Semiconductor devices
A semiconductor device 10 comprises a heterojunction between a lower semiconductor layer 26 made of p-type gallium nitride and an upper semiconductor layer 28 made of n-type AlGaN, wherein the upper semiconductor layer 28 has a larger band gap than the lower semiconductor layer 26 . The semiconductor device 10 further comprises a drain electrode 32 formed on a portion of a top surface of the upper semiconductor layer 28 , a source electrode 34 formed on a different portion of the top surface of the upper semiconductor layer 28 , and a gate electrode 36 electrically connected to the lower semiconductor layer 26 . The semiconductor device 10 can operate as normally-off.
1. A normally-off heterojunction semiconductor device comprising:
a lower semiconductor layer comprising a first type of semiconducting material;
an upper semiconductor layer comprising a second type of semiconducting material that has a larger band gap than the first type of semiconducting material, wherein the upper semiconductor layer is attached to a top surface of the lower semiconductor layer;
a first electrode formed on a portion of a top surface of the upper semiconductor layer;
a second electrode formed on a different portion of the top surface of the upper semiconductor layer; and
a gate electrode electrically connected to the lower semiconductor layer,
wherein a conductivity type of the lower semiconductor layer is p-type,
an electric potential of the lower semiconductor layer can vary in accordance with an electric potential of the gate electrode, and
a two-dimensional electron gas layer is generated at a boundary between the lower semiconductor layer and the upper semiconductor layer when a gate-on voltage is applied to the gate electrode so that the electrons flow between the first electrode and the second electrode via the two-dimensional electron gas layer.
2. The normally-off heterojunction semiconductor device of claim 1 , wherein the first and the second types of semiconducting materials include III-V semiconductors.
3. The normally-off heterojunction semiconductor device of claim 2 , wherein the first and the second types of semiconducting materials contain Al X Ga Y In 1−X−Y N (0≦X≦1, 0≦Y≦1, 0≦(1−X−Y)≦1), wherein the X of the first type of semiconducting material is smaller than the X of the second type of semiconducting material, and/or the (1−X−Y) of the first type of semiconducting material is larger than the (1−X−Y) of the second type of semiconducting material.
4. The normally-off heterojunction semiconductor device of claim 1 , wherein a conductivity type of the upper semiconductor layer is n-type.
5. The normally-off heterojunction semiconductor device of claim 1 , wherein a part of the top surface of the lower semiconductor layer is not covered by the upper semiconductor layer at a portion other than an area extending between the first electrode and the second electrode, and the gate electrode is formed on that uncovered region.
6. The normally-off heterojunction semiconductor device of claim 5 , wherein the gate electrode extends parallel to a direction along which the first electrode and the second electrode are separated.