Field effect transistor and multilayered epitaxial film for use in preparation of field effect transistor

Abstract

In a group III nitride-type field effect transistor, the present invention reduces a leak current component by conduction of residual carriers in a buffer layer, and achieves improvement in a break-down voltage, and enhances a carrier confinement effect (carrier confinement) of a channel to improve pinch-off characteristics (to suppress a short channel effect). For example, when applying the present invention to a GaN-type field effect transistor, besides GaN of a channel layer, a composition-modulated (composition-gradient) AlGaN layer in which aluminum composition reduces toward a top gradually or stepwise is used as a buffer layer (hetero buffer). To gate length Lg of a FET to be prepared, a sum a of layer thicknesses of an electron supply layer and a channel layer is selected so as to fulfill Lg/a≥5, and in such a case, the layer thickness of the channel layer is selected in a range of not exceeding 5 times (about 500 Å) as long as a de Broglie wavelength of two-dimensional electron gas accumulated in the channel layer in room temperature.

1 . A semiconductor device comprising:
 a substrate;   a multilayered epitaxial grown layer formed on the substrate, having:
 an AlGaN buffer layer; 
 a GaN channel layer formed on the AlGaN buffer layer; and 
 an AlGaN electron supply layer formed on the GaN channel layer; 
   source and drain electrodes formed on a surface of the AlGaN electron supply layer; and   a gate electrode formed on the surface of the AlGaN electron supply layer between the source and drain electrodes,   wherein an Al composition of the AlGaN buffer layer positioned at vicinity of the GaN channel layer is relatively lower than an Al composition of the AlGaN buffer layer positioned at vicinity of the substrate, and   wherein an Al composition of the AlGaN buffer layer is monotonically reduced in a direction from the substrate toward the GaN channel layer.   
     
     
         2 . The semiconductor device according to  claim 1 , wherein an Al composition of the AlGaN buffer layer is step-wisely reduced in a direction from the substrate toward the GaN channel layer. 
     
     
         3 - 6 . (canceled)