| Summary: | 碩士 === 國立中央大學 === 電機工程研究所 === 99 === Recently a new type of heterojunction bipolar transistors (HBTs) with InGaAsSb base was proposed by our group. This novel transistor exhibited low turn-on voltage, high collector current density, high average electron velocity, low specific contact resistivity, and high current gain. In order to elevate the operation frequency of HBTs, the dimension of emitter must be scaled down to the sub-micron level to reduce the parasitic delays. In this work, efforts are focused on developing sub-micron InP-based HBTs.
InGaAs/InP HBTs with a 0.8x9 um2 emitter are fabricated by e-beam lithography. The highest current gain cutoff frequency (fT) is 290 GHz, which is superior than the 198 GHz observed on the conventional devices with an emitter of 1x10 um2. Devices with a base thickness of 40 nm and collector thickness of 200 nm exhibit a base-collector capacitance as low as 7.9 fF as the emitter dimension is scaled down to 0.4x9 um2. Meanwhile its fT and power gain cutoff frequency (fMAX) are 272 and 176 GHz, which are 1.424 and 1.558 times higher than those of the 1x10 um2 device, respectively.
In the course of this study, many difficulties in fabricating sub-micron InGaAsSb base DHBT are encountered and resolved. Besides, to further reduce parasitic delays, some more processes, such as 0.27 um-emitter metal, hybrid dry/wet etch for emitter, low emitter and base resistance, are developed.
As device size is scaled down to deep sub-micron level, surface recombination current increases and device performance degrades. Hence, this work is also involved in the investigation of emitter size effect (ESE). Compared to InGaAs base DHBT, the degradation of current gain is less significant for InGaAsSb base DHBT as device is scaled down. The normalized periphery surface recombination current density (KB,surf) for the InP/InGaAsSb DHBTs fabricated in this work is one of the lowest values among the reported InP-based DHBTs. It means that complicated Emitter/Base junction structures, growth processes and additional device process steps are no longer necessary for the HBTs with an InGaAsSb base. Furthermore, KB,surf as low as 7.37x10^-6 uA/um at Jc = 0.1 A/cm2 has been obtained as the Sb-content in the InGaAsSb base is increased to 28 %.
The results obtained in this work provide critical guidelines and routes for achieving THz InGaAsSb base HBTs.
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