Guan-Ting Chen

MgO/p-GaN enhancement mode metal-oxide semiconductor field-effect transistors

We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor (MOSFET) utilizing Si+ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion. The breakdown voltage for an 80-nm-thick MgO gate dielectric was ∼14 V, corresponding to a breakdown field strength of 1.75 MV cm−1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage >15 V. Inversion of the channel was achieved for gate voltages above 6 V. The maximum transconductance was 5.4 μS mm−1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs.

Effect of external strain on the conductivity of AlGaN/GaN high-electron-mobility transistors

The changes in conductance of the channel of AlGaN/GaN high electron mobility transistor structures during application of both tensile and compressive strain were measured. For fixed Al mole fraction, the changes in conductance were roughly linear over the range up to 2.7x 10 8 N.cm -2 , with coefficients for planar devices of -6.0 +/- 2.5 x 10 -10 S.N -1 .m -2 for tensile strain and +9.5+/-3.5 x10 -10 S.N -1 .m -2 for compressive strain .For mesa-isolated structures, the coefficients were smaller due to the reduced effect of the AlGaN strain, with values of 5.5 +/- 1.1 x10 -13 S.N -1 .m -2 for tensile strain and 4.8 x10 -13 S.N -1 .m -2 for compressive strain. The large changes in conductance demonstrate that simple AlGaN/GaN heterostructures are promising for pressure and strain sensor applications.

AlGaN/GaN HEMT based liquid sensors

Abstract An AlGaN/GaN high electron mobility transistor structure was used for sensing different liquids present in the gate region. The forward current showed significant decreases upon exposure of the gate area to solvents (water, acetone) or acids (HCl). The pH sensitivity is due to changes in net surface charge that affects the relative depletion in the channel of the transistor. The results indicate that nitride-based heterostructures may have application in integrated chemical, gas and fluid monitoring sensors.

Performance enhancement by using the n/sup +/-GaN cap layer and gate recess technology on the AlGaN-GaN HEMT fabrication

Due to the low mobility and wide bandgap characteristics of the undoped AlGaN layer used in the conventional AlGaN-GaN HEMT as a cap layer, the RF performance of this device will be limited by its high contact resistance and high knee voltage. In this letter, we propose using the n/sup +/-GaN cap layer and the selective gate recess etching technology on the AlGaN-GaN HEMTs fabrication. With this n/sup +/-GaN instead of the undoped AlGaN as a cap layer, the device contact resistance is reduced from 1.0 to 0.4 /spl Omega//spl middot/mm. The 0.3 /spl mu/m gate-length device demonstrates an I/sub ds,max/ of 1.1 A/mm, a g/sub m,max/ of 220 mS/mm, an f/sub T/ of 43 GHz, an f/sub max/ of 68 GHz, and an output power density of 4 W/mm at 2.4 GHz.

Luminescence efficiency of InGaN multiple-quantum-well ultravioletlight-emitting diodes

The electroluminescence efficiency of In0.06Ga0.94N∕GaN multiple-quantum-well UV light-emitting diodes (LEDs) with emission wavelength of 400nm has been investigated and compared with blue (470nm) LEDs. Based on their injection current-dependent characteristics under dc and pulsed operation, it can be concluded that carrier overflow is the dominant factor that affects the external quantum efficiency of UVLED before thermal effects take over. It is experimentally shown that increasing the number of quantum wells is necessary to alleviate the carrier overflow issue and improve the luminescence efficiency of the UVLEDs.

On the origin of spin loss in GaMnN/InGaN light-emitting diodes

Spin polarization of GaMnN/InGaN light-emitting diodes grown by molecular beam epitaxy is analyzed. In spite of the ferromagnetic behavior of the GaMnN spin injector, the diodes are shown to exhibit very low efficiency of spin injection. Based on resonant optical orientation spectroscopy, the spin loss in the structures is shown to be largely due to fast spin relaxation within the InGaN spin detector, which itself destroys any spin polarization generated by optical spin orientation or electrical spin injection.

Laser emission from GaN photonic crystals

In this study, photonic crystals have been designed, fabricated, and characterized in GaN bulk materials. The energy dependent measurement showed that the emission peak width can be significantly reduced as the pumping pulse energy was larger than 0.7μJ at room temperature. The mode at the wavelength of 371nm emitted from the defect due to the structure disorder unintentionally introduced during the fabrication process of the GaN photonic crystals can be obtained.

Current–voltage and reverse recovery characteristics of bulk GaN p-i-n rectifiers

p-i-n rectifiers were fabricated on epitaxial layers grown on free-standing GaN substrates. The forward turn-on voltage, VF was ∼5 V at 300 K and displayed a positive temperature coefficient. The specific on-state resistance (RON) was ∼5 mΩ cm2 at 300 K, with an ideality factor of ∼2 and activation energy for low forward current density of ∼1.6 eV. This is consistent with carrier recombination in the space charge region via a midgap deep level. The figure-of-merit, VB2/RON, where VB is the reverse breakdown voltage, was 0.32 MW cm−2. The reverse recovery time was ⩽600 ns at 300 K. The improved forward characteristics relative to previous heteroepitaxial p-i-n GaN rectifiers show the advantages of employing a GaN substrate to make a true vertical transport geometry device.

Optical piezoelectric transducer for nano-ultrasonics

Piezoelectric semiconductor strained layers can be treated as piezoelectric transducers to generate nanometer-wavelength and THz-frequency acoustic waves. The mechanism of nano-acoustic wave (NAW) generation in strained piezoelectric layers, induced by femtosecond optical pulses, can be modeled by a macroscopic elastic continuum theory. The optical absorption change of the strained layers modulated by NAW through quantum-confined Franz-Keldysh (QCFK) effects allows optical detection of the propagating NAW. Based on these piezoelectric-based optical principles, we have designed an optical piezoelectric transducer (OPT) to generate NAW. The optically generated NAW is then applied to one-dimensional (1-D) ultrasonic scan for thickness measurement, which is the first step toward multidimensional nano-ultrasonic imaging. By launching a NAW pulse and resolving the returned acoustic echo signal with femtosecond optical pulses, the thickness of the studied layer can be measured with <1 nm resolution. This nano-structured OPT technique will provide the key toward the realization of nano-ultrasonics, which is analogous to the typical ultrasonic techniques but in a nanometer scale.

Low damage, Cl/sub 2/-based gate recess etching for 0.3-/spl mu/m gate-length AlGaN/GaN HEMT fabrication

The traditional dry etching for GaN using the Ar/Cl/sub 2/ mixture gas in the reactive ion etching system has been developed. In order to reduce the surface damage, the additional CH/sub 4/ gas is introduced. However, this approach still has the problems of the residual surface damage and low etching selectivity between the AlGaN and GaN materials. Therefore, the following rapid thermal annealing (RTA) at 700/spl deg/C is necessary to recover the surface properties. In this study, we proposed the Ar/Cl/sub 2//CH/sub 4//O/sub 2/ for the GaN gate-recess etching in AlGaN/GaN HEMTs fabrication, which achieves a low surface damage and a high etching selectivity simultaneously. The 0.3 /spl mu/m gate-length AlGaN/GaN HEMTs present a transconductance of 230 mS/mm, an f/sub T/ of 48 GHz, and f/sub max/ of 60 GHz, respectively.