Photoconductive device

1. A semiconductor structure comprising:

a GaAs or InP substrate;

an InxGa1-xAs epitaxial layer grown on the substrate, x being greater than about 0.01 and less than about 0.53;

a wider bandgap epitaxial layer grown as a cap layer on top of the In x Ga1- x As epitaxial layer;

wherein the structure is patterned and etched down to the substrate except for a small region or mesa that remains unetched;

wherein the mesa is substantially round in shape; and

wherein the diameter of the mesa is in the range between about 10 and 100 μm.

2. The semiconductor structure of claim 1 further comprising an electrically insulating, photo-defined dielectric film deposited over the surface of the semiconductor structure except in a region, defining a window, centered over the top surface of the mesa.

3. The semiconductor structure of claim 2 wherein the photo-defined window in the dielectric film is substantially round in shape and smaller in diameter than the top diameter of the mesa.

4. The semiconductor structure of claim 3 wherein the window in the dielectric film has a diameter in the range between about 5 and 90 μm.

5. The semiconductor structure of claim 4 further comprising electrical contacts having two thin film electrodes deposited on the dielectric film and having a gap between them that is positioned such that the gap and portions of each electrode are in the window of the dielectric film and make contact with the top semiconductor structure surface of the mesa.

6. The semiconductor structure of claim 5 wherein the semiconductor surface between the two electrodes is coated with an antireflective coating.

7. The semiconductor structure of claim 5 wherein the structure generates pulsed or sinusoidally varying electrical signals when an electrical bias is applied across the two electrodes and an optical pulse or sinusoidally varying optical signal is incident in the gap and excites carriers in the photoconductive gap.

8. The semiconductor structure of claim 5 further comprising a radiating antenna electrically connected to the two electrodes, the electrical signals having subterahertz to terahertz bandwidths that radiate from the antenna.

9. The semiconductor structure of claim 5 wherein a voltage from a pulsed or sinusoidally varying electrical waveform is applied across the two electrodes and an optical pulse or sinusoidally varying optical signal is incident in the gap and excites carriers in the photoconductive gap to measure the electrical waveform.

10. The semiconductor structure of claim 9 further comprising a receiving antenna electrically connected to the two electrodes, the electrical waveform having subterahertz to terahertz bandwidths being coupled to the photoconductive gap through the receiving antenna.

11. The semiconductor structure of claim 5 wherein the substrate side is coated with an antireflective coating and light enters from the substrate side to excite carriers in the photoconductive gap.

12. The semiconductor structure of claim 1 , wherein the semiconductor structure is a photoconductive device.