High electron mobility transistor and method for forming the same

1. A high electron mobility transistor, comprising:

a substrate;

a mesa structure disposed on the substrate, wherein the mesa structure comprises a channel layer and a barrier layer on the channel layer;

a passivation layer disposed on the mesa structure;

at least a contact structure disposed in the passivation layer and the mesa structure, wherein the contact structure comprises a body portion and a plurality of protruding portions, the body portion penetrates through the passivation layer and a portion of the barrier layer, the plurality of protruding portions penetrate through the barrier layer and a portion of the channel layer, wherein a bottom surface of the body portion is lower than an upper surface of the barrier layer and higher than a bottom surface of the barrier layer; and

a gate structure disposed on the mesa structure and between the contact structure and another one of the contact structure, wherein the gate structure comprises:

a semiconductor gate layer directly disposed on the barrier layer of the mesa structure and covered by the passivation layer; and

a gate metal layer through the passivation layer to directly contact a top surface of the semiconductor gate layer.

2. The high electron mobility transistor according to claim 1 , wherein the plurality of protruding portions are arranged along a first direction and a second direction to form an array in a top plane view, the first direction and the second direction are perpendicular.

3. The high electron mobility transistor according to claim 2 , wherein the plurality of protruding portions are aligned along the second direction and are staggered along the first direction.

4. The high electron mobility transistor according to claim 1 , wherein the plurality of protruding portions of the contact structure and the gate structure extend along a same direction in a top plane view.

5. The high electron mobility transistor according to claim 1 , wherein the plurality of protruding portions of the contact structure extend along a direction that is perpendicular to a direction along that the gate structure extends.

6. The high electron mobility transistor according to claim 1 , the body portion of the contact structure overlaps two edges of the mesa structure.

7. The high electron mobility transistor according to claim 1 , the body portion and the plurality of protruding portions of the contact structure are of a one-piece structure.

8. The high electron mobility transistor according to claim 1 , wherein a material of the contact structure is selected from a group consisting of gold (Au), tungsten (W), cobalt (Co), nickel (Ni), titanium (Ti), molybdenum (Mo), copper (Cu), aluminum (Al), tantalum (Ta), Palladium (Pd), platinum (Pt), a compounds of the above materials, a composite layer of the above materials, and an alloy of the above materials.

9. The high electron mobility transistor according to claim 1 , wherein a material of the channel layer is selected from a group consisting of gallium nitride (GaN), aluminum gallium nitride (AlGaN), aluminum indium nitride (AlInN), indium gallium nitride (InGaN), aluminum gallium indium nitride (AlGaInN), and a combination thereof.

10. The high electron mobility transistor according to claim 1 , wherein a material of the barrier layer is selected from a group consisting of aluminum gallium nitride (AlGaN), aluminum indium nitride (AlInN), aluminum gallium indium nitride (AlInGaN), aluminum nitride (AlN), and a combination thereof.

11. The high electron mobility transistor according to claim 1 , wherein a material of the passivation layer is selected from a group consisting of aluminum nitride (AlN), aluminum oxide (Al 2 O 3 ), boron nitride (BN), silicon nitride (Si 3 N 4 ), silicon oxide (SiO 2 ), zirconia (ZrO 2 ), hafnium oxide (HfO 2 ), lanthanum oxide (La 2 O 3 ), lutetium oxide (Lu 2 O 3 ), lanthanum oxide (LaLuO 3 ), and a combination thereof.

12. A method for forming a high electron mobility transistor, comprising:

providing a substrate;

forming a mesa structure on the substrate, wherein the mesa structure has a channel layer and a barrier layer disposed on the channel layer;

forming a semiconductor gate layer on the barrier layer of the mesa structure;

forming a passivation layer covering the substrate and the mesa structure and the semiconductor gate layer;

forming at least a first opening at a side of the semiconductor gate layer, wherein the first opening penetrates through the passivation layer and extends into part of the barrier layer, wherein a bottom surface of the first opening is lower than an upper surface of the barrier layer and higher than a bottom surface of the barrier layer;

forming a plurality of second openings, wherein the plurality of second openings are connected to a bottom surface of the first opening and penetrate through the barrier layer and a portion of the channel layer;

forming a metal layer in the first opening and the plurality of second openings thereby forming a contact structure; and

forming a gate metal layer through the passivation layer to directly contact a top surface of the semiconductor gate layer.

13. The method for forming a high electron mobility transistor according to claim 12 , wherein the plurality of second openings are arranged along a first direction and a second direction to form an array in a top plane view, the first direction and the second direction are perpendicular.

14. The method for forming a high electron mobility transistor according to claim 12 , wherein the first opening overlaps two edges of the mesa structure.