Field-effect transistor

Abstract

A field-effect transistor includes a carrier transport layer made of nitride semiconductor, a gate electrode having first and second sidewall surfaces on first and second sides, respectively, an insulating film formed directly on the gate electrode to cover at least one of the first and second sidewall surfaces, first and second ohmic electrodes formed on the first and second sides, respectively, a passivation film including a first portion extending from the first ohmic electrode toward the gate electrode to cover a surface area between the first ohmic electrode and the gate electrode and a second portion extending from the second ohmic electrode toward the gate electrode to cover a surface area between the second ohmic electrode and the gate electrode, wherein the insulating film is in direct contact with at least the first and second passivation film portions, and has a composition different from that of the passivation film.

1 . A field-effect transistor, comprising:
 a semiconductor multilayer structure including a carrier transport layer made of nitride semiconductor;   a gate electrode formed on the semiconductor multilayer structure at a position corresponding to a channel region of the carrier transport layer, the gate electrode having a first sidewall surface on a first side thereof and a second sidewall surface on a second side thereof;   an insulating film formed directly on the gate electrode to cover at least one of the first sidewall surface and the second sidewall surface;   a first ohmic electrode formed on the first side of the gate electrode on the semiconductor multilayer structure;   a second ohmic electrode formed on the second side of the gate electrode on the semiconductor multilayer structure; and   a passivation film including a first portion extending from the first ohmic electrode toward the gate electrode to cover a surface area of the semiconductor multilayer structure between the first ohmic electrode and the gate electrode and a second portion extending from the second ohmic electrode toward the gate electrode to cover a surface area of the semiconductor multilayer structure between the second ohmic electrode and the gate electrode,   wherein the insulating film is in direct contact with at least the first and second portions of the passivation film, and has a composition different from that of the passivation film.   
   
   
       2 . The field-effect transistor as claimed in  claim 1 , wherein the insulating film comprises an oxide, nitride, or fluoride of a metal element constituting the gate electrode. 
   
   
       3 . The field-effect transistor as claimed in  claim 1 , wherein the insulating film is a multilayer film including layers each comprising an oxide, nitride, or fluoride of a metal element constituting the gate electrode. 
   
   
       4 . The field-effect transistor as claimed in  claim 1 , wherein the insulating film comprises aluminum oxide, aluminum nitride, gallium oxide, nickel oxide, nickel fluoride, or a copper oxide. 
   
   
       5 . The field-effect transistor as claimed in  claim 1 , wherein the insulating film has a film thickness no smaller than 0.5 nm and no larger than 500 nm. 
   
   
       6 . The field-effect transistor as claimed in  claim 1 , wherein the insulating film is formed to seamlessly cover the first and second sidewall surfaces and upper surface of the gate electrode. 
   
   
       7 . The field-effect transistor as claimed in  claim 1 , wherein the passivation film comprises a silicon nitride film or silicon oxide film. 
   
   
       8 . The field-effect transistor as claimed in  claim 1 , wherein the electron transport layer comprises GaN, AlN, or InN. 
   
   
       9 . The field-effect transistor as claimed in  claim 1 , wherein the field-effect transistor is a HEMT, and the semiconductor multilayer structure includes a carrier supply layer made of nitride semiconductor on the carrier transport layer, with 2-dimensional carrier gas formed in the carrier transport layer. 
   
   
       10 . The field-effect transistor as claimed in  claim 9 , wherein a nitride semiconductor layer including no Al is formed between the carrier supply layer and the gate electrode in the semiconductor multilayer structure. 
   
   
       11 . A method of making a field-effect transistor having a gate electrode, a source electrode, and a drain electrode on a semiconductor multilayer structure including a carrier transport layer, comprising:
 forming the gate electrode on the semiconductor multilayer structure;   forming a passivation film to cover the gate electrode on the semiconductor multilayer structure;   forming an opening in the passivation film to expose the gate electrode; and   forming an insulating film having a composition different from that of the passivation film on the exposed gate electrode such that the insulating film covers at least a sidewall surface of the gate electrode on a same side as the drain electrode.   
   
   
       12 . A method of making a field-effect transistor having a gate electrode, a source electrode, and a drain electrode on a semiconductor multilayer structure including a carrier transport layer, comprising:
 forming the gate electrode on the semiconductor multilayer structure;   forming an insulating film on the exposed gate electrode such that the insulating film covers at least a sidewall surface of the gate electrode on a same side as the drain electrode; and   forming a passivation film having a composition different from that of the insulating film to cover the gate electrode with the insulating film formed thereon on the semiconductor multilayer structure.   
   
   
       13 . The method of making a field-effect transistor as claimed in  claim 12 , wherein the insulating film is formed by oxidizing treatment, nitriding treatment, or fluorination treatment of the gate electrode.