Design and development of radiation-resistant and low-noise semiconductor transistors for applications in high frequency communication systems
In the frame of this work, the fundamental properties of GaN/AlGaN HEMT structures have been investigated in order to optimize their performance in high power microwave devices. A number of properties have been studied, starting from DC transport properties in bare 2DEG channels up to investigation of phase noise of a X-band HEMT-based oscillator. This work is separated into five parts. In the first part, basics of fluctuation phenomena in semiconductors are described along with a discussion of theoretical models, which are most applicable in case of investigation of noise in HEMT devices. In the second part, a study of 2DEG transport at low and moderate electric fields is performed. The gateless TLM structures have been suggested for adequate analysis of the transport effects. The observed current saturation through a TLM structure confirms that both thermal overheating and hot electron effects are influencing the current. A theoretical model has been developed for the separate investigation of both relevant effects. We performed microsecond pulse measurements that show a 60% reduction of self-heating influence compared to DC measurements, which is in agreement with our calculations. In order to remove Joule heating effects and to investigate only the fundamental transport properties, a nanosecond pulse measurement technique was developed. By applying 10-30 ns electrical pulses to AlGaN/GaN gateless heterostructures, we experimentally measured low ?eld and high ?eld transport phenomena, including the drift velocity dependence on the applied electric ?eld up to the record height of 150 kV/cm for GaN-based heterostructures. Besides the self-heating, another strong limitation of high frequency devices is low- frequency noise. In the third part, an adequate method for the low-frequency noise sources characterisation from different device regions is proposed and tested for nanoscale Group-III- nitride heterostructures. The temperature dependence of the noise revealed a characteristic transition frequency from the 1/f dependence to a stronger one. The process has activation energy of 0.4 eV, which can be related to traps in the AlGaN barrier. In the forth part of this work, transport and noise properties of AlGaN/GaN based HEMTs with gate length from 150 nm to 350 nm have been studied before and after gamma irradiation up to 1x109 Rad doses. The observed changes in the measured HEMTs characteristics were interpreted in terms of additional defects produced by gamma radiation. The noise spectra demonstrate the increasing role of leakage current after high gamma irradiation dose. The Hooge parameters, determined separately for passive and active regions of the device, show values three to five times larger after a radiation dose of 2x108 Rad (~1x10-3 for the passive region and about 5x10-4 for the active region). Still, even after the highest dose, devices can operate on an acceptable level. In general, the results confirm, that due to their high radiation hardness, GaN-based HEMTs have good prospects for stability and reliability of devices intended for application in radiation environment, particularly for space applications. In the final part of this work, the experience, gained during the investigation of transport and noise properties of the transistors, was used to optimize the parameters of the integrated circuit oscillator. As a result, a low phase noise GaN MMIC oscillator has been presented, in conjunction with the study of the low frequency noise. The oscillator has demonstrated remarkably good performance with low phase noise and high output power. The oscillator delivers an output power of 28 dBm with DC-to-RF efficiency of 16%. The measured phase noise was -105 dBc/Hz at a 100 kHz offset from 9.35 GHz carrier with only 15 MHz/V up conversion factor, which is better then the noise measured for MMIC oscillators built on GaAs HEMTs. In conclusion, the most important results were summarized, and a perspective for further work towards the development and design of radiation-resistant and low-noise semiconductor components for applications in high frequency communication systems were given.
info:eu-repo/classification/ddc/530, HEMT, Heterostruktur-Bauelement, Galliumnitrid, Aluminiumnitrid, Elektronischer Transport, Rauscheigenschaft, Physik, Wide band gap semiconductors, 1/f noise, phase noise, loop oscillators, III-Nitride heterostructures, hot electron effects
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