Norio Onojima

Reduction in potential barrier height of AlGaN∕GaN heterostructures by SiN passivation

SiN passivation on AlGaN∕GaN heterostructures was carried out using catalytic or plasma-enhanced chemical vapor deposition (Cat-CVD or PECVD), which has been found to increase two-dimensional electron gas (2DEG) density. The 2DEG density can be closely related to AlGaN surface properties via polarization effects. AlGaN potential barrier heights of AlGaN∕GaN heterostructures with and without SiN passivation were systematically investigated using x-ray photoelectron spectroscopy (XPS) and capacitance-voltage (C-V) measurements. The results for the XPS and C-V measurements were consistent and demonstrated that a reduction in the AlGaN potential barrier height was actually induced by SiN passivation. Furthermore, Cat-CVD SiN passivation lowered the AlGaN potential barrier height more significantly than PECVD SiN passivation did, suggesting that the passivation method can influence the AlGaN potential barrier height.

Effects of SiN passivation by catalytic chemical vapor deposition on electrical properties of AlGaN∕GaN heterostructure field-effect transistors

We investigated the effects of SiN passivation by catalytic chemical vapor deposition (Cat-CVD) on the electrical properties of AlGaN∕GaN heterostructure field-effect transistors. The two-dimensional electron density (Ns) greatly increased after the Cat-CVD SiN deposition, and the tendency of the increase was enhanced with decreasing AlGaN barrier thickness. As a result of the large increase in Ns, the sheet resistance (Rsh) significantly decreased after the deposition, and it had low values of 320–460Ω∕◻ for extremely thin AlGaN barriers of 4–10nm. The increase in Ns showed little dependence on SiN thickness, indicating that the stress applied to the AlGaN barrier by SiN cannot be the origin of the increase. Cat-CVD SiN also improved the in-plane uniformity of mobility for extremely thin-barrier structures, which in turn improved the uniformity of Rsh. Moreover, we found that Cat-CVD was more effective than plasma-enhanced chemical vapor deposition in increasing Ns. A comparison of theoretical calculatio...

High Off-state Breakdown Voltage 60-nm-Long-Gate AlGaN/GaN Heterostructure Field-Effect Transistors with AlGaN Back-Barrier

We fabricated 60-nm-long-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) with an AlGaN back-barrier structure and investigated the high frequency device characteristics and three-terminal off-state breakdown characteristics as a function of the source-to-drain distance. These devices, with source-to-drain distances of 2 to 5 µm, showed very high current-gain cutoff frequencies of more than 118 GHz. The off-state breakdown characteristics were largely dependent on the source-to-drain distance compared to the high frequency device characteristics, and the devices with source-to-drain distances of 4 and 5 µm exhibited very high off-state breakdown voltages of more than 110 V while keeping very high cutoff frequencies. These good breakdown characteristics might be the result of the double-barrier structure (i.e., AlGaN/GaN/AlGaN), which prevents electron spillover to the AlGaN back-barrier at high power conditions.

Ultrathin AlN∕GaN heterostructure field-effect transistors with deposition of Si atoms on AlN barrier surface

We deposited Si atoms on the AlN barrier surface of an ultrathin AlN∕GaN heterostructure field-effect transistor (HFET). This induced a remarkable change in the electrical properties of the two-dimensional electron gas. A 2-nm-thick Si layer reduced the sheet resistance of an AlN∕GaN HFET (AlN barrier, 2nm) from 60356to388Ω∕sq. The effect on the Ohmic contact was also significant: the presence of an undermost layer of Si atoms under Ohmic contacts produced a low specific contact resistance of 1.7×10–6Ωcm2. A 50-nm-gate AlN∕GaN HFET with a Si layer exhibited excellent device characteristics with a current-gain cutoff frequency of 106GHz.

Effects of Si Deposition on AlGaN Barrier Surfaces in GaN Heterostructure Field-Effect Transistors

We observed that Si deposition on AlGaN/GaN heterostructure field-effect transistors (HFETs) can increase the two-dimensional electron gas (2DEG) density up to a level comparable to that achieved as a result of SiN passivation. Silicon atoms located at the AlGaN barrier surface might act as positively ionized donors, which can partially neutralize the negative polarization charges of the AlGaN surface and thereby increase the 2DEG density through polarization effects. This phenomenon is not only interesting from the scientific point of view, but also promising from the practical point of view. By using a 2-nm-thick Si insert layer in the ohmic contact, we obtained a contact resistance value 2.5 times lower than that obtained without the Si insert layer (conventional ohmic contact). GaN HFETs with Si deposited on the AlGaN barrier surface exhibited excellent device characteristics owing to a reduction in the source and drain sheet resistances and the ohmic contact resistances.