Field effect transistor with enhanced insulator structure
View Patent ↗A III-nitride based field effect transistor obtains improved performance characteristics through manipulation of the relationship between the in-plane lattice constant of the interface of material layers. A high mobility two dimensional electron gas generated at the interface of the III-nitride materials permits high current conduction with low ON resistance, and is controllable through the manipulation of spontaneous polarization fields obtained according to the characteristics of the III-nitride material. The field effect transistor produced can be made to be a nominally on device where the in-plane lattice constants of the material forming the interface match. A nominally off device may be produced where one of the material layers has an in-plane lattice constant that is larger than that of the other layer material. The layer materials are preferably InAlGaN/GaN layers that are particularly tailored to the characteristics of the present invention.
1. A method for constructing a III-nitride device, comprising:
providing a first III-nitride material layer having an in-plane lattice constant A;
overlaying and contacting the first III-nitride material with a second III-nitride material layer having an in-plane lattice constant B and having a non-uniform thickness;
providing an interface between said first III-nitride material layer and said second III-nitride material layer; and
selecting the composition of the first or second III-nitride material such that constant B is greater than constant A, thereby:
substantially producing a nominally OFF III-nitride device;
controlling a density of charge accumulation at said interface by spontaneous polarization fields, wherein piezoelectric polarization fields do not contribute to said charge accumulation.
2. The method according to claim 1 , further comprising providing GaN as one of the first or second III-nitride materials.
3. The method according to claim 2 , further comprising providing InAlGaN as the other of the first or second III-nitride materials.
4. The method according to claim 1 , further comprising obtaining a bandgap value for one of the III-nitride materials that is greater than that of the other of the III-nitride materials.
5. A method for fabricating a III-nitride based field effect transistor (FET), comprising:
providing a first III-nitride material layer having a first composition;
forming a second III-nitride material layer having a non-uniform thickness and having a second composition different from the first composition overlaying and contacting the first III-nitride material layer;
wherein the III-nitride based FET is nominally OFF and the interface between the first and second III-nitride material layers is such that the spontaneous and piezoelectric polarization fields substantially cancel one another out.
6. The method in claim 5 , wherein said first composition comprises GaN and said second composition comprises InAIGaN.
7. The method according to claim 5 , further comprising obtaining a bandgap value for one of the III-nitride material layers that is greater than that of the other of the III-nitride material layers.
8. The method in claim 1 further comprising:
forming a source, a gate, and a drain electrode, wherein a channel is formed between said source and said drain electrode, and wherein said gate electrode controls said channel.
9. The method in claim 8 , wherein the non-uniform thickness of said second III-nitride material layer comprises a different thickness under the gate electrode than under the source or drain electrodes.
10. The method in claim 9 , wherein:
the second III-nitride material layer forms a good ohmic contact.
11. The method in claim 5 , wherein there is substantially no net charge at the interface.
12. A method for constructing a III-nitride device, comprising:
providing a first III-nitride material layer having an in-plane lattice constant A;
overlaying and contacting the first III-nitride material layer with a second III-nitride material layer having an in-plane lattice constant B and having a non-uniform thickness;
providing an interface between said first III-nitride material layer and said second III-nitride material layer;
selecting the composition of the first or second III-nitride material, wherein:
if constant B is greater than constant A:
said III-nitride device is nominally OFF;
a density of charge accumulation at said interface is controlled by spontaneous polarization fields;
piezoelectric polarization fields do not contribute to said charge accumulation;
if said constant B is substantially equal to constant A:
said III-nitride device is nominally ON.
13. The method according to claim 12 , wherein strain induced piezoelectric polarization fields of said III-nitride device are substantially eliminated.
14. The method according to claim 12 , wherein said second III-nitride material comprises InAlGaN.
15. The method according to claim 12 , wherein said first III-nitride material comprises GaN.
16. The method according to claim 12 , further comprising obtaining a bandgap value for said second III-nitride material layer that is greater than that of said first III-nitride material layer.