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Title: Silicon heterojunction bipolar transistors with wide band-gap amorphous semiconductor emitters
Author: Garner, D. M.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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Firstly, the motivation behind the silicon HBT is discussed, followed by an investigation of tetrahedral amorphous carbon (ta-C) as a possible heteroemitter material. Silicon HBTs with n-type ta-C as a heteroemitter in npn transistors were studied first. Transistor action occurred, but the current gain was only 10-6. This was found to be due to the low electron affinity of ta-C creating a barrier to electron injection into the base, opposite to how an HBT should behave. The low electron affinity of ta-C motivated the fabrication of pnp transistors with p-type ta-C as the heteroemitter. These performed better, having a current gain approaching unity. The lack of thorough investigation of the silicon HBT with an amorphous silicon (a-Si:H) heteroemitter prompted an investigation into the static and dynamic characteristics of this device. Numerical simulation using device modelling software such as MEDICI is well-established for crystalline semiconductor materials but less so for amorphous materials. Therefore a physics-based model for a a-Si:H was incorporated into MEDICI and used to model the a-Si: emitter HBT under a range of collector and base dopings. The main observation from this work was that the low drift mobility of a-Si:H was a major impediment on its frequency response, limiting it to only 2.6GHz. The heteroemitter material properties were then varied to study their effect on transistor dynamic and static characteristics. The requirements on heteroemitter material parameters were found which produced a silicon HBT with superior performance to a silicon homojunction bipolar transistor. Silicon HBTs with a-Si:H emitters were fabricated, and the experimental characteristics compared well with the stimulations, verifying the a-Si:H model and the use of device simulation software to calculate the characteristics of silicon HBTs with amorphous emitters. Finally, a new 'pseudo'-HBT (PHBT) structure which has an n-type doped crystalline silicon emitter region between the heteroemitter material and the p-type base was elucidated and studied. This was found to increase the maximum cutoff frequency using an a-Si:H as the heteroemitter material from 2.8GHz to 6.4GHz.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available