Nitride based transistors for millimeter wave operation
View Patent ↗Field effect transistors having a power density of greater than 5 W/mm when operated at a frequency of at least 30 GHz are provided. The power density of at least 5 W/mm may be provided at a drain voltage of 28 V. Transistors with a power density of at least 8 W/mm when operated at 40 GHz at a drain voltage of 28 V are also provided.
1. A field effect transistor, comprising:
a Group III-nitride channel layer;
a spacer layer on the Group III-nitride channel layer;
a gate contact on the Group III-nitride channel layer and configured to modulate a conductivity of the channel layer in response to a voltage applied to the gate contact, the gate contact having a gate length that is sufficient to permit modulation of the conductivity of the channel layer at frequencies exceeding 30 GHz;
source and drain contacts on the Group III-nitride channel layer; and
a lower field plate electrically connected to the gate contact and extending across the spacer layer a distance L FD toward the drain contact, wherein L FD is at least about 0.1 μm.
2. The field effect transistor of claim 1 , further comprising a Group III-nitride barrier layer on the channel layer, wherein the gate contact and the spacer layer are on the barrier layer, and the barrier layer and the channel layer cooperatively induce a two-dimensional electron gas in the channel layer near an interface between the barrier layer and the channel layer.
3. The field effect transistor of claim 2 , wherein the barrier layer comprises a first barrier sublayer on the channel layer and a second barrier sublayer on the first barrier sublayer, the first barrier sublayer comprising AlN and the second barrier sublayer comprising Al x Ga l−x N, wherein 0.15≦x≦0.45.
4. The field effect transistor of claim 3 , wherein the first barrier sublayer has a thickness of about 0 to about 4 nm and the second barrier sublayer has a thickness of about 10 to about 50 nm.
5. The field effect transistor of claim 1 , wherein the channel layer comprises a first channel sublayer and a second channel sublayer on the first channel sublayer, the first channel sublayer comprising GaN and having a concentration of Fe dopants of at least about 1×10 17 /cm 3 , the second channel sublayer comprising GaN and having a concentration of Fe dopants therein that decreases with distance from the first channel sublayer.
6. The field effect transistor of claim 1 , wherein L FD is about 0.25 μm.
7. The field effect transistor of claim 6 , wherein the spacer layer comprises SiN.
8. The field effect transistor of claim 1 , wherein the field plate extends across the spacer layer toward the source contact by a distance L FS of about 0.2 μm.
9. The field effect transistor of claim 1 , wherein the power density of at least 5 W/mm is provided at a drain voltage of 28 V.
10. The field effect transistor of claim 1 , wherein the field effect transistor has a power added efficiency greater than about 30%.
11. The field effect transistor of claim 1 , wherein the field effect transistor is configured to exhibit a power density of greater than about 8 W/mm when operated at a frequency of at least 30 GHz.
12. The field effect transistor of claim 1 , wherein the field effect transistor is configured to exhibit a power density of greater than about 5 W/mm when operated at a frequency of at least 40 GHz.