Field effect transistor

Abstract

A dielectric film 4 made of a high dielectric material with a relative permittivity of 8 or more is laid between a field plate section 9 and a channel layer 2 . Tantalum oxide (Ta 2 O 5 ), for example, may be used as the high dielectric material.

What is claimed is:  
     
         1 . A field effect transistor; comprising 
 a semiconductor substrate with a channel layer being formed on its surface;    a source electrode and a drain electrode being formed at a distance on said semiconductor substrate; and    a gate electrode being placed between said source electrode and said drain electrode and making a Schottky junction with said channel layer; wherein:    said gate electrode is provided with an overhanging field plate section; and    between said field plate section and said channel layer, there is laid a dielectric film made of a high dielectric material with a relative permittivity of 8 or more.    
     
     
         2 . The field effect transistor according to  claim 1 , wherein said high dielectric material is a material selected from the group consisting aluminium oxide (Al 2 O 3 ), aluminium nitride, tantalum oxide (Ta 2 O 5 ), strontium titanate (SrTiO 3 ), barium titanate (BaTiO 3 ), barium titanate strontium (Ba x Sr 1-x TiO 3  (0<x<1)) and bismuth tantalate strontium (SrBi 2 Ta 2 O 9 )  
     
     
         3 . The field effect transistor according to  claim 1 , wherein the surface of said channel layer is partially or entirely covered with a silicon oxide film and said dielectric film is laid between said silicon dioxide film and said field plate section.  
     
     
         4 . The field effect transistor according to  claim 1 , wherein the film thickness of said dielectric film is 100 to 1500 nm.  
     
     
         5 . The field effect transistor according to  claim 1 , wherein said dielectric film is formed only in a region directly under said field plate section.  
     
     
         6 . The field effect transistor according to  claim 1 , wherein the electrostatic capacitance per unit area of a capacitor that consists of said field plate section and said channel layer separated by said dielectric film decreases with distance from the gate electrode.  
     
     
         7 . The field effect transistor according to  claim 1 , wherein the thickness of said dielectric film directly under said field plate section is less on the side of the gate electrode than on the side of the drain electrode.  
     
     
         8 . The field effect transistor according to  claim 1 , wherein one or more openings are formed in said field plate section.  
     
     
         9 . The field effect transistor according to  claim 1 , wherein the edge section of said field plate section on the side of the drain electrode is comb-shaped.  
     
     
         10 . The field effect transistor according to  claim 1 , wherein the permittivity of said dielectric film directly under said field plate section decreases with distance from said gate electrode.  
     
     
         11 . The field effect transistor according to  claim 1 , wherein a float electrode is set under said field plate section.  
     
     
         12 . The field effect transistor according to  claim 1 , wherein a field control electrode is set, in addition, over the dielectric film on said channel layer, between said gate electrode and said drain electrode.  
     
     
         13 . The field effect transistor according to  claim 1 , wherein a sub electrode is set, in addition, over the dielectric film on said channel layer, between said gate electrode and said source electrode.  
     
     
         14 . The field effect transistor according to  claim 1 , wherein said channel layer is made of a group III-V compound semiconductor.  
     
     
         15 . A field effect transistor; comprising: 
 a semiconductor substrate with a channel layer being formed on its surface;    a source electrode and a drain electrode being formed at a distance on said semiconductor substrate; and    a gate electrode being placed between said source electrode and said drain electrode and making a Schottky junction with said channel layer; wherein:    said gate electrode is provided with an overhanging field plate section; and    between said field plate section and said channel layer, there is laid a dielectric film; and    when the relative permittivity and the film thickness of the dielectric film are denoted by ε and t (nm), respectively, one of the following conditions (1) and (2) is satisfied.    1<ε<5, and 25 <t/ε< 70  (1)5=ε<8, and 100 <t< 350  (2)    
     
     
         16 . The field effect transistor according to  claim 15 , wherein said dielectric film is formed only in a region directly under said field plate section.  
     
     
         17 . The field effect transistor according to claim  15 , wherein the electrostatic capacitance per unit area of a capacitor that consists of said field plate section and said channel layer separated by said dielectric film decreases with distance from the gate electrode.  
     
     
         18 . The field effect transistor according to  claim 15 , wherein the thickness of said dielectric film directly under said field plate section is less on the side of the gate electrode than on the side of the drain electrode.  
     
     
         19 . The field effect transistor according to  claim 15 , wherein one or more openings are formed in said field plate section.  
     
     
         20 . The field effect transistor according to  claim 15 , wherein the edge section of said field plate section on the side of the drain electrode is comb-shaped.  
     
     
         21 . The field effect transistor according to  claim 15 , wherein the permittivity of said dielectric film directly under said field plate section decreases with distance from said gate electrode.  
     
     
         22 . The field effect transistor according to  claim 15 , wherein a float electrode is set under said field plate section.  
     
     
         23 . The field effect transistor according to  claim 15 , wherein a field control electrode is set, in addition, over the dielectric film on said channel layer, between said gate electrode and said drain electrode.  
     
     
         24 . The field effect transistor according to  claim 15 , wherein a sub electrode is set, in addition, over the dielectric film on said channel layer, between said gate electrode and said source electrode.  
     
     
         25 . The field effect transistor according to  claim 15 , wherein said channel layer is made of a group III-V compound semiconductor.