Nitride semiconductor device such as transverse power FET for high frequency signal amplification or power control

A nitride semiconductor device comprising:

• a non-doped first aluminum gallium nitride (AlxGa1-xN (0≦x≦1)) layer which is formed as a channel layer;
• a non-doped or n type second aluminum gallium nitride (AlyGa1-yN (0≦y≦1, x <y)) layer which is formed on the first aluminum gallium nitride layer as a barrier layer;
• a gate electrode which is formed on a portion of the second aluminum gallium nitride layer as a control electrode;
• an aluminum nitride (AlN) film which is formed on the second aluminum gallium nitride layer to cover the gate electrode as an insulating film lower layer, the aluminum nitride film being selected from a polycrystalline film and an amorphous film;
• an aluminum oxide (Al2O3) film which is formed on the aluminum nitride film as an insulating film upper layer;
• and
• a source electrode and a drain electrode which are formed as first and second main electrodes to be electrically connected to the second aluminum gallium nitride layer, respectively.