Seikoh Yoshida

GaN Power Transistors on Si Substrates for Switching Applications

In this paper, GaN power transistors on Si substrates for power switching application are reported. GaN heterojunction field-effect transistor (HFET) structure on Si is an important configuration in order to realize a low loss and high power devices as well as one of the cost-effective solutions. Current collapse phenomena are discussed for GaN-HFETs on Si substrate, resulting in suppression of the current collapse due to using the conducting Si substrate. Furthermore, attempts for normally off GaN-FETs were examined. A hybrid metal-oxide-semiconductor HFET structure is a promising candidate for obtaining devices with a lower on-resistance (Ron) and a high breakdown voltage (Vb).

High power AlGaN/GaN HFET with a high breakdown voltage of over 1.8 kV on 4 inch Si substrates and the suppression of current collapse

In this paper, we successfully demonstrate an AlGaN HFET with a high breakdown voltage of over 1.8 kV on 4 inch Si substrates. In order to obtain the high breakdown voltage and to improve the crystalline quality of GaN layers, a thick GaN epitaxial layer including a buffer layer with a total thickness of over 6 mum was grown. The breakdown voltage and the maximum drain current were achieved to be over 1.8 kV and 120 A, respectively. Furthermore, the suppression of the current collapse phenomenon is examined. The on-resistance is not so significantly increased up to the high drain off-bias-stress of 1.0 kV.

High-temperature reliability of GaN metal semiconductor field-effect transistor and bipolar junction transistor

High-quality GaN was grown using gas-source molecular beam epitaxy (GSMBE). We fabricated a GaN metal semiconductor field-effect transistor (MESFET) and an n–p–n bipolar junction transistor. The high-temperature reliability of the GaN MESFET and bipolar junction transistor was investigated. That is, the life performance of the FET at 400 °C was examined during continuous current injection at 400 °C. We confirmed that the FET performance did not change at 400 °C for 300 h. No degradation of the metal–semiconductor interface was observed by secondary ion-mass spectrometry and transmission electron microscopy. Furthermore, an n–p–n bipolar junction transistor using GaN grown by GSMBE was demonstrated. The bipolar junction transistor was operated at 300 °C. The reliability of a GaN MESFET and the bipolar junction transistor at high temperature was thus confirmed.

Characterization of a GaN Bipolar Junction Transistor after Operation at 300 for over 300 h

GaN with an n-p-n structure was grown using gas-source molecular beam epitaxy (GSMBE). We fabricated an n-p-n GaN bipolar junction transistor. The high-temperature reliability of the bipolar junction transistor (BJT) was investigated. That is, the lifetime performance of the BJT at 300°C was examined during continuous current injection at 300°C. We confirmed that the performance of the bipolar transistor at 300°C did not change for over 300 h. No degradation of the metal-semiconductor interface was observed by transmission electron microscopy (TEM). The reliability of the GaN BJT at 300°C was thus confirmed.

Normally Off n-Channel GaN MOSFETs on Si Substrates Using an SAG Technique and Ion Implantation

We have demonstrated n-channel gallium nitride (GaN) MOSFETs using a selective area growth (SAG) technique and ion implantation on a silicon substrate. Both MOSFETs realized good normally off operations. The MOSFET using the SAG technique showed a large drain current of 112 mA/mm, a lower leakage current, and a high field mobility of 113 cm2/V . s, which is, to our knowledge, the best for a GaN MOSFET on a silicon substrate.

Reliability of GaN Metal Semiconductor Field-Effect Transistor at High Temperature

We have grown a high quality GaN for fabricating a metal semiconductor field effect transistor (MESFET) using gas-source molecular beam epitaxy (GSMBE). A GaN MESFET for high temperature operation was developed. We used Au/Pt as a Schottky gate, and Au/Ti/Al as a source-drain. The endurance of high temperatures by GaN MESFET was investigated. It was confirmed for the first time, that the FET performance was satisfactory even after the FET was heated at 400°C for over 700 h, thereby demonstrating, for the first time, the reliability of GaN MESFET. The device continued to be operative even when, heated further up to 600°C.

Growth of hexagonal GaN on Si(111) coated with a thin flat SiC buffer layer

Hexagonal GaN films were grown on Si(111) covered with a thin flat SiC buffer layer under both N- and Ga-rich growth conditions. A flat 2.5-nm-thick SiC layer was an effective buffer layer for GaN growth. The growth mode and microstructure of GaN depended strongly on the Ga/N flux ratios. Under N-rich growth conditions, the growth mode was three dimensional; GaN showed statistical roughening of the surface and a characteristic columnar structure. Under Ga-rich conditions, the GaN growth mode was two dimensional; GaN films with a flat surface and an almost stacking-fault-free microstructure were obtained. The two-dimensional growth mode was facilitated by strong wetting between Ga and SiC(111) at the first Ga-layer deposition on SiC.

Switching Characteristics of GaN HFETs in a Half Bridge Package for High Temperature Applications

AlGaN/GaN heterojunction field effect transistors (HFETs) are expected to be a good candidate for power switching application at high temperatures. We designed and fabricated a discrete HFET package and a half bridge module using the AlGaN/GaN HFETs and SiC Schottky barrier diodes (SBDs) for high temperature applications. The half bridge module exhibited good reliability after 250 C and 400 h high temperature storage. Switching characteristics of the AlGaN/GaN HFET were investigated. Qg x Ron, which shows a figure of merit of switching operation, was more than 10 times better than commercial Si MOSFETs. The switching characteristics of the HFET showed no significant degradation up to 225 C.

Reliability of metal semiconductor field-effect transistor using GaN at high temperature

The reliability of a GaN metal semiconductor field-effect transistor (MESFET) was investigated. We used Au/Pt as a Schottky gate and Ti/Al as a source drain of a GaN MESFET. The thermal stress test of the GaN MESFET at high temperature was investigated. It was found that no change of FET characteristics was observed even after the device was heated at 400 °C for 1000 h. Furthermore, using a GaN MESFET which was heated at 400 °C for 1000 h, a life of FET at 350 °C was examined by a continuously current injection at 350 °C. We confirmed that the FET performance did not change at 350 °C for 150 h. No degradation of metal–semiconductor interface was observed by secondary ion mass spectrometry and a transmission electron microscope. The reliability of the GaN MESFET as a high-temperature operation transistor was fully demonstrated.

Heteroepitaxial growth of cubic GaN on Si(001) coated with thin flat SiC by plasma-assisted molecular-beam epitaxy

High-quality cubic GaN films were grown on Si(001) coated with flat ultrathin SiC under different Ga/N flux ratios. The 2.5-nm-thick cubic SiC film proved to be an effective buffer layer for cubic GaN growth on Si(001). Under a Ga-rich condition, films with local atomically flat surfaces were obtained, and the x-ray diffraction full-width at half maximum of (002) peak was 19 min for a 0.82-μm-thick film. The reduced SiC surface roughness decreased the defect density in the GaN epilayers. Under a N-rich condition, the GaN films showed statistical roughening of the surface and a characteristic columnar structure. Under the Ga-rich condition, the columns grew up and then laterally coalesced, so that an atomically flat surface with flat areas in size from 0.05 to 0.40 μm was formed.