Study of bulk and elementary screw dislocation assisted reversebreakdown in low-voltage (<250 V) 4H-SiC p+-n junctiondiodes. II. Dynamic breakdown properties

For Part I see ibid., vol.46, no.3, pp.478-84 (Mar. 1999). This paper outlines the dynamic reverse-breakdown characteristics of low-voltage (<250 V) small-area <5×10^-4 cm² 4H-SiC p⁺-n diodes subjected to nonadiabatic breakdown-bias pulsewidths ranging from 0.1 to 20 μs 4H-SiC diodes with and without elementary screw dislocations exhibited positive temperature coefficient of breakdown voltage and high junction failure power densities approximately five times larger than the average failure power density of reliable silicon pn rectifiers. This result indicates that highly reliable low-voltage SiC rectifiers may be attainable despite the presence of elementary screw dislocations. However, the impact of elementary screw dislocations on other more useful 4H-SiC power device structures, such as high-voltage (>1 kV) pn junction and Schottky rectifiers, and bipolar gain devices (thyristors, ICBT's, etc.) remains to be investigated     

Progress in silicon carbide semiconductor electronics technology

Silicon carbide’s demonstrated ability to function under extreme high-temperature, high-power, and/or high-radiation conditions is expected to enable significant enhancements to a far-ranging variety of applications and systems. However, improvements in crystal growth and device fabrication processes are needed before SiC-based devices and circuits can be scaled-up and incorporated into electronic systems. This paper surveys the present status of SiC-based semiconductor electronics and identifies areas where technological maturation is needed. The prospects for resolving these obstacles are discussed. Recent achievements include the monolithic realization of SiC integrated circuit operational amplifiers and digital logic circuits, as well as significant improvements to epitaxial and bulk crystal growth processes that impact the viability of this rapidly emerging technology.     

Molecular anatomy of the perivascular sheath in human placental stem villi: the contractile apparatus and its association to the extracellular matrix

In previous studies, we have shown that smooth muscle cells and myofibroblast subpopulations of the perivascular stem villous sheath of the human placenta contain focal adhesion plaques and talin immunoreactivity. The close association of these cells to elastic and collagen fibres have led to the assumption of a functional myofibroelastic unit within the perivascular stem villous sheath. Interactions between the extracellular matrix and smooth muscle cells depend on a variety of structural protein assemblies. In the present study, we examined, by immunocytochemistry, whether the molecular assembly of extracellular matrix proteins and molecules of focal adhesions, known to be essential for signal transduction in smooth muscle cells, are also found in smooth muscle cells of the perivascular stem villous sheath of the human placenta. Vascular and extravascular smooth muscle cells were immunoreactive for alpha-actinin, vinculin, paxillin and tensin, the integrin chains alpha1 and beta1, and the basement membrane components laminin and heparan/-chondroitin sulfate proteoglycan perlecan. pp125(FAK) did not react. In the extracellular matrix of blood vessel walls and the perivascular stem villous sheath, we found immunoreactivity of fibronectin and collagen types I, VI and undulin (collagen type XIV). From our data we conclude that within the perivascular stem villous sheath, there exists a system of signal transduction molecules, indicating a cross talk between the smooth muscle cells of this sheath and their surrounding extracellular matrix.     

Precision crystal corner cube arrays for optical gratings formed by (100) silicon planes with selective epitaxial growth

High-quality, micrometer scale, corner cube arrays were grown on (111) silicon substrates by selective epitaxial growth (SEG) techniques. Sixteen different arrays were produced that had periodic corner spacing ranging from 3 to 50 μm. The arrays were formed by suppressing silicon SEG in a regular geometric pattern, producing the three mutually perpendicular (100) smooth crystal planes. For coherent light of 633-nm wavelengtha sharp diffraction pattern of threefold symmetry was observed out to 7 maxima, as well as a retroreflection component.     

Cross-sectional TEM and KOH-Etch Studies of Extended Defects in 3C-SiC <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">n</Emphasis> Junction Diodes Grown on 4H-SiC Mesas

This article presents cross-sectional transmission electron microscopy and molten-potassium hydroxide etching studies of (111) 3C-SiC diodes which we previously reported to be free of forward-voltage drift despite abundant electroluminescent linear features presumed to be defects. Our results show that the majority of linear features are stacking faults lying in inclined {111} planes. Additionally, high densities of isolated etch pits (10⁶–10⁸ cm⁻²) are observed in 3C films grown on stepped 4H mesas, while 3C films nucleated on step-free 4H mesas exhibited orders of magnitude fewer etch pits and stacking faults. Defect formation mechanisms whose impetuses are steps on the 4H-SiC pregrowth mesa are discussed.     

High-temperature electronics - a role for wide bandgap semiconductors?

The fact that wide bandgap semiconductors are capable of electronic functionality at much higher temperatures than silicon has partially fueled their development, particularly in the case of SiC. It appears unlikely that wide bandgap semiconductor devices will find much use in low-power transistor applications until the ambient temperature exceeds approximately 300/spl deg/C, as commercially available silicon and silicon-on-insulator technologies are already satisfying requirements for digital and analog VLSI in this temperature range. However practical operation of silicon power devices at ambient temperatures above 200/spl deg/C appears problematic, as self-heating at higher power levels results in high internal junction temperatures and leakages. Thus, most electronic subsystems that simultaneously require high-temperature and high-power operation will necessarily be realized using wide bandgap devices, once they become widely available. Technological challenges impeding the realization of beneficial wide bandgap high ambient temperature electronics, including material growth, contacts, and packaging, are briefly discussed.     

Vertical bipolar transistors fabricated in local silicon oninsulator films prepared using confined lateral selective epitaxialgrowth (CLSEG)

A new technique called the confined lateral selective epitaxial growth (CLSEG) process has been used successfully to produce thin local silicon-on-insulator (SOI) films of high material quality. Two different vertical bipolar transistor structures are fabricated in local SOI to evaluate the material quality and to demonstrate the versatility of the CLSEG technique. The first bipolar structure emitter is formed by ion implantation silicon and demonstrates maximum DC current gains (β _max) of 400 with junction ideality factors of less than 1.08. A second bipolar structure is fabricated which simultaneously forms both the emitter and subcollector regions. The subcollector is formed on the underside of the local SOI film by exposing it during the emitter phosphorus diffusion and serves to reduce parasitic collector resistance (r'_C). These nonoptimized underdiffused devices have measured β_max=158 and lower r'_C. A PISCES simulation accurately predicts the measured r'_C value and indicates values at least as low as 74 Ω in an optimized layout     

Significant long-term reduction in n-channel MESFET subthresholdleakage using ammonium-sulfide surface treated gates

Ammonium-sulfide ((NH₄)₂S) treated gates have been employed in the fabrication of GaAs MESFETs that exhibit a remarkable reduction in subthreshold leakage current. A greater than 100-fold reduction in drain current minimum is observed due to a decrease in Schottky gate leakage. The electrical characteristics have remained stable for over a year during undesiccated storage at room temperature, despite the absence of passivation layers     

One-transistor GaAs MESFET- and JFET-accessed dynamic RAM cells forhigh-speed medium density applications

Theoretical and experimental work that presents the possibility for a high-speed, low-power one-transistor room-temperature dynamic RAM (DRAM) technology in GaAs is discussed. Isolated storage capacitors have demonstrated over 20 min of storage time at room temperature with charge densities comparable to that obtained with planar silicon technology. One-transistor MESFET- and JFET-accessed DRAM cells have been fabricated and operated at room temperature and above. The standby power dissipation of these first cells is only a small fraction of the power dissipated by the best commercial GaAs static RAM (SRAM) cells