Shigeharu Yamagami

Novel Power Si/4H-SiC Heterojunction Tunneling Transistor (HETT)

We demonstrate a novel power Si/4H-SiC heterojunction tunneling transistor (HETT) on the basis of theoretical analysis and experimental results. The HETT is an insulated gate drive device and has a unique switching mechanism. In the off-state, the heterojunction barrier prevents current flow between the Si source region and the 4H-SiC drift region. In the on-state, the width of the heterojunction barrier is controlled by the gate bias to allow tunneling current to flow. The HETT has a zero channel length structure that is more independent of channel mobility compared with a conventional 4H-SiC MOSFET. As a result, the HETT is expected to have low on-resistance. A HETT was fabricated with n+-type polycrystalline silicon on an n--type 4H-SiC epitaxial wafer for power devices. The fabricated HETT shows a low specific on-resistance of 6.8 mcm2 (at Jd=500 A/cm2).

Negative Field Reliability of ONO Gate Dielectric on 4H-SiC

It was experimentally shown that an ONO gate dielectric carefully formed on 4H-SiC has extremely high reliability even under a negative electric field at least up to a junction temperature of 300°C, making it promising for power MOS and CMOS applications. Medium charge to failure of –30 C/cm2 was achieved for fully processed polycrystalline Si gate MONOS capacitors with an equivalent SiO2 thickness of teq = 44 nm and a 200-μm diameter. The medium time to failure of these capacitors projected for –3 MV/cm exceeds 86 and 6.3 thousand years at room temperature and 300°C, respectively. A parasitic memory action did not appear even when Eox of -6.6 MV/cm was applied for 5000 seconds.

Novel SiC Power Devices utilizing a Si/4H-SiC Heterojunction

We have developed novel SIC devices, both a diode and a transistor, utilizing a Si/4H-SiC heteroj unction. A heterojunction diode (HJD) was fabricated with P+ polycrystalline silicon on an N- epitaxial layer of 4H-SiC. The HJD achieved lower Von and higher reverse blocking voltage than a commercial Schottky barrier diode (SBD) of SiC. Switching charcteristics of the HJD indicated almost zero reverse recovery similar to that of a SBD. A hetero junction tunneling transistor (HETT) was driven by an insulated gate electrode. The width of the heterojunction barrier was controlled by the gate bias to allow tunneling current to flow. The HETT was fabricated with N+ polycrystalline silicon on an N- 4H-SiC epitaxial layer. The channnel length of the HETT was almost zero and was expected to have low on-resistance.

SiC Trench MOSFET with an Integrated Low Von Unipolar Heterojunction Diode

We demonstrate a SiC trench MOSFET with an integrated low Von unipolar heterojunction diode (MOSHJD). A region of the heterojunction diode (HJD) was fabricated in a trench with p+-type poly-crystalline silicon on an n--type epitaxial layer of 4H-SiC. The measured on-resistance (Ron) of the transistor action was 15 mΩcm2. The measured Von of the diode action was 2.2 V at a forward current density of 100 A/cm2. The fabrication process of the MOSHJD is simple. First, the trenches of the MOSFET region and the HJD region are formed simultaneously; then poly-crystalline silicon is deposited to form the gate electrode of the MOSFET region and the anode electrode of the HJD region at the same time.

Novel Low VON Poly-Si/4H-SiC Heterojunction Diode Using Energy Barrier Height Control

We experimentally investigated a method of controlling the energy barrier height (ΦB) of polycrystalline silicon (poly-Si)/4H-SiC heterojunction diodes (HJDs) and conducted a numerical simulation of a novel low Von and low reverse recovery current diode using ΦB control. The ΦB of the HJD with arsenic-doped n+-poly-Si was 0.79 eV and that of the HJD with boron-doped p+-poly-Si was 1.59 eV. The ΦB can be controlled over a wide range by varying the dopant and ion implantation dose of poly-Si. A novel merged HJD (M-HJD) with two different ΦB values obtained by using ΦB control is also presented. The numerical simulation results show that the M-HJD reduces Von without increasing reverse leakage current at high reverse voltage.