Threshold voltage uniformity and characterization of microwaveperformance for GaAs/AlGaAs high electron-mobility transistors grown onSi substrates

We report on DC and microwave characteristics for high electron-mobility transistors (HEMT's) grown on Si substrates by metal-organic chemical vapor deposition (MOCVD). Threshold voltage (V _th) distribution in a 3-in wafer shows standard deviation of V_th (σV_th) of 36 mV with V_th of -2.41 V for depletion mode HEMT's/Si and σV_th of 31 mV with V_th of 0.01 V for enhancement mode, respectively. The evaluation of V_th in a 1.95×1.9 mm² area shows high uniformity for as-grown HEMT's/Si with σV_th of 9 mV for V_th of -0.10 V, which is comparable to that for HEMT's/GaAs. Comparing the V_th distribution pattern in the area with that for annealed HEMT's/Si, it is indicated that the high uniformity of V_th is obtained irrelevant of a number of the dislocations existing in the GaAs/Si. From microwave characteristic evaluation for HEMT's with a middle-(10~50 Ω·cm) and a high-(2000~6000 Ω·cm) resistivity Si substrate using a new equivalent circuit model, it is demonstrated that HEMT's/Si have the disadvantage for parasitic capacitances and resistances originated not from the substrate resistivity but from a conductive layer at the Si-GaAs interface. The parasitic parameters, especially the capacitances, can be overcome by the reduction of electrode areas for bonding pads and by the insertion of a dielectric layer under the electrode, which bring high cut-off frequency (f_T) and maximum frequency of operation (f_max) of 24 GHz for a gate length of 0.8 (μm). These results indicate that HEMT's/Si are sufficiently applicable for IC's and discrete devices and have a potential to be substituted for HEMT's/GaAs     

Dislocation processes during SiC bulk crystal growth

Dislocation processes during physical vapor transport (PVT) growth of silicon carbide (SiC) single crystals were investigated by defect selective etching. It was found that foreign polytype inclusions introduced a high density of dislocations at the polytype boundary. In the polytype-transformed areas of the crystal, very few medium size hexagonal etch pits due to threading screw dislocations were observed, indicating that the polytype transformation ceased the propagation of threading screw dislocations. The oval-shaped etch pit arrays observed on the etched vicinal (0 0 0 1)Si surface, indicative of the dislocation multiplication in the basal plane, showed characteristic distribution around micropipes. Based on the results, we have argued the dislocation behavior in PVT grown SiC crystals, suggesting that dislocation interaction and conversion are relevant processes to understanding the behavior.     

Tunneling Atomic Force Microscopy Studies on Surface Growth Pits Due to Dislocations in 4H-SiC Epitaxial Layers

The morphological and electrical properties of surface growth pits caused by dislocations in 4H-SiC epitaxial layers were characterized using tunneling atomic force microscopy. The characteristic distribution of the tip current between the metal-coated atomic force microscopy tip and the SiC was observed within a large surface growth pit caused by a threading screw dislocation. The current was highly localized inside the pit and occurred only on the inclined surface in the up-step direction near the pit bottom. This paper discusses the causes and possible mechanisms of the observed tip current distribution inside surface growth pits.     

Abundance of native fishes,wild‐introduced salmonids and escaped farmed rainbow trout in a Patagonian reservoir

The introduction of salmonids in Patagonia has resulted in significant impacts on its lakes, as well as a major impact on streams, in which native fishes seem to have been displaced almost completely by rainbow trout (Oncorhynchus mykiss). Another perspective is that the introduced salmonid species have resulted in wild fish populations that sustain an economically important sport fishery. The wide distribution and high abundance of escaped farmed rainbow trout, and a clear decrease in the abundance of native and successfully introduced salmonid species in Alicura Reservoir were all observed, based on comparison of recent data and data from 1993 to 1995 corresponding to littoral gillnet captures. Thus, both native fish and introduced salmonid populations seem to have been drastically reduced in the presence of farmed fish escapees. The results of the present study regarding fish escapes deserve major consideration when making decisions about fish cage culture activities for other Patagonian reservoirs.     

A small-signal linear equivalent circuit of HEMTs fabricated onGaAs-on-Si wafers

A new small-signal linear equivalent circuit for high electron mobility transistors (HEMTs) fabricated on GaAs-on-Si wafers, HEMTs-on-Si, has been proposed. The new equivalent circuit describes the microwave characteristics of HEMTs-on-Si much better than the conventional metal-semiconductor field-effect transistor (MESFET) equivalent circuit does. Influences of the pads, the GaAs-Si interface, and the Si substrate on the microwave characteristics are included in the circuit. It also has a great advantage in that it can separately analyze the intrinsic device characteristics and influences of Si substrate and GaAs-Si interface. Analyzes using the new equivalent circuit show that the crucial problem of HEMTs-on-Si is the larger values of the pad capacitances and the drain-source capacitances than those of HEMTs fabricated on GaAs bulk wafers, HEMTs-on-GaAs, and that the substrate resistivity is not an important factor for microwave performances of HEMTs-on-Si. The microwave performance was improved by the reduction of the pad capacitances     

Super low-noise HEMTs with a T-shaped WSix gate

A T-shaped quarter-micron gate structure composed of WSi_x /Ti/Pt/Au has been developed for low-noise AlGaAs/GaAs HEMTs. The gate resistance R_g was reduced to 0.3 Ω for devices with 200 μm-wide gates despite using WSi_x, and the source resistance R_s reached 0.28 Ω mm by minimising the source-gate distance using a self-alignment technique. This HEMT exhibited the lowest reported noise figure of 0.54 dB with an associated gain of 12.1 dB at 12 GHz