The DRC included a late paper by Cree on their observation of electrically pumped laser emission in the nitrides; a paper on UV detectors; and 3 papers on nitride-based FETs for microwave and high temperature applications. Nitrides were also highlighted in one of the Rump Sessions.
The laser Late Paper (co-authored by Cree, North Carolina State University and Brown University, with a total of 15 authors) described an electrically driven blue laser based on an InGaN/GaN MQW structure grown on 6H-SiC. Room temperature laser emission at 403 nm was observed, with a pulsed threshold current of 1.2A, corresponding to a current density of 48 kA/cm2 for the 5µm wide stripe device (which had cleaved facets). The high thermal conductivity of the SiC substrate aided in getting the laser action. The layer structure included 8 quantum wells of In0.14Ga0.86N/GaN; Al0.06Ga0.94N and Al0.13Ga0.87N layers were used for waveguiding and cladding.
The UV photodetector paper, by University of Illinois, Air Force Wright Laboratory and Honeywell, describes detectors with internal quantum efficiencies up to 50%, and low dark currents (below 100pA in modest size mesa diodes). Growth was by reactive MBE, using NH3 as a nitrogen source.
University of Illinois and Wright Laboratory combined also to describe AlGaN/GaN HFETs with high current and high transconductance at high temperature. Devices were tested up to 500C. Despite a large gate length of 1.75µm, the FETs exhibited very large full channel current (up to 1.1A/mm at room temperature, 0.5A/mm at 500C) and transconductance (up to 270mS/mm and 80 mS/mm at the two temperatures, respectively). Drain-source leakage was moderate at the high temperature.
A paper by Yi-Feng Wu et al. of the UCSB group, on "High Speed and High Power AlGaN/GaN MODFETs", provided a review of their accomplishments in demonstrating ultrahigh FET fT, breakdown voltage, and microwave power density in these devices. FT up to 50GHz was shown, for 0.2-0.25µm gate length devices; fmax was up to 92GHz. Devices with Lg=0.7µm provided drain-source breakdown voltage above 200V. 10 GHz power density results were as high as 2.6W/mm.
An additional Late Paper was given by the UCSD / Rockwell group, in which the importance of the piezoelectric effect on the characteristics of AlGaN/GaN HFETs was highlighted. The piezoelectric effect was shown to be the basis for the large sheet carrier densities observed in undoped AlGaN/GaN devices; for the fact that parallel MESFET channels in the AlGaN are not found even at high sheet charge density; for the behavior of HFETs after recess etching in the gate and ohmic regions; for the reduction of sheet charge under conditions where there is a buried AlGaN layer; and for the anomalously high breakdown voltage in AlGaN/GaN HFETs.
The Rump Session of interest to the nitrides was amusingly entitled, "Does SiC have a choke-hold on switching and RF Power?". It evolved into a discussion about whether GaN-based HFETs would compete successfully against SiC SITs and MESFETs, which are already emerging into the 10GHz range, with power density of 3.3W/mm in a 6W device, and power-added efficiency up to about 35%. Umesh Mishra of UCSB was eloquent on behalf of GaN FETs - highlighting, among other things, the fact that blue light emission will carry GaN research and manufacturing forward to high volume and low cost; FETs will get a free ride, and will take over the application slots.
Peter Asbeck and Edward Yu University of California, San Diego Department of Electrical and Computer Engineering, MS 0407 La Jolla, CA 92093-0407 email: asbeck@ece.ucsd.edu, ety@ece.ucsd.edu
last updated August 6, 1997 2:41:35 PM.
© 1997 The Materials Research Society
