Development of a Heterojunction Structure Detector in the Submillimeter Wave Region

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A Heterojunction Bipolar Transistor (HBT) structure detector has been studied as a new type detector/mixer in the millimeter (MM) and submillimeter (SMM) wave region. The detector may have much higher sensitivity compared to the Schottky Barrier Diode in the frequency range from 100GHz to THz.Primarily, we have studied the HBT detector as a theoretical simulation and a model experiment using a Si bipolar transistor in the microwave region. The result has suggested a feasibility of special design of the emitter, base, and collector structure for MM and SMM wave detector/mixers.We designed and fabricated the emitter-base structure using AlAs/GaAs heterojunction first, and have obtained good I-V characteristics successfully. Next, we started to design and fabricate base-collector structure. However, selective wet etching process of collector was not easy to obtain precise etching profile, and we could not observe good collector current versus collector-emitter voltage influenced by large leakage current. As a new fabrication process, an ECR plasma etching has been introduced to improve it. The leakage current was reduced a little, but it was not enough to operate as a transistor. Another problem of device lifetime appeared when the collector current applied up to several tens mA.A reduced area base structure could be necessary taking into account of the base current density to improve such problems. Now we are going on redesigning the base structure and fabrication of the HBT detector device.