GaN for THz Sources

In this work we investigate two different approaches to generate THz radiation by the use of the unique electrical and thermal properties of GaN. One method is heterodyne photomixing, a compact and inexpensive approach to generate continuous electromagnetic radiation in the terahertz range, with tuneable frequency. It uses two lasers with slightly different wavelengths that illuminate an ultrafast photoconductor. The interference of both laser beams generates a beat frequency of the illumination intensity in the terahertz range. One drawback of the conventionally used LT GaAs as ultrafast photoconductor material is the relatively low THz power in the nW to ?W range. The aim of our work is to increase the output power by replacing the LT GaAs with GaN. This semiconductor is rather known as basic material for blue LEDs and lasers, but it has also remarkable electrical and thermal properties that allow higher laser power and bias voltage. A more conventional, electronic approach to generate THz radiation consists in the fabrication of an oscillator circuit based on ultrafast transistors, e.g. Hetero Field Effect Transistors based on InGaAs. These circuits can be designed up to about 100 GHz oscillation frequency. The THz region is achieved by frequency multipliers, e.g. realized by very small-sized Schottky diodes. However, each multiplier stage considerable reduces the output power. In this field we investigate GaN based transistor devices to profit from the much better power performance of this material, compared to classical semiconductors. Devices in this material system are usually used for high power applications at moderate frequencies, but the very high electron saturation velocity of GaN allows the application above 100 GHz as well.