Patent ID: 7432538

Claim:
A field-effect transistor prepared by forming a plurality of semiconductor crystal layers containing group-III element nitride based semiconductors by crystal growth, the field-effect transistor comprising: a channel layer having a channel; a carrier supply layer, disposed on the channel layer, containing a semiconductor represented by the formula Al x Ga 1-x N; and a source electrode, a gate electrode and a drain electrode which are formed on said carrier supply layer, wherein the channel is formed near a first interface between the channel layer and the carrier supply layer or depleted, the carrier supply layer has a band gap energy greater than that of the channel layer, x in the formula Al x Ga 1-x N decreases monotonically at least one of stepwise and continuously with an increase in the distance from said first interface, said carrier supply layer has a first composition ratio x 1 at said first interface and x 1 in the formula Al x1 Ga 1-x1 N is greater than or equal to 0.15 and less than or equal to 0.40 and said carrier supply layer has a second interface located on the side opposite to said first interface and has a second composition ratio x 2 at said second interface and x 2 in the formula Al x2 Ga 1-x2 N is greater than or equal to 0.05 and less than or equal to 0.20.