2DEG Hot Electron Microbolometer For Millimeter and Sub-millimeter Wavelength Heterodyne Sensing
Development of low noise and high bandwidth detectors and receivers for millimeter and submillimeter wavelength range has been an important goal for the applications in the areas such as astronomy, biological and medical science, chemical sensing, and high resolution imaging, etc. Most of the research on the detectors in this frequency range has been focused on the superconducting hot electron bolometers (HEB), which requires complex cryogenic cooling system, and the bandwidth achieved is relatively low due to the small diffusion constant in superconducting materials. In this work, a different concept of hot electron bolometer is introduced, which makes use of a nonlinear element based on the two-dimensional electron gas (2DEG) medium at a heterojunction interface, for example formed between AlGaAs and GaAs. In this dissertation, I report the fabrication procedures and experimental results of the proposed 2DEG HEB mixer. The AlGaAs/GaAs 2DEG sample has an electron mobility of 2.3 x 10 5 cm 2 /Vs and an electron concentration about 3.6 x 10 11 cm -2 at 77K. The critical cooling mechanisms dictated by the material parameters such as electron-phonon relaxation time, diffusion constant and electron mean-free path have been clearly distinguished for different device channel lengths. A responsivity of 22 A/W is measured with a bias voltage of 50 mV. Intermediate frequency (IF) bandwidth greater than 8.1 GHz was obtained from the mixer with a 5 μm channel length, due to the fast diffusion of hot electrons into the contact pads. The interplay between optimum bias current and LO power required to achieve lowest conversion loss of 22 dB in our mixers is demonstrated. The two-band noise temperature of the mixer is estimated to be ∼ 1760K for the best device. Also presented in this dissertation are the experiment results of AlGaN/GaN 2DEG mixers, aiming at the future room temperature 2DEG mixer applications. The reason for using AlGaN/GaN instead of AlGaAs/GaAs for room temperature applications is discussed. The bandwidth obtained with a 5 ìm channel length GaN 2DEG mixer is 2 GHz. The THz direct detection measurement has been performed with a FIR gas laser as the radiation source. The highest responsivity achieved at 1.84 THz is around 0.17 mA/W, and at 2.55 THz it is lower, 0.13 mA/W. Further development should demonstrate the performance of this type of mixers at higher frequencies, and with higher bandwidth and lower conversion loss. This research shows that semiconductor 2DEG hot electron bolometer is very promising for millimeter and THz applications.