Reliable normally-off III-nitride active device structures, and related methods and systems

1. A field-effect transistor, comprising:

a first gate having a first connection to a variable voltage source;

a second gate having a second connection to a fixed voltage source and held at a substantially fixed potential in a cascode configuration; and

a semiconductor channel having

an enhancement mode portion, gated to be turned on and off by said first gate, which has been modified to operate in enhancement mode, and

a depletion mode portion, gated by said second gate, which has been modified to operate in depletion mode, and which is operative to shield the first gate from voltage stress.

2. The field-effect transistor of claim 1 , wherein said enhancement mode portion further has a fixed charge positioned to deplete the semiconductor channel near the first gate.

3. The field-effect transistor of claim 2 , wherein:

the depletion mode portion is situated between the enhancement mode portion and a drain terminal of the field-effect transistor; and

said fixed charge is at least partially shielded by the depletion mode portion of the semiconductor channel to mitigate fixed charge migration from the enhancement mode portion in response to an electrical-field at the depletion mode portion.

4. The field-effect transistor of claim 1 , wherein the field-effect transistor has a substantially stable threshold voltage under electrical-field stress at said depletion mode portion.

5. The field-effect transistor of claim 1 , wherein the second connection to the fixed voltage source is electrically coupled to a source terminal of the field effect transistor.

6. An active device structure in a III-N-type semiconductor material, comprising:

a first conductive source and drain region and a second conductive source and drain region, positioned to connect to different locations of a III-N-type semiconductor channel; and

first and second gate terminals in a cascode configuration, each capacitively coupled to a different respective portion of said III-N-type semiconductor channel between said first conductive source and drain region and the second conductive source and drain region, and wherein one of said gate terminals has a first connection to a variable voltage source, and the other has a second connection to a fixed voltage source;

said III-N-type semiconductor channel having different respective threshold voltages, of opposite signs, with respect to said first and second gate terminals.

7. The structure of claim 6 , wherein said channel consists essentially of Gallium Nitride.

8. The structure of claim 6 , wherein both said gate terminals are separated from said channel by a semiconductor layer having a wider bandgap than said channel.