High Speed Performance of Si Homo-and Si/Si1-xGex Heterojunction Bipolar Transistors
One intention of our investigations was to compare devices with base profiles resulting from different types of base formation. The first type of base profiles used in the simulations (type I) is fabricated conventionally by ion implantation , the second by epitaxial growth of Si (type II). An epitaxially grown base offers the possibility of profiles which show a significantly increased gradient of the base dopant concentration towards the collector. This is the feature which we are mainly interested in. In terms of numbers this means 17 nm/decade in the case of an epitaxial base (even 10 nm/dec are possible) compared to a minimum of 40 nm/dec with an implanted one, as seen in Fig. 1.
A larger collector-side gradient of the base profile results in a thinner base necessary to reach a certain doping level starting from a given level at the collector-base junction, where we assumed a figure of 1.8.1017 cm-3 for all our devices. A thinner base reduces base transit time. The grading also causes an accelerating electric field in the quasi-neutral base, which further reduces base transit time.
In fixing the value of the base sheet resistance, the base transit time τB can be reduced by raising the doping level in the base and simultaneously shrinking its width. This is explained by the fact that the base transit time depends much more strongly on the base
KeywordsDoping Level Bipolar Transistor Heterojunction Bipolar Transistor Base Profile High Speed Performance
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