Showing papers by "Evelyn L. Hu published in 2003"

n-AlGaAs/p-GaAs/n-GaN heterojunction bipolar transistor wafer-fused at 550–750 °C

Abstract: We recently reported an initial AlGaAs/GaAs/GaN heterojunction bipolar transistor (HBT), formed via wafer fusion of a p-GaAs base to an n-GaN collector. The device was formed by fusion at a high temperature (750 °C) and demonstrated low output current (∼100 A/cm2) and low common-emitter current gain (0.5). This letter describes a systematic variation of fusion temperature (550–750 °C) in the formation of the HBT, and reveals the correlation between fusion temperature, base–collector leakage, and emitter–base degradation. With reduced fusion temperatures, devices demonstrate improvements in leakage, output current (∼1 kA/cm2), and common-emitter current gain (>1). Optimization of device structure should further improve performance.

Wafer-fused n-AlGaAs/p-GaAs/n-GaN Heterojunction Bipolar Transistor with uid-GaAs Base-Collector Setback

Abstract: Recently we reported the first AlGaAs-GaAs-GaN heterojunction bipolar transistor (HBT), a device that potentially combines the high-breakdown voltage of an n-GaN collector with the high mobility of an AlGaAs-GaAs emitter-base.1,2 Because of the high degree of lattice mismatch between GaAs (lattice constant of 5.65A) and GaN (3.19A), we formed these devices through wafer fusion, also called direct wafer bonding. Measurements on the first generations of wafer fused HBTs revealed good current modulation, with modest output current (0.83 KA/cm2) and a current gain of 1.2.3 Limitations to the current gain may be related to traps and defects introduced by the fusion process, or may be a consequence of the natural conduction band offset between GaAs and GaN, which is not well known. This paper describes our new HBT structure that included a thin (20nm) uid-GaAs base-collector “setback” layer. The setback layer shifted the fused GaAs-GaN interface slightly into the collector. This new HBT structure also incorporated a reduced base thickness of 100 nm. HBTs with setback layers demonstrate increased output current (1.7 KA/cm2) and increased current gain (1.9).

Analysis of Leakage Currents in AlGaN/GaN Current Aperture Vertical Electron Transistors (CAVETs)

Abstract: A complete analysis of leakage currents in AlGaN/GaN Current Aperture Vertical Electron Transistors (CAVETs) with regrown aperture and source regions was carried out. The total observed leakage current was found to be a combination of both gate leakage and source leakage. Two paths for source leakage have been identified; electrons passing directly through the insulating layer to the drain region as well as electrons traveling through the aperture but underneath the 2DEG at the AlGaN/GaN interface. Source leakage through the insulating layer resulted from pits formed at the onset of regrowth, as the sample was heated to growth temperature, and was successfully eliminated by optimizing regrowth conditions. Source leakage underneath the 2DEG occurred when the unintentionally doped (UID) GaN layer above the insulating layer was not fully depleted and could be eliminated by reducing the thickness of the UID GaN layer. Gate leakage has been attributed to the enhanced incorporation of n-type impurities inside as well as above the aperture region during regrowth, resulting in a narrowing ofthegateSchottkybarrier.

InAs quantum dot microdisk injection laser

Abstract: An airbridge contacted quantum dot microdisk diode with continuous-wave lasing at 5 K is reported. The threshold current is as low as 43 /spl mu/A in 4 /spl mu/m diameter microdisks. Peak shifts indicate thermal heating of the structure.

Systematic transmission characterization of 2D InP photonic crystal membrane structures

Abstract: Two-dimensional photonic crystal membranes integrated with ridge-waveguides were fabricated in InP/InGaAsP and characterized by optical transmission measurements A photonic band-gap with at least 20 dB extinction was observed. Accurate band-edge tuning by varying r/a is demonstrated.