimated according to the design information provided by the gasification companies. The reuse water treatment systems were reviewed. The work has revealed that the weakest portions of the designs rest in water reuse and sulfur control technology. In the authors' judgements, there is a good probability that existing technology can be utilized to meet the zero liquid discharge and existing gaseous emission requirements for the proposed gasificationmore » facilities with minimal upset. However, the technology for internal water reuse is not well established, and these streams can be expected to have characteristics other than those projected. Stretford and Claus technology for sulfur removal exists and, in our judgement, has a good probability of meeting existing guidelines and regulations. Some operating data are available for high CO/sub 2/, low H/sub 2/S, streams; these data are not available in the literature at this time. Recommendations for research have been made.« less