This is an ambitious project aiming to the progress of a broad range of high-frequency semiconductor devices, which will be modelled, characterized and fabricated. The objectives of the research will be to improve the energy efficiency of the devices and to enlarge their functionalities by reaching the THz frequency range. The mid-term goal is to transfer the developments made on the different technologies targeted in this project to systems with practical applications in the fields of ultra-fast communications (for both transmitters and receptors) and sensors for THz spectroscopy. Both classical architectures, such as Schottky barrier diodes, Gunn diodes and HEMT transistors, and novel structures, such as selfswitching diodes (SSDs) and gated-SSDs (G-SSDs), will be addressed, with the common feature that all of them are based on III-V semiconductors, specifically InGaAs as narrow-bandgap semiconductor (oriented to low-power, ultrahigh-frequency applications, above 300 GHz) and GaN as wide bandgap one (oriented to high-power applications below 300 GHz). Two approaches for THz emitters will be studied. Gunn diodes (on both InGaAs and GaN) and frequency-multiplied sources based on Schottky barrier diodes. In the latter case, high-power GaN diodes will be used, so that one of the main issues will be the study of selfheating effects and the optimization of the breakdown voltage. Gated Gunn diodes with the G-SSDs architecture will straightforwardly allow for the modulation of the emitted signal (with simple on-off keying), so that their application to high-speed communications can be envisaged. Moreover, they would allow for wideband tunability; thus, THz spectroscopy is other important field of potential application of the outcomes of the project. The THz detection/receptor side will be tackled by means of the classic Schottky barrier diode technology (with the novelty of the high-power capability of GaN) and with the more innovative TERAFET concept (field effect transistors, FETs, employed as THz detectors), using HEMTs and G-SSDs.

The main objective of GaNGUN is to reach for the first time a GaN-based Gunn oscillator, at the mm/sub-mm frontier, i.e. around 300 GHz. The design rules provided by the simulations performed at USAL will be used for the growth of optimized epilayers with doped GaN as active region at NTU, and the device processing at both IEMN and NTU. MC simulations may become necessary in order to explain possible discrepancies with the expected results or to propose further improvements of the technology that will be implemented in a second run of epilayer growth and device fabrication. Two different device fabrication processes (mainly with different isolation techniques) will be used at the clean rooms of both NTU and IEMN. Detectors based on GaN SSDs will be also fabricated, aiming to the fabrication of a compact room temperature sub-THz communication system. In the last phase of the project, GaN SSDs will be integrated in micromechanical supports and waveguides to implement demonstrators for both emitters and receivers. The final objectives of this project will be the experimental characterization of the fabricated devices and their benchmarking against state-of-the-art technologies.

The millimeter and submillimeter frequency range (reaching THz) is of huge potential in a wide variety of fields related to information and communication technologies. Self-heating and temperature variation of the electrical characteristics of electronic devices operating in this frequency range are determinant in their performance in many of their applications, mainly when, due to their small size, they manage high power densities.

This project aims at studying the electrothermal behavior of electronic nanodevices based on different technologies, able to operate in the sub-THz band, with the final objective of optimizing their performance as emitters or detectors in communication systems. Different types of advanced devices will be analyzed, some with novel architectures, like asymmetric semiconductor nanodiodes (ungated and gated, called SSDs and G-SSDs, respectively) and other with traditional designs, like Schottky diodes and heterojunction field-effect transistors (HESMTs); some fabricated with high-mobility semiconductors, like InGaAs (for low-power, very high-frequency operation), and others with wide band-gap semiconductors, like GaN (for high power, lower-frequency operation).

The strength of our approach is the combination of complementary activities in the fields of fabrication, characterization and simulation by means of the Monte Carlo technique. Initially, specific test structures will be fabricated and characterized as a function of temperature in order to obtain basic properties of the semiconductors (mobility or velocity-field characteristic), of the wafers (doping profile or sheet resistance) or of the technology (ohmic contact resistance), that will be used to fine-tune the electrothermal Monte Carlo simulator. Exhaustive measurement campaigns of the different devices will be performed as a function of temperature (both in DC and high frequency). Such measurements will allow the improvement of the models and guarantee the predictive capability or the simulator, that will be finally used in the derivation of design rules for the performance optimization of the different devices.