Kengo Kobayashi

Ultrahigh sensitive sub-terahertz detection by InP-based asymmetric dual-grating-gate high-electron-mobility transistors and their broadband characteristics

We report on room-temperature plasmonic detection of sub-terahertz radiation by InAlAs/InGaAs/InP high electron mobility transistors with an asymmetric dual-grating-gate structure. Maximum responsivities of 22.7 kV/W at 200 GHz and 21.5 kV/W at 292 GHz were achieved under unbiased drain-to-source condition. The minimum noise equivalent power was estimated to be 0.48 pW/Hz0.5 at 200 GHz at room temperature, which is the record-breaking value ever reported for plasmonic THz detectors. Frequency dependence of the responsivity in the frequency range of 0.2–2 THz is in good agreement with the theory.

A new process approach for slant field plates in GaN-based high-electron-mobility transistors

A new process approach to realize a slant field plate — a field plate (FP) gradually separated from the semiconductor surface from the gate edge toward the drain — is reported. A multistep SiCN dielectric film is used to make a sloped sidewall in the dielectric film consisting of a slant FP. The sidewall shape is controlled by the number of steps in SiCN. AlGaN/GaN high-electron-mobility transistors (HEMTs) with slant and conventional FPs are prepared using this technique. The advantage of slant FPs over conventional FPs is experimentally confirmed as a result of reduced current collapse, higher current gain cutoff frequency, and higher off-state breakdown voltage.

Improved breakdown voltage and RF characteristics in AlGaN/GaN high-electron-mobility transistors achieved by slant field plates

We experimentally demonstrate the efficacy of using slant field plates (field plates with the plate-to-channel gap gradually increasing away from the gate edge) on the breakdown voltage. We develop a new fabrication process using a multi-step SiCN film such that both slant and conventional field plates are fabricated simultaneously. Consequently, we fabricate 230-nm-gate AlGaN/GaN HEMTs with several types of field plates. The slant field plate increases the breakdown voltage by 66% more than that of the conventional field plate. The advantages of using slant field plates to increase the breakdown voltage are experimentally confirmed for the first time.

Scanning Tunneling Microscopy and Barrier-Height Imaging of Subsurface Dopants on GaAs(110)

Subsurface dopant atoms are investigated by scanning tunneling microscopy (STM) and barrier height (BH) imaging. The obtained BH image shows a local reduction in BH -(1–2) eV at positively charged donor sites while it shows a local increase in BH (1–5) eV at negatively charged surface Ga vacancies. Our results indicate that the charge-sensitive imaging can be accomplished by utilizing the STM–BH technique. A simple one-dimensional simulation of tunneling current through a STM junction on the GaAs(110) surface indicates that the results of BH imaging well reflect the charge state of the subsurface dopant or charged surface defects under appreciable electron accumulation conditions.

Scanning Tunneling Microscopy Barrier Height Imaging of Subsurface Dopant Atoms on Hydrogen-Terminated Si(111)

Dopant atoms beneath a hydrogen-terminated Si(111) 1×1 surface are investigated by scanning tunneling microscopy (STM) and barrier height (BH) imaging. Because of the weak screening effect of the hydrogen-terminated Si(111) surface, subsurface donors and acceptors can be imaged by STM. From BH images, we can deduce a local change in BH at subsurface dopant sites. Both the sign and magnitude of the observed BH change are in good agreement with calculations assuming a screened Coulomb potential for a dopant potential.

Extremely-high sensitive terahertz detector based on dual-grating gate InP-HEMTs

We report on an extremely-high sensitive terahertz (THz) detector based on our original asymmetric dual-grating gate high electron mobility transistors (A-DGG HEMTs) designed and fabricated using InAlAs/InGaAs/InP material systems. The obtained responsivity is 22.7 kV/W at 200 GHz. To the best of our knowledge, this value is the record responsivity ever reported for this frequency range at room temperature.

Ultrahigh Sensitive Non-Resonant and Resonant Terahertz Detection by Asymmetric Dual-Grating Gate HEMTs

We have proposed terahertz (THz) detectors based on our original asymmetric dual-grating gate high electron mobility transistors (A-DGG HEMTs) designed and fabricated using InAlAs/InGaAs/InP material systems. We report on a record responsivity of 22.7 kV/W at 200 GHz and a superior low noise equivalent power (NEP) of 0.14 pW/Hz 0.5 at 292 GHz at room temperature. In addition, a resonant mode detection was observed at low temperature.

Local Tunneling Barrier Height at and around Subsurface Dopant Sites on p-GaAs(110)

The local tunneling barrier height at and around subsurface Zn acceptor atoms on GaAs(110) has been investigated by scanning tunneling microscopy (STM) and local barrier height (LBH) imaging. The measured LBH showed the bias dependence, which was in good agreement with the theoretical simulation that takes into account the effect of tip-induced band bending (TIBB). Above the acceptor sites, the bias dependence of LBH shifted to the positive-bias side as compared with that of the area distant from acceptors. We explained the observed shift by the additional local band bending that is induced by ionized acceptors.