Bridge circuits and their components

1. A circuit, comprising:

a transistor comprising a gate, a source, a drain, and a channel, the transistor being configured to block substantial voltage during a first mode of operation of the circuit, to conduct substantial current through the channel in a first direction during a second mode of operation of the circuit, and to conduct substantial current through the channel in a second direction during a third mode of operation of the circuit; wherein

during the third mode of operation, the gate of the transistor is biased relative to the source of the transistor at a voltage lower than a threshold voltage of the transistor.

2. The circuit of claim 1 , wherein the circuit comprises a half bridge, and the transistor is part of the half bridge.

3. The circuit of claim 2 , further comprising a plurality of the half bridges of claim 2 .

4. The circuit of claim 1 , wherein the transistor is further configured to conduct substantial current through the channel in the second direction during a fourth mode of operation of the circuit, wherein during the fourth mode of operation the gate of the transistor is biased relative to the source of the transistor at a voltage higher than the threshold voltage of the transistor.

5. The circuit of claim 4 , wherein the transistor is in a lower power mode of operation during the fourth mode of operation of the circuit than during the third mode of operation of the circuit.

6. The circuit of claim 1 , wherein the transistor is a III-N transistor.

7. A method of operating a transistor in a circuit, the transistor comprising a gate, a source, a drain, and a channel, the method comprising:

blocking a substantial voltage across the transistor during a first mode of operation of the circuit;

conducting substantial current through the channel in a first direction during a second mode of operation of the circuit; and

conducting substantial current through the channel in a second direction during a third mode of operation of the circuit; wherein

during the third mode of operation, the gate of the transistor is biased relative to the source of the transistor at a voltage lower than a threshold voltage of the transistor.

8. The method of claim 7 , wherein the circuit comprises a half bridge, and the transistor is part of the half bridge.

9. The method of claim 7 , further comprising conducting substantial current through the channel in a second direction during a fourth mode of operation of the circuit, wherein during the fourth mode of operation the gate of the transistor is biased relative to the source of the transistor at a voltage higher than the threshold voltage of the transistor.

10. The method of claim 9 , wherein the transistor is in a lower power mode of operation during the fourth mode of operation of the circuit than during the third mode of operation of the circuit.

11. The method of claim 10 , wherein the circuit comprises a half bridge, and the transistor is part of the half bridge.

12. The method of claim 7 , wherein the transistor is a III-N transistor.

13. A method of operating a circuit comprising a first transistor, a second transistor having a channel, and an inductive component coupled between the first transistor and second transistor, the method comprising:

biasing the first transistor on and biasing the second transistor off, allowing current to flow through the first transistor and the inductive component, whereby a blocking voltage is present across the second transistor;

changing the first transistor to an off bias, allowing current to flow through the channel of the second transistor and through the inductive component while the second transistor remains biased off; and

after changing the first transistor to an off bias, changing the second transistor to an on bias; wherein

after changing the second transistor to an on bias, current continues to flow through the channel of the second transistor and through the inductive component.

14. The method of claim 13 , wherein the circuit comprises a half bridge, and the first and second transistors are part of the half bridge.

15. The method of claim 14 , wherein the first transistor is between a high-voltage supply and the second transistor, and the second transistor is between an electrical ground and the first transistor.

16. The method of claim 15 , wherein a time between the step of changing the first transistor to an off bias and changing the second transistor to an on bias is sufficient to prevent shoot-through currents from the high-voltage supply to the electrical ground.

17. The method of claim 14 , wherein the second transistor is between a high-voltage supply and the first transistor, and the first transistor is between an electrical ground and the second transistor.

18. The method of claim 17 , wherein a time between the step of changing the first transistor to an off bias and changing the second transistor to an on bias is sufficient to prevent shoot-through currents from the high-voltage supply to the electrical ground.

19. The method of claim 13 , wherein the first transistor or the second transistor is a III-N transistor.

20. A circuit, comprising:

a depletion mode transistor comprising a first gate, a first source, a first drain, and a first channel; and

an enhancement mode transistor having a threshold voltage, the enhancement mode transistor comprising a second gate, a second source, a second drain, a second channel, and a parasitic diode anti-parallel to the second channel, the second drain being electrically connected to the first source; wherein

the circuit is configured such that during a first mode of operation of the circuit, the first drain is held at a higher voltage than the second source, a voltage greater than the threshold voltage of the enhancement mode transistor is applied to the second gate relative to the second source, and current flows from the first drain to the second source, wherein during the first mode of operation the current flows through the first channel and through the second channel;

during a second mode of operation of the circuit, a voltage of the first drain is lower than a voltage of the second source, a voltage less than the threshold voltage of the enhancement mode transistor is applied to the second gate relative to the second source, and current flows from the second source to the first drain, wherein during the second mode of operation, current flows through the first channel, but most of the current does not flow through the second channel; and

during a third mode of operation of the circuit, a voltage of the first drain is lower than a voltage of the second source, a voltage greater than the threshold voltage of the enhancement mode transistor is applied to the second gate relative to the second source, and current flows from the second source to the first drain, wherein during the third mode of operation current flows through the first channel and through the second channel.

21. The circuit of claim 20 , wherein during the second mode of operation most of the current flows through the parasitic diode.

22. The circuit of claim 20 , wherein the third mode of operation of the circuit is a lower power mode of operation than the second mode of operation.

23. The circuit of claim 20 , further comprising an additional diode connected in parallel to the parasitic diode.

24. The circuit of claim 23 , wherein during the second mode of operation most of the current flows through the additional diode.

25. The circuit of claim 24 , wherein the third mode of operation of the circuit is a lower power mode of operation than the second mode of operation.

26. The circuit of claim 20 , wherein the depletion mode transistor is a III-N HEMT.

27. The circuit of claim 26 , wherein the enhancement mode transistor is a Si MOS transistor.

28. The circuit of claim 20 , wherein the circuit comprises a half bridge having a switch, and the enhancement mode transistor and the depletion mode transistor are part of the switch.

29. A method of operating a circuit comprising a depletion mode transistor and an enhancement mode transistor, the depletion mode transistor comprising a first gate, a first source, a first drain, and a first channel, the enhancement mode transistor having a threshold voltage, the enhancement mode transistor comprising a second gate, a second source, a second drain, a second channel, and a parasitic diode anti-parallel to the second channel, the second drain being electrically connected to the first source, the method comprising:

during a first mode of operation of the circuit, holding the first drain at a higher voltage than the second source, and applying a voltage greater than the threshold voltage of the enhancement mode transistor to the second gate relative to the second source, allowing current to flow from the first drain to the second source, wherein during the first mode of operation current flows through the first channel and through the second channel;

during a second mode of operation of the circuit, holding the first drain at a lower voltage than the second source, and applying a voltage less than the threshold voltage of the enhancement mode transistor to the second gate relative to the second source, allowing current to flow from the second source to the first drain, wherein during the second mode of operation current flows through the first channel, but most of the current does not flow through the second channel; and

during a third mode of operation of the circuit, holding the first drain at a lower voltage than the second source, and applying a voltage greater than the threshold voltage of the enhancement mode transistor to the second gate relative to the second source, allowing current to flow from the second source to the first drain, wherein during the third mode of operation current flows through the first channel and through the second channel.

30. The method of claim 29 , wherein during the second mode of operation most of the current flows through the parasitic diode.

31. The method of claim 30 , wherein the third mode of operation of the circuit is a lower power mode of operation than the second mode of operation.

32. The method of claim 29 , wherein the circuit further comprises an additional diode connected in parallel to the parasitic diode.

33. The method of claim 32 , wherein during the second mode of operation, most of the current flows through the additional diode.

34. The method of claim 33 , wherein the third mode of operation of the circuit is a lower power mode of operation than the second mode of operation.

35. The method of claim 29 , wherein the depletion mode transistor is a III-N HEMT.

36. The method of claim 35 , wherein the enhancement mode transistor is a Si MOS transistor.

37. The method of claim 29 , wherein the circuit comprises a half bridge having a switch, and the enhancement mode transistor and the depletion mode transistor are part of the switch.

38. A circuit, comprising:

a depletion mode transistor comprising a first gate, a first source, a first drain, and a first channel; and

an enhancement mode transistor having a threshold voltage, the enhancement mode transistor comprising a second gate, a second source, a second drain, a second channel, and a parasitic diode anti-parallel to the second channel, the second drain being electrically connected to the first source; wherein

the circuit is configured such that during a first mode of operation of the circuit, the first drain is held at a higher voltage than the second source, a voltage less than the threshold voltage of the enhancement mode transistor is applied to the second gate relative to the second source, and the depletion mode transistor blocks a substantial voltage;

during a second mode of operation of the circuit, a voltage of the first drain is lower than a voltage of the second source, a voltage less than the threshold voltage of the enhancement mode transistor is applied to the second gate relative to the second source, and current flows from the second source to the first drain, wherein during the second mode of operation current flows through the first channel, but most of the current does not flow through the second channel; and

during a third mode of operation of the circuit, a voltage of the first drain is lower than a voltage of the second source, a voltage greater than the threshold voltage of the enhancement mode transistor is applied to the second gate relative to the second source, and current flows from the second source to the first drain, wherein during the third mode of operation current flows through the first channel and through the second channel.

39. A method of operating a circuit comprising a depletion mode transistor and an enhancement mode transistor, the depletion mode transistor comprising a first gate, a first source, a first drain, and a first channel, the enhancement mode transistor having a threshold voltage, the enhancement mode transistor comprising a second gate, a second source, a second drain, a second channel, and a parasitic diode anti-parallel to the second channel, the second drain being electrically connected to the first source, the method comprising:

during a first mode of operation of the circuit, holding the first drain at a higher voltage than the second source, and applying a voltage less than the threshold voltage of the enhancement mode transistor to the second gate relative to the second source, causing the depletion mode transistor to block a substantial voltage;

during a second mode of operation of the circuit, holding the first drain at a lower voltage than the second source, and applying a voltage less than the threshold voltage of the enhancement mode transistor to the second gate relative to the second source, allowing current to flow from the second source to the first drain, wherein during the second mode of operation current flows through the first channel, but most of the current does not flow through the second channel; and

during a third mode of operation of the circuit, holding the first drain at a lower voltage than the second source, and applying a voltage less than the threshold voltage of the enhancement mode transistor to the second gate relative to the second source, allowing current to flow from the second source to the first drain, wherein during the third mode of operation current flows through the first channel and through the second channel.