Transistor including an active region and methods for forming the same

A semiconductor structure comprising a thin film transistor overlying a substrate, wherein the thin film transistor comprises:

• an active layer including a compound semiconductor material comprising oxygen, at least one acceptor-type element, and at least one heavy post-transition metal element, wherein each of the at least one acceptor-type element is selected from a group consisting of gallium (Ga) and tungsten (W), wherein each of the at least one heavy post-transition metal element is selected from a group consisting of indium (In) and tin (Sn), and wherein an atomic percentage of the at least one heavy post-transition metal element at a top surface of the active layer is higher than an atomic percentage of the at least one heavy post-transition metal element at a bottom surface of the active layer;
• and
• a top gate stack comprising a top gate dielectric straddling the active layer and a top gate electrode located on a top surface of the top gate dielectric,
• wherein the active layer comprises a vertical stack of a front channel layer in contact with the top gate dielectric, a back channel layer comprising a bottom surface of the active layer, and a bulk semiconductor layer located between the front channel layer and the back channel layer;
• wherein one of the front channel layer, the back channel layer, and the bulk semiconductor layer comprises an inhomogeneous compound semiconductor material having a vertical composition gradient such that an atomic concentration of the at least one acceptor-type element decreases continuously throughout said one of the front channel layer, the back channel layer, and the bulk semiconductor layer with a vertical distance from the substrate, and an atomic concentration of the at least one heavy post-transition metal element increases continuously throughout said one of the front channel layer, the back channel layer, and the bulk semiconductor layer with the vertical distance from the substrate;
• and
• wherein another of the front channel layer, the back channel layer, and the bulk semiconductor layer comprises a homogeneous compound semiconductor material having a uniform material composition throughout.

A transistor comprising:

• an active layer located over a substrate and including a compound semiconductor material comprising oxygen, at least one acceptor-type element, and at least one heavy post-transition metal element, wherein each of the at least one acceptor-type element is selected from a group consisting of gallium (Ga) and tungsten (W), wherein each of the at least one heavy post-transition metal element is selected from a group consisting of indium (In) and tin (Sn);
• and
• a gate stack comprising a gate dielectric and a gate electrode, wherein the gate dielectric contacts a middle portion of a first horizontal surface of the active layer, and the active layer comprises a second horizontal surface located at an opposite side of the first horizontal surface, and wherein an atomic percentage of the at least one heavy post-transition metal element at the first horizontal surface of the active layer is higher than an atomic percentage of the at least one heavy post-transition metal element at the second horizontal surface of the active layer,
• wherein the active layer comprises a vertical stack of a front channel layer in contact with the top gate dielectric, a back channel layer comprising a bottom surface of the active layer, and a bulk semiconductor layer located between the front channel layer and the back channel layer;
• wherein one of the front channel layer, the back channel layer, and the bulk semiconductor layer comprises an inhomogeneous compound semiconductor material having a vertical composition gradient such that an atomic concentration of the at least one acceptor-type element decreases continuously throughout said one of the front channel layer, the back channel layer, and the bulk semiconductor layer with a vertical distance from the substrate, and an atomic concentration of the at least one heavy post-transition metal element increases continuously throughout said one of the front channel layer, the back channel layer, and the bulk semiconductor layer with the vertical distance from the substrate;
• and
• wherein another of the front channel layer, the back channel layer, and the bulk semiconductor layer comprises a homogeneous compound semiconductor material having a uniform material composition throughout.