We review our recent advances of GaN-based high voltage power transistors. These are promising owing to low on-state
resistance and high breakdown voltage taking advantages of superior material properties. However, there still remain a
couple of technical issues to be solved for the GaN devices to replace the existing Si-based power devices.
The most critical issue is to achieve normally-off operation which is strongly desired for the safety operation, however, it
has been very difficult because of the built-in polarization electric field. Our new device called GIT (Gate Injection
Transistor) utilizing conductivity modulation successfully achieves the normally-off operation keeping low on-state
resistance. The fabricated GIT on a Si substrate exhibits threshold voltage of +1.0V. The obtained on-state resistance and
off-state breakdown voltage were 2.6mΩ•cm2 and 800V, respectively.
Remaining technical issue is to further increase the breakdown voltage. So far, the reported highest off-state breakdown
voltage of AlGaN/GaN HFETs has been 1900V. Overcoming these issues by a novel device structure, we have
demonstrated the world highest breakdown voltages of 10400V using thick poly-crystalline AlN as a passivation film
and Via-holes through sapphire which enable very efficient layout of the lateral HFET array avoiding any undesired
breakdown of passivation films. Since conventional wet or dry etching cannot be used for chemically stable sapphire,
high power pulsed laser is used to form the via-holes.
The presented GaN power devices demonstrate that GaN is advantageous for high voltage power switching applications
replacing currently used Si-based power MOSFETs and IGBTs.