A nitride semiconductor device includes a substrate, a first electron transport layer above the substrate, a first electron supply layer above the first electron transport layer, a first nitride semiconductor layer above the first electron supply layer, a first opening passing through the first nitride semiconductor layer and the first electron supply layer and reaching the first electron transport layer, a second electron transport layer disposed above the first nitride semiconductor layer and along the inner surface of the first opening, a second electron supply layer disposed above the second electron transport layer and covering the first opening, a gate electrode disposed above the second electron supply layer and covering the first opening, a source electrode connected to the first nitride semiconductor layer and the second electron transport layer, and a drain electrode.

A nitride semiconductor device includes a substrate, a first electron transport layer above the substrate, a first electron supply layer above the first electron transport layer, a first nitride semiconductor layer above the first electron supply layer, a first opening passing through the first nitride semiconductor layer and the first electron supply layer and reaching the first electron transport layer, a second electron transport layer disposed above the first nitride semiconductor layer and along the inner surface of the first opening, a second electron supply layer disposed above the second electron transport layer and covering the first opening, a gate electrode disposed above the second electron supply layer and covering the first opening, a source electrode connected to the first nitride semiconductor layer and the second electron transport layer, and a drain electrode.

A trench-type MESFET includes an n-type semiconductor layer including a Ga.sub.2O.sub.3-based single crystal and including plural trenches opening on one surface, first insulators respectively buried in bottom portions of the plural trenches, gate electrodes respectively buried in the plural trenches so as to be placed on the first insulators and so that side surfaces thereof are in contact with the n-type semiconductor layer, a source electrode connected to a mesa-shaped portion between the adjacent trenches of the n-type semiconductor layer, second insulators respectively buried in the plural trenches so as to be placed on the gate electrodes to insulate the gate electrodes and the source electrode, and a drain electrode directly or indirectly connected to the n-type semiconductor layer on a side opposite to the source electrode.

A semiconductor device is provided with one or more gate fingers (20) that are provided in an active region on a semiconductor substrate (1), and a source finger (30) and a drain finger (40) that are provided in the active region and arranged alternately to allow each gate finger to be sandwiched between the source and drain fingers. The semiconductor device includes terminal circuit (60) that has inductive impedance at the frequency of a signal input to an input terminal of the one or more gate fingers, and is directly or indirectly connected to the one or more gate fingers at an area being spaced away from a connecting position of the input terminal (21a) of the one or more gate fingers (20).

A semiconductor substrate (1) includes a front surface and a back surface opposite to each other, and a through-hole (9) penetrating from the back surface to the front surface. A metal film (10) surrounding the through-hole (9) is formed in a ring shape on the front surface. A front-surface electrode (6) includes a wiring electrode (11,12) covering the through-hole (9) and the metal film (10) and is joined to the front surface outside the metal film (10). A back-surface electrode (15) is formed on the back surface and inside the through-hole (9) and connected to the wiring electrode (11,12). The metal film (10) has a lower ionization tendency and a higher work function than the wiring electrode (11,12).

A gate electrode includes a main portion formed of a gate electrode material, and a gate electrode barrier layer disposed between the main portion and a barrier layer and formed of a conductive material that prevents the gate electrode material from diffusing into the barrier layer. A surface of the main portion in a region above a first insulating layer faces a periphery without a layer of the conductive material being formed.

A gate electrode includes a main portion formed of a gate electrode material, and a gate electrode barrier layer disposed between the main portion and a barrier layer and formed of a conductive material that prevents the gate electrode material from diffusing into the barrier layer. A surface of the main portion in a region above a first insulating layer faces a periphery without a layer of the conductive material being formed.

A Ga.sub.2O.sub.3-based semiconductor device includes a Ga.sub.2O.sub.3-based crystal layer including a donor, and an N-doped region formed in at least a part of the Ga.sub.2O.sub.3-based crystal layer.

A Ga.sub.2O.sub.3-based semiconductor device includes a Ga.sub.2O.sub.3-based crystal layer including a donor, and an N-doped region formed in at least a part of the Ga.sub.2O.sub.3-based crystal layer.

A semiconductor test structure includes a field-effect transistor and a metal connection structure. The field-effect transistor includes a substrate with first doping type, a gate located on a surface of the substrate, and a source region with a second doping type and a drain region with the second doping type in the substrate, the source region and the drain region are located on two sides of the gate, respectively. The metal connection structure is connected with the gate; the metal connection structure forms a Schottky contact with the substrate.