Ryun-Hwi Kim

Normally Off Single-Nanoribbon

A single-nanoribbon Al₂O₃/GaN metal-insulator-semiconductor field-effect transistor (MISFET) has been fabricated. The fabricated device exhibits normally off operation with a threshold voltage of 2.1 V. The device also exhibits superior performances such as a maximum drain current density of 1.51 A/mm, a maximum transconductance of 580 mS/mm, and a field-effect mobility of 293 cm²·V^-1·s^-1. This is because the electron concentration in the GaN channels can be increased due to the enhanced gate controllability, which, thus, effectively screens the field lines from the interface traps or the defects near the channels to improve the electron mobility in the channel. The nanoribbon Al₂O₃/GaN MISFET is a very promising candidate for high-performance normally off GaN FETs.

\hbox{Al}_{2} \hbox{O}_{3}\hbox{/GaN}

Heavily doped GaN nanochannel FinFET has been proposed and fabricated, for the first time, which does not have any p-n junction or heterojunction. In spite of its easy and simple epitaxial growth and fabrication process, the fabricated device with nanochannel width of 80 nm and gate length of 1 μm exhibited excellent off-state performances such as extremely low off-state leakage current of ~ 10-11 mA with BV of ~ 300 V and subthreshold slope of 68 mV/decade, very close to the theoretically limited value, which leads to high Ion/Ioff ratio of 107 ~ 109. The device also exhibited high on-state performances such as maximum drain current of 562 mA/mm and maximum transconductance of 138 mS/mm. The proposed nanochannel GaN FinFET can be very promising candidate not only for high performance, but also high power applications.

MISFET

AlGaN/GaN-based fin-shaped field-effect transistors (FinFETs) with very steep sidewall and various fin-widths (Wfin) have been fabricated by utilizing electron-beam lithography and additional anisotropic sidewall wet etch in tetramethyl ammonium hydroxide (TMAH) solution. The device with Wfin of 180 nm exhibits normally-on performance with threshold voltage of -3.5 V and extremely broad transconductance (gm) characteristic ranging from -2 to ~ 3 V at VD = 5 V which is essential for high linearity device performance. This broad gm characteristic is because the current from the side-wall MOS channel becomes comparable to that from the two-dimensional electron gas (2DEG) channel and hence significantly contributes to the total device current. On the other hand, devices with smaller Wfin = 50 and 100 nm exhibit normally-off performance with positive threshold voltage of 2.0 and 0.6 V, respectively, and less broad gm characteristics because the current from the side-wall MOS channel dominates the total device current.

First demonstration of heterojunction-free GaN nanochannel FinFETs

The current-voltage (I-V) characteristics of Au/polyvinyl alcohol (PVA)/n-InP Schottky diode have been measured at temperature range 175-425 K. It is found that the series resistance (RS) values of Au/PVA/n-InP Schottky diode estimated from Cheungs and Nordes methods, are strongly temperature dependent. The values of barrier height and RS have very different especially towards to the lower temperatures. This is attributed to non-ideal I-V characteristics of the MIS structure and non-pure thermionic emission (TE) mechanism due to the low temperature effects.

Fabrication of high performance AlGaN/GaN FinFET by utilizing anisotropic wet etching in TMAH solution

This letter presents the MISHFET with si-doped AlGaN/GaN heterostructure for power amplifier. The device grown on 6H-SiC(0001) substrate with a gate length of 180 nm has been fabricated. The fabricated device exhibited maximum drain current density of 837 mA/mm and peak transconductance of 177 mS/mm. A unity current gain cutoff frequency was 45.6 GHz and maximum frequency of oscillation was 46.5 GHz. The reported output power density was 1.54 W/mm and A PAE(Power Added Efficiency) was 40.24 % at 9.3 GHz.

Effect of temperature on series resistance determination of Au/polyvinyl alcohol/n-InP Schottky structures

We have fabricated normally-off two different GaN MOSFETs with Al₂O₃ gate insulator with thickness of 38- and 54-nm and investigated the difference in device performance. From the C-V measurement, relatively higher threshold voltage and smaller threshold voltage shift were observed in the device with 54-nm-thick Al₂O₃ gate insulator compared to those of the device with 38-nm-thick Al₂O₃ gate insulator. Due to the decreased Cox with increased Al₂O₃ thickness, the device with 54-nm-thick Al₂O₃ gate insulator exhibited a little degraded DC performances compared to those of the device with 38-nm-thick Al₂O₃ gate insulator. Also, the device with 54-nm-thick Al₂O₃ exhibited better off-state characteristic with higher breakdown voltage than the device with 38-nm-thick Al₂O₃. However, no significant differences in the gate leakage characteristics were observed with low gate leakage current.