Study Of Radiation Effects Of GaN-based Semiconductor Materials And HEMTs

Rapid developments of the broadband communication infrastructure haveunderscored the need for high efficiency devices that can operate reliably at highfrequencies and handle high power. Electronic devices based on the III-nitride materialssystem, like high electron mobility transistors (HEMTs), they have emerged aspromising candidates for high power applications at microwave frequencies. Moreover,GaN-based materials and devices are more radiation tolerant than GaAs and Si-basedbecause of higher displacement threshold energy. To assess the device behavior in spaceenvironment, it becomes extremely important to understand the effects of radiation andthe mechanisms underlying device degradation after exposure to radiation.In the dissertation, the radiation effects on GaN-based materials and AlGaN/GaNHEMTs are systematically studied, both experimentally and theoretically. The principalfocus of this work is on the characterization of the radiation response and degradationmechanisms, and the establishment of physical and mathematical models. The mainresults of the study are as follows:1. The total dose effects of60Co gamma radiation on static and high-frequencycharacteristics of AlGaN/GaN HEMTs are investigated experimentally in detail. Inorder to simulate the actual working status of devices in space, the devices appliedvoltages were tested on line. The results show that, the anti-irradiation property ofAlGaN/GaN HEMTs depends on the sensitivity of surface states to gamma ray.2. The effects of proton irradiation on the AlGaN/GaN heterostructures are studiedin detail, which provides a basis for the study of HEMTs. The materials before and afterproton irradiation are characterized exactly in terms of the crystal quality, electrical andoptical properties. It is observed that the concentration and mobility of2DEG decreaseafter irradiation. The Raman spectra and XRD indicate that the strain, carrierconcentration and dislocations are not affected by the proton. PL tests show that theintensity of yellow band increases with the proton fluence. Therefore, Ga vacancies orGa vacancies-related complex defects introduced by the proton radiation might causethe degradation of optical property of the AlGaN/GaN heterostructures.3. The effects of high energy proton on AlGaN/GaN HEMTs are researchedexperimentally. The DC and RF characteristics of devices were measured after theproton irradiation with the different energy and dose. It is found that only high dose ofthe proton can lead to the degradation of the parameters. Compared with the energy of 10MeV, the degradations of devices induced by3MeV proton irradiation are much moreserious, which is because the non ionizing energy loss caused by the proton with lowerenergy is much larger. Through device simulation soft and mathematical model, thedegradation mechanism of proton radiation effect is discussed in detail. Adding thesimple vacancies into Silvaco trapping model, we discover that being an acceptor-likedefect, Ga vacancies play an important part in worsening the device performances, andsimulation results match well with the trends of experimental data. The influences of theacceptor-like defects on2DEG are analyzed by introducing the charge control model. Alcomposition of the AlGaN layer, conduction band discontinuous and the dopingconcentration of AlGaN layer do not contribute to the2DEG concentration degradation.It is concluded that the Ga vacancies or Ga vacancies-related complex defectsintroduced in GaN layer maybe the primary reason for the degradation of AlGaN/GaNHEMTs performance.4. The neutron irradiation effects on the AlGaN/GaN heterostructures are studied indetail. The materials before and after neutron irradiation are characterized in terms ofthe crystal quality, electrical and optical properties.5. The neutron radiation effects on AlGaN/GaN HEMTs are studied. It is observedthat drain saturation current and transconductance decrease dramatically, and thresholdvoltage shifts slightly to the forward direction after neutron irradiation. The mobilitydegradation in the channel is mainly responsible for the decrease of deviceperformances. With the same radiation dose, the parameter degradation induced by theneutron is weaker than that by the proton.6. The HVPE GaN before and after proton and neutron irradiation arecharacterized exactly in terms of the resurface topography, crystal quality and opticalproperties. PL tests show that YL/BE increases slightly after proton irradiation, whileBL/BE decreases obviously. In comparison with the proton irradiation, HVPE GaNshows more tolerant of the neutron irradiation.