High electron mobility transistors and power amplifiers including said transistors having improved performance and reliability

1. A method of forming a high electron mobility transistor (HEMT), comprising:

forming a semiconductor structure on a substrate;

forming an ohmic source contact on an upper surface of the semiconductor structure; and

forming a via extending from a lower surface of the substrate to the ohmic source contact,

wherein a length of a cross-section of the via taken parallel to the lower surface of the substrate exceeds a width of the cross-section of the via, and

wherein the via contacts a portion of the ohmic source contact that is in between a gate contact and a drain contact.

2. The method of claim 1 , wherein a width of the via decreases as the via extends in a depth direction from the lower surface of the substrate to the ohmic source contact, the method further comprising thinning the substrate after forming the semiconductor structure thereon.

3. The method of claim 2 , wherein a thickness of the substrate is 75 μm or less after thinning the substrate.

4. The method of claim 1 , wherein forming the semiconductor structure on the substrate comprises:

forming a channel layer on the substrate; and

forming a barrier layer on the channel layer.

5. The method of claim 4 , wherein the ohmic source contact is in a recess in an upper surface of the barrier layer.

6. The method of claim 1 , further comprising depositing a backmetal layer within the via.

7. The method of claim 6 , wherein the backmetal layer directly contacts the lower surface of the ohmic source contact, and

wherein a contact area between the backmetal layer and the lower surface of the ohmic source contact is fifty percent or more of an area of the lower surface of the ohmic source contact.

8. The method of claim 6 , wherein the via has an oval shape in the cross-sections taken parallel to the lower surface of the substrate.

9. The method of claim 8 , wherein a maximum width of the via in a direction parallel to the lower surface of the substrate is less than 16 μm.

10. A method of forming a transistor, comprising:

forming a semiconductor structure on a substrate;

forming a first ohmic source contact and a second ohmic source contact on an upper surface of the semiconductor structure;

forming a drain contact on the upper surface of the semiconductor structure between the first ohmic source contact and the second ohmic source contact, the first ohmic source contact and the drain contact forming a first unit cell transistor and the second ohmic source contact and the drain contact forming a third unit cell transistor; and

forming a first via and a second via extending between a bottom surface of the substrate and the first ohmic source contact and the second ohmic source contact, respectively, the first and second vias on opposed sides of the drain contact,

wherein the first ohmic source contact is shared between the first unit cell transistor and a second unit cell transistor,

wherein the second ohmic source contact is shared between the third unit cell transistor and a fourth unit cell transistor, and

wherein a distance between a center of the first via and a center of the second via is 75 μm or less.

11. The method of claim 10 , wherein a width of the first via decreases as the first via extends from the bottom surface of the substrate to the first ohmic source contact, and wherein the first via has non-circular cross-sections in planes parallel to the bottom surface of the substrate.

12. The method of claim 10 , wherein the transistor is a high electron mobility transistor (HEMT), and

wherein forming the semiconductor structure comprises:

forming a channel layer on the substrate; and

forming a barrier layer on the channel layer.

13. The method of claim 10 , wherein a maximum width of the first via in a direction parallel to a lower surface of the substrate is less than 16 μm.

14. The method of claim 10 , further comprising:

forming a well region in an upper portion of the semiconductor structure,

wherein the first ohmic source contact is on the well region.

15. The method of claim 10 , wherein the first ohmic source contact is in a recess in the upper surface of the semiconductor structure.

16. A transistor, comprising:

a semiconductor structure on a substrate;

a first ohmic source contact and a second ohmic source contact on an upper surface of the semiconductor structure;

a drain contact on the upper surface of the semiconductor structure between the first ohmic source contact and the second ohmic source contact; and

a first via and a second via extending between a bottom surface of the substrate and the first ohmic source contact and the second ohmic source contact, respectively, the first and second vias on opposed sides of the drain contact,

wherein a length of a cross-section of the first via taken in a plane parallel to the bottom surface of the substrate that has the largest area exceeds a width of the cross-section, and the length of the cross-section is 40 microns or less and the width of the cross-section is 16 microns or less, and

wherein a distance between a center of the first via and a center of the second via is 75 μm or less.

17. The transistor of claim 16 , further comprising a gate contact on the semiconductor structure,

wherein the first via is between, in plan view, the drain contact and the gate contact.

18. The transistor of claim 16 , further comprising a well region in an upper portion of the semiconductor structure,

wherein the first ohmic source contact is on the well region.

19. The transistor of claim 16 , wherein a width of the first via decreases as the first via extends from the bottom surface of the substrate to the first ohmic source contact.

20. The transistor of claim 10 , wherein a cross-section of the first via taken parallel to the lower surface of the substrate is an oval cross-section.

21. The method of claim 10 , wherein a distance between the first ohmic source contact and the second ohmic source contact is 60 μm or less.

22. The transistor of claim 16 , wherein the first ohmic source contact is in a recess in the upper surface of the semiconductor structure.

23. The transistor of claim 16 , wherein the first via is shared between a first unit cell transistor and a second unit cell transistor.

24. The transistor of claim 1 , wherein the cross-section is an oval cross-section.