research  review Trapezoidal wells combat LED droop


WIDER WELLS, slimmer barriers and polarization-matched active regions have all been used over the past few years to minimize droop, the decline in LED efficiency at higher drive currents. Inserting trapezoidal wells into the active region can now be added to this growing list, thanks to the efforts of a team of researchers from Korea.


These scientists from Gwangju Institute of Science and Technology and Samsung LED company claim that the addition of trapezoidal wells increases electron-hole overlap, which in turn cuts droop.


Two of the hallmarks of the Korean LED are a slight reduction in forward voltage and series resistance. Seong-Ju Park from Gwangju Institute of Science and Technology claims that these attributes stem from superior carrier transport, which is the result of a graded indium-composition in the trapezoid well layer. The Korean team compared the performance of their blue- emitting trapezoidal LED with a conventional equivalent. They made both of the devices by depositing a GaN-based epitaxial stack on sapphire by MOCVD, and then etching


550 µm by 550 µm mesas with an inductively coupled plasma. Deposition of ITO added transparent contacts.


Trapezoidal wells had a 0.5 nm-thick InGaN core sandwiched between two 1.5 nm-thick layers that were linearly graded in alloy composition to 7.5 nm-thick GaN barriers. Identical barriers featured in the conventional LED, which had a 2.5 nm-thick InGaN well. External quantum efficiency (EQE) measurements revealed that the trapezoidal LED produces a 19 percent higher output at 35 A cm-2


percent improvement at 70 A cm-2


, rising to a 20 .


The EQE of the novel LED overtook the control LED at 5 A cm-2


. This is a very low


value compared with the crossover current densities reported in previous studies by other groups, according to the team. Both of the devices were devoid of any light extraction technology. This accounts for the low values of EQE, which peaked at about 30 percent.


Modeling both device architectures with LED simulator SiLENSe uncovered a relationship between electron-hole overlap


HRL speeds E-mode transistor


HRL Laboratories claims to have simultaneously broken three records for an enhancement-mode, GaN HFET: maximum transconductance; highest cut-off frequency; and the highest value for maximum oscillation frequency.


The AlN/GaN/AlGaN double heterojunction FET developed by HRL had a peak transconductance of 700 mS/mm, a cut-off frequency of 112 GHz and maximum oscillation frequency of 215 GHz.


Although these values are inferior to best figures produced by their far more common depletion-mode cousins, the E-mode version has one major advantage: it is normally off. This simplifies circuit design; aids power switching, by reducing power consumption and increasing safety; and enables creation of E/D-mode logic for digital circuits and mixed-signal applications.


HRL’s motivation for its efforts is focused on the development of GaN transistors and


38 www.compoundsemiconductor.net October 2010


integrated circuit technology for high- performance analog-to-digital converters that can be deployed in future advanced electronic systems. This work is funded by DARPA’s NEXT program.


The engineers at HRL attribute their success to aggressive reduction of parasitic resistances and vertical scaling.


Fabrication of the double-heterostructure FETs involved the growth of a GaN-based epitaxial stack on 3-inch SiC. This comprised an Al0.08


Ga0.92 N buffer, a 20-40 nm GaN channel, a 2 nm AlN barrier and a 2.5 nm cap.


Mesas were formed in the material, before plasma-enhanced CVD deposited a SiO2 mask. This was patterned and etched to within 20 nm of the sample surface in the source and drain regions. MBE growth of a 50 nm, heavily silicon-doped GaN layer was added to facilitate the formation of low- resistance ohmic contacts, before the silicon mask was removed.


and droop. Switching from the conventional well to the trapezoidal one smoothed conduction and valence bands profiles, increased electron overlap from 37.2 percent to 41.6 percent, and cut the distance between the maxima of the electron and hole wavefunctions from 1.5 mm to 1.1 mm.


“Since the volume of the trapezoidal well is 20 percent smaller than the standard well, our study shows that overlap of the electron-hole wavefunction in the trapezoidal well is more important than the non-radiative Auger process,” claims Park.


Interestingly, Park can account for the improved EQE of the double heterostructure LEDs, which have been pioneered by Lumileds as an approach for cutting droop. He says that in this wide-well LED there is very little band-bending of the InGaN layer. “This result indicates that the electron-hole wavefunction overlap in the double heterostructure can be much higher than in the quantum well.”


S.-H. Han et al. J. Phys. D. Appl. Phys. 43 354004 (2010)


After Ti/Al/Pt contacts were deposited, T- shaped Pt/Au gates were added by: depositing a sacrificial mask layer; patterning the gate foot with e-beam lithography; and performing gate top lithography, metallization, liftoff and mask removal.


These record-breaking E-mode double heterostructure FETs had a 2 x 37.5 µm gate periphery and produced a maximum drain current of 0.92 mA/mm at 2 V gate bias. On-resistance was 1.06 Ω mm.


Cut-off frequency and maximum oscillation frequency records were realized at a drain bias of 2.0 V. According to HRL’s engineers, previous claims for these records have tended to employ higher drain biases, typically 5-10 V, which were needed to overcome the excess voltage drop across parasitic resistances.


The team claims that optimized lateral-device scaling could spur further improvements in its E-mode FET performance.


A. Corrion et al. to appear in Electron. Dev. Lett. (2010)


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