Regrowth method for fabricating wide-bandgap transistors, and devices made thereby
Methods are provided for fabricating a HEMT (high-electron-mobility transistor) that involve sequential epitaxial growth of III-nitride channel and barrier layers, followed by epitaxial regrowth of further III-nitride material through a window in a mask layer. In examples, the regrowth takes place over exposed portions of the channel layer in the source and drain regions of the device, and the regrown material has a composition different from the barrier layer. In other examples, the regrowth takes place on the barrier layer, only in the access region or regions. Devices made according to the disclosed methods are also provided.
1. A method for fabricating a high electron mobility transistor, comprising:
epitaxially growing a III-nitride channel layer on a substrate;
epitaxially growing a III-nitride barrier layer on the channel layer;
depositing a dielectric mask layer on the barrier layer;
opening a window through the dielectric mask layer in a source region and in a drain region;
etching through the source region window and through the drain region window so as to remove barrier layer material and thereby leave a void in the barrier layer reaching down to an exposed upper surface of the channel layer; and
filling the voids in the source and drain regions by epitaxial regrowth of a III-nitride contact material that differs in composition from the barrier layer;
wherein the III-nitride contact material is regrown with a graded composition that initially matches the channel layer composition but progressively decreases in bandgap during the regrowth.
2. The method of claim 1 , wherein the III-nitride contact material has an Al x Ga 1−x N composition, x is at least 0 and at most 1, and x decreases during the regrowth.
3. The method of claim 2 , further comprising removing the III-nitride contact material from an upper surface of the dielectric mask layer after the regrowth has finished, and then removing the dielectric mask layer.
4. The method of claim 3 , wherein the step of removing the dielectric mask layer is performed by etching the dielectric mask layer with a potassium hydroxide based etchant.
5. A high electron mobility transistor, comprising:
an epitaxial III-nitride channel layer of Al 0.85 Ga 0.15 N on a substrate;
an epitaxial III-nitride barrier layer on the channel layer; and
an epitaxial III-nitride contact-region regrowth, wherein:
the contact-region regrowth consists of contact material grown on the channel layer;
the contact-region regrowth fills voids in the barrier layer in a source region and in a drain region of the transistor;
the contact-region regrowth is different in composition from the barrier layer; and
the contact-region regrowth has a graded composition that decreases in bandgap with distance from the channel layer.
6. A high electron mobility transistor, comprising:
an epitaxial III-nitride channel layer of Al 0.85 Ga 0.15 N on a substrate;
an epitaxial III-nitride barrier layer on the channel layer; and
an epitaxial III-nitride contact-region regrowth, wherein:
the contact-region regrowth consists of contact material grown on the channel layer;
the contact-region regrowth fills voids in the barrier layer in a source region and in a drain region of the transistor; and
the contact-region regrowth has a smaller bandgap than the barrier layer and a smaller bandgap than the channel layer.