Bipolar transistor and preparation method thereof

The invention provides a bipolar transistor and a preparation method thereof. The bipolar transistor comprises a substrate, an epitaxial layer arranged on the substrate, a base region arranged in the epitaxial layer and located at the top of the epitaxial layer, an emitter region arranged in the base region and located at the top of the base region, and an emitter region and a collector region which are arranged in the epitaxial layer and surround the base region. A bipolar transistor structure formed by an annular collector field plate surrounding the base region and the emitter junction forms an annular electric field when being powered on, and the annular electric field can enable minority carriers injected into the base region by the emitter junction to be collected by a collector junction along transmission paths in multiple directions, so that the carrier concentration on the current transmission path is reduced. The diffusion of minority carriers in the base region to the interface position is weakened