Sachiko Kawaji

New 3-D lateral power MOSFETs with ultra low on-resistance

This paper presents a new 3D lateral power MOSFET which has a double gate and a trench gate/drain structure. The double gate structure decreased its channel resistance, and the trench gate/drain structure decreased its n drift resistance. We realized this structure using solid phase epitaxy and a conventional trench technology. From experimental results, a breakdown voltage of 49.5 V and a specific on-resistance of 42 m/spl Omega//spl middot/mm/sup 2/ were obtained.

A normally-off bipolar mode static induction transistor (BSIT) with high current gains

The DC current gain of a normally-off bipolar mode static induction transistor with surface-gate structure can be improved by the reduction of the source- contact-area. To explain the current gain increase, a simple device model was introduced. It was found from this model that the DC current gains were in proportion to the inversion of the source-contact-area. The model is able to explain the experimental results well. In addition, the adequacy of the model was verified qualitatively by computer simulation. Based on the model, a high power BSIT module with high current gains has been developed.

Improved Evaluation Method for Channel Mobility in SiC Trench MOSFETs

We proposed an improved method for evaluating the effective channel mobility (μeff), involving an appropriate definition of the threshold voltage (Vth) based on the ideal gate bias voltage – drain current (VG-ID) characteristics. Using this method, the dependence of μeff on the effective field (Eeff) could be evaluated even for SiC trench MOSFETs with large interface state density (Dit) values. The dominant influence on μeff in the low Eeff region was found to be Coulomb scattering caused by interface states at the SiC/SiO2 interfaces.

Impact of Al Doping Concentration at Channel Region on Mobility and Threshold Voltage Instability in 4H-SiC Trench N-MOSFETs

The effect of Al doping concentration (NA) at channel regions ranging from 1.0×1017 to 4.0×1017 cm-3 on the effective channel mobility of electron (μeff) and the threshold voltage (Vth) instability under the positive bias-temperature-stress conditions has been investigated througu the use of trench-gate 4H-SiC MOSFETs with m-face (1-100) channel regions. It was found that μeff degraded with an increase in NA. On the other hand, the increase of NA enlarged the Vth instability. These results indicate that NA has a large impact not only on the Vth value but also on the channel resistance and reliability in 4H-SiC trench MOSFETs.

A new power MOSFET having excellent avalanche capability

In this paper, we explore a new concept for improvement of an avalanche capability of a power MOSFET. The concept is to make parasitic bipolar transistors in all over the chip turn on, and to suppress breakdown current crowding. The avalanche capability of an UMOSFET applied this new concept was 1500 mJ. This value was about one order higher than that of a conventional UMOSFET.

Low loss static induction devices (transistors and thyristors)

To decrease the power loss, a current gain of a bipolar mode Static Induction Transistor (BSIT) is examined with quasi-one-dimensional analysis. The current gain dependence on source contact area is presented with experimental, analyzed and simulated results. Concerning Static Induction Thyristor (SIThy), the switching speed improvement methods are presented to reduce the switching loss of device. A proton irradiation, or a shorting method is a useful technique to decrease the switching time and switching loss of SIThy.

A PT-IGBT with a p-/n+ buffer layer

A novel punch-through insulated gate bipolar transistor with a p-/n+ buffer layer was proposed to improve the characteristics of conventional high power IGBT used in motor control inverters at high voltages operation. The new structure with p-floating layer inserted between n- epi and n+ buffer layer shows higher breakdown voltage than that of conventional IGBT structures. We also demonstrate, for the first time, the performance of 900V-200A class IGBTs using this p- floating/n+ buffer structure. As a result of the measurements, the IGBT proposed here shows an on-state voltage of 1.9V at 250A/cm/sup 2/ and the fall time of 350 nsec.

Impact of optical phonon scattering on inversion channel mobility in 4H-SiC trenched MOSFETs

Temperature characteristics of the channel mobility were investigated for 4H-SiC trenched MOSFETs in the range from 30 to 200 °C. The conventional model of channel mobility limited by carrier scattering is based on Si-MOSFETs and shows a greatly different channel mobility from the experimental value, especially at high temperatures. On the other hand, our improved mobility model taking into account optical phonon scattering yielded results in excellent agreement with experimental results. Moreover, the major factors limiting the channel mobility were found to be Coulomb scattering in a low effective field (<0.7 MV/cm) and optical phonon scattering in a high effective field.