A bipolar transistor includes a base structure in a hollow space on a single crystal silicon collector region defined by a silicon oxide layer, and the base structure has an extrinsic base provided around a single crystal silicon emitter region and an intrinsic base layer of single crystal...http://www.google.de/patents/US5698890?utm_source=gb-gplus-sharePatent US5698890 - Semiconductor device having bipolar transistor free from leakage current across thin base region 
