State-of-the-art technologies of GaN-based power switching transistors are reviewed, in which normally-off operation and heat spreading as technical issues. We demonstrate a new operation principle of GaN-based normally-off transistor called Gate Injection Transistor (GIT). The GIT utilizes hole-injection from p-AlGaN to AlGaN/GaN hetero-junction which increases electron density in the depleted channel resulting in dramatic increase of the drain current owing to conductivity modulation. The fabricated GIT on Si substrate exhibits the threshold voltage of +1.0V with high maximum drain current of 200mA/mm. The obtained on-state resistance (Ron·A) and off-state breakdown voltage (BVds) are 2.6mΩ·cm2 and 800V, respectively. These values are the best ones ever reported for GaN-based normally-off transistors. In addition, we propose the use of poly-AlN as surface passivation. The AlN has at least 200 times higher thermal conductivity than conventional SiN so that it can effectively reduce the channel temperature.