P type gallium nitride conformal epitaxial structure over thick buffer layer

A method of forming a semiconductor device, comprising:

• forming a buffer layer of III-N semiconductor material over a silicon substrate, the buffer layer having a top surface;
• wherein:
• the buffer layer includes a columnar region having a first buffer thickness, a transition region surrounding the columnar region, and an inter-columnar region having a second buffer thickness, the first buffer thickness being greater than the second buffer thickness;
• the top surface in the columnar region has a first vertical range;
• and
• the top surface in the transition region extends from the columnar region to the inter-columnar region and has a second vertical range that is greater than the first vertical range;
• forming a barrier layer of a gallium nitride field effect transistor (GaN FET) over the top surface, the barrier layer having III-N semiconductor material including aluminum and nitrogen;
• and
• forming a gate layer of p-type III-N semiconductor material over the barrier layer by a gate metal organic vapor phase epitaxy (MOVPE) process using gas reagents and a carrier gas;
• wherein:
• the gas reagents include a gallium-containing gas reagent and a nitrogen-containing gas reagent;
• the carrier gas includes a gas selected from the group consisting of nitrogen gas and a noble gas, and includes zero to 40 percent hydrogen gas by volumetric flow rate;
• and
• the gate layer has a lower aluminum content than the barrier layer.