Semiconductor device

A semiconductor device which comprises a group III nitride semiconductor layer formed on a single crystalline sapphire substrate, a source electrode, a drain electrode and pad electrodes thereof formed apart from one another on the surface of said group III nitride semiconductor layer, and a gate electrode formed between said source electrode and said drain electrode;

• wherein
• said group III nitride semiconductor layer is formed on a plane lying parallel to a C axis of said single crystalline sapphire substrate;
• and
• a thickness of said single crystalline sapphire substrate tsub satisfies the following Equation (1).
• 6 t sub ≦ 10 ⁢ ϵ sub ⁢ S pad ϵ epi ⁢ S gate ⁢ t act , ( 1 )
• where
• Spad is an area of the pad electrode;
• Sgate is an area of the gate electrode;
• εsub is a relative permittivity of the sapphire substrate in the direction of the thickness;
• εepi is a relative permittivity of the group III nitride semiconductor layer in the direction of the thickness;
• tsub is a thickness of the sapphire substrate;
• and
• tact is an effective thickness of the group III nitride semiconductor layer.