Takuma Nanjo

Remarkable breakdown voltage enhancement in AlGaN channel high electron mobility transistors

The channel layer substitution of a wider bandgap AlGaN for a conventional GaN in high electron mobility transistors (HEMTs) is an effective method of enhancing the breakdown voltage. We demonstrated a remarkable breakdown voltage enhancement in these AlGaN channel HEMTs. The obtained maximum breakdown voltages were 463 and 1650V in the Al0.53Ga0.47N∕Al0.38Ga0.62N HEMT with the gate-drain distances of 3 and 10μm, respectively. This result is very promising for the further higher power operation of high-frequency HEMTs.

Highly resistive GaN layers formed by ion implantation of Zn along the c axis

Highly resistive layers are formed by the implantation of Zn ion along the c axis of GaN and AlGaN/GaN epitaxial layers. Heavy ions such as Zn have been desirable for the formation of highly resistive layers, because ions effectively transferred their energy to the crystal atoms rather than the electrons in GaN. A sheet resistance Rs as high as 3.8×1011 Ω/sq was obtained on GaN layers after the ion implantation. Rs increased up to 2.2×1013 Ω/sq after the annealing at 500 °C for 300 s in an N2 atmosphere. The thermal activation energy Er for this sample was 0.67 eV. It was found that the experimental data in current–voltage characteristics were fitted to the equation included the Poole–Frenkel current and resistive (ohmic) current. The difference of Rs between the as-implanted and 500 °C annealed samples was due to the Poole–Frenkel current. The Poole–Frenkel current overcame the resistive one, and dominated the current mechanism in the case of the samples annealed at 200 °C or less. On the other hand, for...

First Operation of AlGaN Channel High Electron Mobility Transistors

A channel layer substitution of a wider bandgap AlGaN for conventional GaN in high electron mobility transistors (HEMTs) is one possible method of enhancing the breakdown voltage for higher power operation. Wider bandgap AlGaN, however, should also increase the ohmic contact resistance. We utilized a Si ion implantation doping technique to achieve sufficiently low resistive source/drain contacts, and realized the first HEMT operation with an AlGaN channel layer. This result is very promising for the further higher power operation of high-frequency HEMTs.

Improvement of DC and RF Characteristics of AlGaN/GaN High Electron Mobility Transistors by Thermally Annealed Ni/Pt/Au Schottky Gate

A thermally annealed Ni/Pt/Au metal structure was employed as the gate contacts of AlGaN/GaN high electron mobility transistors (HEMTs), and their DC and RF performances were investigated. This gate structure markedly improved the Schottky characteristics such as the Schottky barrier height and leakage current. Regarding the DC characteristics, the maximum drain current and off-state breakdown voltage were increased from 0.78 A/mm (Vg=1 V) to 0.90 A/mm (Vg=3 V) due to the improved applicability of the gate voltage and from 108 V to 178 V, respectively, by annealing the gate metals. In addition, a reduction of the transconductance was not observed. Furthermore, even after the deposition of SiNx passivation film, the off-state breakdown voltage remained at a relatively high value of 120 V. Regarding the RF characteristics, the cut-off frequency and maximum oscillation frequency were also improved from 10.3 GHz to 13.5 GHz and from 27.5 GHz to 35.1 GHz, respectively, by annealing the gate metals whose gate length was 1 µm.

A C-band AlGaN/GaN HEMT with Cat-CVD SiN passivation developed for an over 100 W operation

We applied a Cat-CVD (catalytic chemical vapor deposition) passivation film to AlGaN/GaN HEMTs, to resolve the trade-off between their drain current transient time and gate-drain break down voltage. We did not employ any field plate because it degrades high frequency operation over C-band. The SiN passivation film, deposited after a NH 3 treatment, resulted in less transient time and less gate leakage current than conventional PE-CVD passivation. A T-shaped gate HEMT fabricated by this technique, with Lg = 0.4 μm and Wg = 50.4 mm, delivered an output power over 140 W (2.79 W/mm), which was a record power at C-band.

Effects of a thin Al layer insertion between AlGaN and Schottky gate on the AlGaN∕GaN high electron mobility transistor characteristics

To improve an AlGaN∕GaN high electron mobility transistor, an Al layer as thin as 3 nm was inserted between the AlGaN barrier layer and the gate contact. At our preceded experiments on Schottky diodes, we confirmed significant improvement in capacitance-gate voltage characteristics especially at a low frequency as well as drastic reduction in gate leakage current, which should be interpreted in terms of decrease in oxygen-related trap density at the AlGaN surface. As a result of the trap reduction, the transistor indicates marked improvement of current collapse with no degradation in transconductance.

Enhancement of Drain Current by an AlN Spacer Layer Insertion in AlGaN/GaN High-Electron-Mobility Transistors with Si-Ion-Implanted Source/Drain Contacts

In AlGaN/GaN high-electron-mobility transistors (HEMTs) with an AlN spacer layer, which improves two-dimensional electron gas (2DEG) properties, it is important to decrease ohmic contact resistance because an AlN spacer layer with an extremely wide band gap decreases the contact resistance significantly. We employed Si ion implantation doping to solve this problem and successfully obtained a sufficiently low contact resistance equivalent to that of HEMT without an AlN spacer layer. In the fabricated AlGaN/AlN/GaN HEMTs with Si-ion-implanted source/drain contacts, as another effect of AlN spacer layer insertion, a reduction in the forward Schottky gate current was found, which made it possible to apply a high gate voltage in the transistor operation. Combined with the improvement in 2DEG properties, a marked enhancement in drain current density of 25–30% was observed.

Drivability Enhancement for AlGaN/GaN High-Electron Mobility Transistors with AlN Spacer Layer Using Si Ion Implantation Doping

Although a thin AlN spacer layer between the AlGaN barrier and GaN channel layers effectively increases electron mobility and sheet carrier concentration in a two-dimensional electron gas, the very wide bandgap AlN makes ohmic contacts difficult to form. We overcame this problem using Si ion implantation to attain contact resistance below 2.5×10-6 Ω cm2. Samples without ion implantation had poor ohmic properties. Inserting the thin AlN spacer layer dramatically improved the drain current of high-electron mobility transistors.

A high reliability GaN HEMT with SiN passivation by Cat-CVD

This is the first report of catalytic vapor deposition (Cat-CVD) passivated AlGaN/GaN HEMT. We have found out that the Cat-CVD passivation film with NH3 treatment greatly enhances the reliability of the AlGaN/GaN HEMT. It is rationalized, through the low frequency capacitance-voltage measurement, that the NH3 treatment in the Cat-CVD reactor before the SiN film deposition minimizes the damage at the SiN/AlGaN interface, leading to reducing the surface trap density. The AlGaN/GaN HEMT passivated by the Cat-CVD SiN film suppresses the degradation of an output power to less than 0.4 dB under the RF operation of Vd = 30 V, f = 5 GHz after 200 h.

Vertical GaN merged PiN Schottky diode with a breakdown voltage of 2 kV

In this study, we successfully fabricated vertical GaN merged PiN Schottky (MPS) diodes and comparatively investigated the cyclic p-GaN width (W p) dependence of their electrical characteristics, including turn-on voltage and reverse leakage current. The MPS diodes with W p of more than 6 µm can turn on at around 3 V. Increasing W p can suppress the reverse leakage current. Moreover, the vertical GaN MPS diode with the breakdown voltage of 2 kV was realized for the first time.