GaN-based optoelectronic devices on sapphire and Si substrates

A recessed gate AlGaN/GaN high-electron mobility transistor (HEMT) on sapphire (0 0 0 1), a GaN metal-semiconductor field-effect transistor (MESFET) and an InGaN multiple-quantum well green light-emitting diode (LED) on Si (1 1 1) substrates have been grown by metalorganic chemical vapor deposition. The AlGaN/GaN intermediate layers have been used for the growth of GaN MESFET and LED on Si substrates. A two-dimensional electron gas mobility as high as 9260 cm²/V s with a sheet carrier density of 4.8×10¹² cm⁻² was measured at 4.6 K for the AlGaN/GaN heterostructure on the sapphire substrate. The recessed gate device on sapphire showed a maximum extrinsic transconductance of 146 mS/mm and a drain–source current of 900 mA/mm for the AlGaN/GaN HEMT with a gate length of 2.1 μm at 25°C. The GaN MESFET on Si showed a maximum extrinsic transconductance of 25 mS/mm and a drain–source current of 169 mA/mm with a complete pinch-off for the 2.5-μm-gate length. The LED on Si exhibited an operating voltage of 18 V, a series resistance of 300 Ω, an optical output power of 10 μW and a peak emission wavelength of 505 nm with a full-width at half-maximum of 33 nm at 20 mA drive current.     

Achieving multimedia communications on a heterogeneous network

A heterogeneous intelligent network has been designed and implemented to allow multimedia real-time communications and messaging among local area network workstations with logically associated speakerphones and among ISDN voice/data terminals with transparent tablets on top of bitmap screens and stylus pens. The architecture, presentation, editing, storage, and retrieval of multimedia documents in these two different kinds of terminals are described. The messages and protocol conversion facilities used for multimedia communications are presented, as are the lessons learned in internetworking a local area network with ISDN     

Effects of postdetection selection diversity reception in QDPSK land mobile radio

Postdetection selection diversity reception for Nyquist pulse shaped quadrature differential PSK (QDPSK) signal transmissions is addressed. Average bit error rate (BER) performance with differential and coherent demodulation is analysed for multiple-branch diversity reception under slow multipath Rayleigh fading. Diversity effects in real fading environments are verified by 8 and 16 kbit/s transmissions at 1.45 GHz carrier frequency.<>     

Preparation of monolithic silica gel and glass by the sol-gel method using N,N-dimethylformamide

The use of N,N-dimethylformamide (DMF) was found to be effective in the sol-gel synthesis of silica gel and glass monoliths from tetramethoxysi lane (TMOS) by the sol-gel method. Wet gel bodies prepared by gelation of TMOS-DMF-CH₃OH-H₂O-NH₄OH (or HNO₃) solutions could be dried without the occurrence of fracture or cracks when drying conditions were appropriate. Furthermore, the resultant dried gel monoliths could be converted without cracking or bloating to silica glass on heating up to 1050° C and holding there for 2 h, when the composition of the starting solution was appropriate.     

Degradation of Pentoses and Hexouronic Acids in Subcritical Water

The degradation of arabinose, xylose, ribose and lyxose in subcritical water was measured at 200, 220 and 240 °C. Ribose was the most rapidly degraded among the pentoses tested. The degradation of the glucuronic and galacturonic acids proceeded at lower temperatures than that of the pentoses, and was measured at 140, 150 and 160 °C. The degradation processes of the pentoses and hexouronic acids could be expressed by the Weibull model, and the kinetic parameters were then estimated. The activation energy and frequency factor for the degradation of each substrate were estimated from the temperature dependence of the rate constant. The enthalpy‐entropy compensation held for the degradation of the pentoses as well as the hexoses, which suggests that the degradation of the hexouronic acids proceeded through a mechanism different from that for the pentoses and hexoses. The molar yield of a pentose to furfural was ca. 0.3 at any temperature, irrespective of the pentose type. Acidic compounds were also formed from the pentoses in proportion to the amount of consumed substrates. The formation of acidic compounds resulted in a rapid decrease in pH.