Yoshiyuki Yonezawa

Improved Channel Mobility in 4H-SiC MOSFETs by Boron Passivation

We propose another process for fabricating 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) with high channel mobility. The B atoms were introduced into a SiO2/4H-SiC interface by thermal annealing with a BN planar diffusion source. The interface state density near the conduction band edge of 4H-SiC was effectively reduced by the B diffusion and the fabricated 4H-SiC MOSFETs showed a peak field-effect mobility of 102 cm2/Vs. The obtained high channel mobility cannot be explained by counter doping because B atoms act as acceptors in 4H-SiC. We suggest that the interfacial structural change of SiO2 may be responsible for the reduced trap density and enhanced channel mobility.

Development of Ultrahigh-Voltage SiC Devices

Ultrahigh-voltage silicon carbide (SiC) devices [p-i-n diodes and insulated-gate bipolar transistors (IGBTs)] and switching test have been investigated. As a result, we have succeeded in developing a 13-kV p-i-n diode, 15-kV p-channel IGBT, and 16-kV flip-type n-channel implantation and epitaxial IGBT with a low differential specific on-resistance (Rdiff,on). It was revealed that a power module fabricated using a nanotech resin, Si3N4 ceramic substrate, and W base plate was suitable for ultrahigh voltage and high temperature. A switching test was carried out using a clamped inductive load circuit, which indicated that the energy loss of a circuit with ultrahigh-voltage SiC devices is lower than that of Si devices.

Temperature-dependent analysis of conduction mechanism of leakage current in thermally grown oxide on 4H-SiC

The conduction mechanism of the leakage current of a thermally grown oxide on 4H silicon carbide (4H-SiC) was investigated. The dominant carriers of the leakage current were found to be electrons by the carrier-separation current-voltage method. The current-voltage and capacitance-voltage characteristics, which were measured over a wide temperature range, revealed that the leakage current in SiO2/4H-SiC on the Si-face can be explained as the sum of the Fowler-Nordheim (FN) tunneling and Poole-Frenkel (PF) emission leakage currents. A rigorous FN analysis provided the true barrier height for the SiO2/4H-SiC interface. On the basis of Arrhenius plots of the PF current separated from the total leakage current, the existence of carbon-related defects and/or oxygen vacancy defects was suggested in thermally grown SiO2 films on the Si-face of 4H-SiC.

Fabrication of a P-Channel SiC-IGBT with High Channel Mobility

We fabricated and characterized an ultrahigh voltage (>10kV) p-channel silicon carbide insulated gate bipolar transistor (SiC-IGBT) with high channel mobility. Higher field-effect channel mobility of 13.5 cm2/Vs was achieved by the combination of adopting an n-type base layer with a retrograde doping profile and additional wet re-oxidation annealing (wet-ROA) at 1100°C in the gate oxidation process. The on-state characteristics of the p-channel SiC-IGBT at 200°C showed the low differential specific on-resistance of 24 mΩcm2 at VG = -20 V. The forward blocking voltage of the p-channel SiC-IGBT at 25°C was 10.2 kV a the leakage current density of 1.0 μA/cm2.

Short minority carrier lifetimes in highly nitrogen-doped 4H-SiC epilayers for suppression of the stacking fault formation in PiN diodes

We investigated the dependency of minority carrier lifetimes on the nitrogen concentration, temperature, and the injected carrier concentration for highly nitrogen-doped 4H-SiC epilayers. The minority carrier lifetimes greatly shortened when the nitrogen concentration exceeded 1018 cm−3 through enhancing direct band-to-band and Auger recombination and showed a slight variation in the temperature range from room temperature (RT) to 250 °C. The epilayer with a nitrogen concentration of 9.3 × 1018 cm−3 exhibited a very short minority carrier lifetime of 38 ns at RT and 43 ns at 250 °C. The short minority carrier lifetimes of the highly nitrogen-doped epilayer were confirmed to maintain the values even after the subsequent annealing of 1700 °C. 4H-SiC PiN diodes were fabricated by depositing a highly nitrogen-doped epilayer as a “recombination enhancing layer” between an n− drift layer free from basal plane dislocations and the substrate. The PiN diodes showed no formation of stacking faults and no increase i...

Threshold-voltage instability in 4H-SiC MOSFETs with nitrided gate oxide revealed by non-relaxation method

The threshold-voltage (V th) shift of 4H-SiC MOSFETs with Ar or N2O post-oxidation annealing (POA) was measured by conventional sweep and non-relaxation methods. Although the V th shift values of both samples were almost identical when measured by the sweep method, those for the Ar POA samples were larger than those for the N2O POA samples when measured by the non-relaxation method. Thus, we can say that investigating the exact V th shifts using only the conventional sweep method is difficult. The temperature-dependent analysis of the V th shifts measured by both methods revealed that the N2O POA decreases charge trapping in the near-interface region of the SiO2.

Low V f and highly reliable 16 kV ultrahigh voltage SiC flip-type n-channel implantation and epitaxial IGBT

Flip-type n-channel implantation and epitaxial (IE)-IGBT on 4H-SiC carbon face with an epitaxial p++ collector layer was investigated. In this study, we employed the IEMOSFET as a MOSFET structure with original wet gate oxidation method, to realize high channel mobility. We were able to achieve an ultrahigh blocking voltage of more than 16 kV, extremely low forward voltage drop of 5 V at 100 A/cm2 and small threshold voltage shift (<; 0.1 V). These characteristics are useful for Smart Grid and HVDC systems, the use of which would realize a low carbon emission society.

Effect of Current-Spreading Layer Formed by Ion Implantation on the Electrical Properties of High-Voltage 4H-SiC p-Channel IGBTs

High-voltage SiC p-channel insulated-gate bipolar transistors (p-IGBT) utilizing current-spreading layer (CSL) formed by ion implantation are fabricated and their properties characterized. A high blocking voltage of 15 kV is achieved at room temperature by optimizing the JFET length. An ampere-class p-IGBT exhibited a low forward voltage drop of 8.5 V at 100 A/cm2 and a low differential specific on-resistance of 33 mΩ cm2 at 250 °C, while these values were high at room temperature. For further reduction of the forward voltage drop in the on-state and temperature stability, the temperature dependence of the JFET effect and carrier lifetime in p-IGBTs are investigated. Optimization of the JFET length using an epitaxial CSL, instead of applying ion implantation and lifetime enhancement, could lead to a further reduction of the forward voltage drop.

Characterization of traps at nitrided SiO2/SiC interfaces near the conduction band edge by using Hall effect measurements

The effects of nitridation on the density of traps at SiO2/SiC interfaces near the conduction band edge were qualitatively examined using a simple, newly developed characterization method that utilizes Hall effect measurements and split capacitance–voltage measurements. The results showed a significant reduction in the density of interface traps near the conduction band edge as a result of nitridation, but the interface traps were not completely eliminated by nitridation.

Device Performance and Switching Characteristics of 16 kV Ultrahigh-Voltage SiC Flip-Type n-Channel IE-IGBTs

Ultrahigh-voltage SiC flip-type n-channel implantation and epitaxial (IE)-IGBTs were developed, and the static and dynamic performance was investigated. A large device (8 mm × 8mm) with a blocking voltage greater than 16 kV was achieved, and an on-state current of 20 A was obtained at the low on-state voltage (Von) of 4.8 V. RonAdiff was 23 mΩ·cm2 at Von = 4.8 V. In order to evaluate the switching characteristics of the IE-IGBT, ultrahigh-voltage power modules were assembled. A chopper circuit configuration was used to evaluate the switching characteristics of the IE-IGBT. Smooth turn-off waveforms were successfully obtained at VCE = 6.5 kV and ICE = 60 A in the temperature range from room temperature to 250°C.