Hyun-Seop Kim

Au-free AlGaN/GaN heterostructure field-effect transistor with recessed overhang ohmic contacts using a Ti/Al bilayer

We have developed a low-temperature ohmic contact process with a recessed overhang configuration for Au-free (complementary metal-oxide semiconductor) CMOS-compatible AlGaN/GaN heterostructure field effect transistors (HFETs). The recessed overhang configuration has a Ti/Al bilayer directly in contact with the AlGaN/GaN heterojunction interface at the recessed sidewall overlaid with the AlGaN barrier layer, which allows good and reproducible ohmic formation with low-temperature annealing. The optimum Ti/Al thickness was 40/200 nm, which resulted in an Rc of 0.76 Ω mm with excellent surface morphology when annealed at 550 °C for 1 min. The device fabricated with a Ni/Mo gate exhibited a maximum drain current density of ~500 mA mm−1, a specific on-resistance of 1.35 mΩ cm2 and a breakdown voltage of >1 kV.

Investigation of flat band voltage shift in recessed-gate GaN MOSHFETs with post-metallization-annealing in oxygen atmosphere

We have investigated the effects of post-metallization-annealing (PMA) in oxygen atmosphere on recessed-gate GaN metal-oxide-semiconductor heterostructure field effect transistors (MOSHFETs). The flat band voltage of MOS is a function of bulk and interface charges in the oxide, which strongly depends on a post-annealing process as well as deposition conditions. A positive threshold voltage shift enabling normally-off operation has been achieved by an O2 PMA process where the GaN MOSHFET employed an ICPCVD SiO2 gate oxide with a Ni/Au metal gate. According to the analysis using energy dispersive x-ray spectroscopy in transmission electron microscopy and x-ray photoelectron spectroscopy, it is suggested that the improved SiO2/GaN interface quality with an enhanced metallic-like Ga level was responsible for the positive shift in threshold voltage.

Comparison of photoresponsive drain conduction and gate leakage in n-channel pseudomorphic HEMT and MESFET under electro-optical stimulations

Photoresponsive drain conduction and gate leakage characteristics of n-channel PHEMT and MESFET are comparatively reported as a function of electro optical stimulation (V/sub GS/, V/sub DS/, P/sub opt/; /spl lambda/=830 nm). Both in PHEMT and MESFET, a strong nonlinearity of drain photoresponse (R) with P/sub opt/ was observed and can be modeled empirically as R=/spl kappa/P/sub opt//sup -/spl beta// (/spl kappa//sub PHEMT//spl Gt//spl kappa//sub MESFET/) where model parameters /spl kappa/ and /spl beta/ accommodate differences in device/epitaxial structures and electrical biases. Gate leakage current was linearly increasing with P/sub opt/ while it was independent of V/sub DS/ in both PHEMT and MESFET. However, I/sub G/ was a strong function of V/sub GS/ in PHEMT while it was almost independent in MESFET due to suppressed modulation of photoresponsive depletion width with heavy channel doping. Photonic gate response (R/sub pG/), on the other hand, was observed to be constant in MESFET while it was a strong function of electrical bias in PHEMT.

Effective Channel Mobility of AlGaN/GaN-on-Si Recessed-MOS-HFETs

We have investigated the channel mobility of AlGaN/GaN-on-Si recessed-metal-oxide-semiconductor-heterojunction field-effect transistors (recessed-MOS-HFET) with SiO 2 gate oxide. Both field-effect mobility and effective mobility for the recessed-MOS channel region were extracted as a function of the effective transverse electric field. The maximum field effect mobility was 380 cm 2 /V·s near the threshold voltage. The effective channel mobility at the on-state bias condition was 115 cm 2 /V·s at which the effective transverse electric field was 340 kV/cm. The influence of the recessed-MOS region on the overall channel mobility of AlGaN/GaN recessed-MOS-HFETs was also investigated.