Direct detector for terahertz radiation
View Patent ↗A direct detector for terahertz radiation comprises a grating-gated field-effect transistor with one or more quantum wells that provide a two-dimensional electron gas in the channel region. The grating gate can be a split-grating gate having at least one finger that can be individually biased. Biasing an individual finger of the split-grating gate to near pinch-off greatly increases the detector's resonant response magnitude over prior QW FET detectors while maintaining frequency selectivity. The split-grating-gated QW FET shows a tunable resonant plasmon response to FIR radiation that makes possible an electrically sweepable spectrometer-on-a-chip with no moving mechanical optical parts. Further, the narrow spectral response and signal-to-noise are adequate for use of the split-grating-gated QW FET in a passive, multispectral terahertz imaging system. The detector can be operated in a photoconductive or a photovoltaic mode. Other embodiments include uniform front and back gates to independently vary the carrier densities in the channel region, a thinned substrate to increase bolometric responsivity, and a resistive shunt to connect the fingers of the grating gate in parallel and provide a uniform gate-channel voltage along the length of the channel to increase the responsivity and improve the spectral resolution.
1. A direct detector for terahertz radiation, comprising:
a field-effect transistor formed in a semiconductor substrate, comprising a heterostructure that provides a two-dimensional electron gas in the channel region between the source and the drain of the field-effect transistor, and a periodic split-grating gate comprising a plurality of fingers on a front surface above the channel region, wherein at least one of the fingers of the grating gate is individually biased, to modulate the electron density in the two-dimensional electron gas;
means for applying a gate voltage to the periodic grating gate and a independent gate voltage to the at least one individually biased finger; and
means for detecting an output signal from the field-effect transistor when the front surface is irradiated with terahertz radiation.
2. The direct detector of claim 1 , wherein the at least one individually biased finger is biased to near the pinch-off voltage of the channel.
3. The direct detector of claim 1 , wherein the heterostructure comprises one or more quantum wells.
4. The direct detector of claim 3 , wherein the heterostructure comprises a single quantum well.
5. The direct detector of claim 1 , wherein the heterostructure comprises dissimilar II-V compound semiconductors, II-VI compound semiconductors, or Ge—Si alloys.
6. The direct detector of claim 5 , wherein the dissimilar III-V compound semiconductors comprise GaAs and AlGaAs.
7. The direct detector of claim 1 , wherein the terahertz radiation has a frequency of greater than 100 GHz.
8. The direct detector of claim 1 , wherein the detecting means comprises measuring the photoconductive response of the field-effect transistor.
9. The direct detector of claim 1 , wherein the detecting means comprises measuring the photovoltaic response of the field-effect transistor.
10. The direct detector of claim 1 , further comprising a back gate on the opposite side of the channel region from the grating gate.
11. The direct detector of claim 1 , further comprising a transparent front gate on the front surface above the channel region.
12. The direct detector of claim 1 , wherein the substrate comprises a thinned substrate.
13. The direct detector of claim 1 , further comprising a resistive shunt that connects the fingers of the grating gate in parallel and means for applying a shunt voltage to the resistive shunt.
14. The direct detector of claim 13 , wherein the shunt voltage is approximately equal to the source-drain voltage.
15. A direct detector for terahertz radiation, comprising:
a field-effect transistor formed in a semiconductor substrate, comprising a single quantum well that provides a two-dimensional electron gas in the channel region between the source and the drain of the field-effect transistor, and a periodic grating gate comprising a plurality of fingers on a front surface above the channel region to modulate the electron density in the two-dimensional electron gas;
means for applying a gate voltage to the periodic grating gate; and
means for detecting an output signal from the field-effect transistor when the front surface is irradiated with terahertz radiation.
16. The direct detector of claim 15 , wherein the single quantum well comprises dissimilar II-V compound semiconductors, II-VI compound semiconductors, or Ge—Si alloys.
17. The direct detector of claim 16 , wherein the dissimilar III-V compound semiconductors comprise GaAs and AlGaAs.
18. The direct detector of claim 16 , wherein the detecting means comprises measuring the photoconductive response of the field-effect transistor.
19. The direct detector of claim 16 , wherein the detecting means comprises measuring the photovoltaic response of the field-effect transistor.
20. The direct detector of claim 16 , further comprising a back gate on the opposite side of the channel region from the grating gate.
21. The direct detector of claim 16 , further comprising a transparent front gate on the front surface above the channel region.
22. The direct detector of claim 16 , wherein the substrate comprises a thinned substrate.
23. The direct detector of claim 16 , further comprising a resistive shunt that connects the fingers of the grating gate in parallel and means for applying a shunt voltage to the resistive shunt.
24. A direct detector for terahertz radiation, comprising:
a field-effect transistor formed in a semiconductor substrate, comprising a heterostructure that provides a two-dimensional electron gas in the channel region between the source and the drain of the field-effect transistor, and a periodic grating gate comprising a plurality of fingers on a front surface above the channel region to modulate the electron density in the two-dimensional electron gas;
a resistive shunt that connects the fingers of the grating gate in parallel;
means for applying a shunt voltage to the resistive shunt; and
means for detecting an output signal from the field-effect transistor when the front surface is irradiated with terahertz radiation.
25. The direct detector of claim 24 , wherein the shunt voltage is approximately equal to the source-drain voltage.
26. The direct detector of claim 24 , wherein the heterostructure comprises one or more quantum wells.
27. The direct detector of claim 24 , wherein the detecting means comprises measuring the photoconductive response of the field-effect transistor.
28. The direct detector of claim 24 , wherein the detecting means comprises measuring the photovoltaic response of the field-effect transistor.
29. The direct detector of claim 24 , further comprising a back gate on the opposite side of the channel region from the grating gate.
30. The direct detector of claim 24 , further comprising a transparent front gate on the front surface above the channel region.
31. The direct detector of claim 24 , wherein the substrate comprises a thinned substrate.
32. A direct detector for terahertz radiation, comprising:
a field-effect transistor formed in a semiconductor substrate, comprising a heterostructure that provides a two-dimensional electron gas in the channel region between the source and the drain of the field-effect transistor, and a periodic grating gate comprising a plurality of fingers on a front surface above the channel region to modulate the electron density in the two-dimensional electron gas;
means for applying a gate voltage to the periodic grating gate; and
means for detecting a photovoltaic response of the field-effect transistor when the front surface is irradiated with terahertz radiation.
33. The direct detector of claim 32 , wherein the heterostructure comprises one or more quantum wells.
34. The direct detector of claim 33 , wherein the heterostructure comprises a single quantum well.
35. The direct detector of claim 31 , further comprising a back gate on the opposite side of the channel region from the grating gate.
36. The direct detector of claim 31 , further comprising a transparent front gate on the front surface above the channel region.
37. The direct detector of claim 31 , wherein the substrate comprises a thinned substrate.
38. A direct detector for terahertz radiation, comprising:
a field-effect transistor formed in a thin semiconductor substrate, comprising a heterostructure that provides a two-dimensional electron gas in the channel region between the source and the drain of the field-effect transistor, and a periodic grating gate comprising a plurality of fingers on a front surface above the channel region to modulate the electron density in the two-dimensional electron gas;
means for applying a gate voltage to the periodic grating gate; and
means for detecting an output signal from the field-effect transistor when the front surface is irradiated with terahertz radiation.
39. The direct detector of claim 38 , wherein the thickness of the substrate is less than 10 microns.
40. The direct detector of claim 38 , wherein the thin semiconductor substrate is suspended by a plurality of legs from an unthinned portion of the substrate.