Field-effect transistor and method therefor

A method comprising:

• etching a trench in a semiconductor substrate, the trench having a first sidewall and a second sidewall;
• depositing a first conductive material to form a vertical field plate in the trench;
• depositing a second conductive material to form a control gate in the in the trench, the control gate disposed between the first sidewall and the vertical field plate;
• forming a dielectric region in the trench between the first sidewall and a first edge of the vertical field plate, the dielectric region having a first dielectric constant;
• and
• forming air cavity region in the trench between the second sidewall and a second edge of the vertical field plate, the air cavity region having a second dielectric constant lower than the first dielectric constant.

A transistor comprising:

• a trench formed in a semiconductor substrate, the trench having a first sidewall and a second sidewall;
• a vertical field plate formed in the trench, the vertical field plate located between the first sidewall and the second sidewall;
• a gate dielectric formed on at least a portion of the first sidewall in the trench;
• a gate electrode formed in the trench, a first edge of the gate electrode adjacent to the gate dielectric and a second edge of the gate electrode proximate to the vertical field plate;
• a dielectric material formed in the trench between the first sidewall and the vertical field plate;
• and
• an air cavity formed in the trench between the vertical field plate and the second sidewall, the air cavity having a dielectric constant lower than that of the dielectric material.