Lateral bipolar transistor structure with base layer of varying horizontal width and methods to form same

A lateral bipolar transistor structure comprising:

• a semiconductor layer on an insulator;
• an emitter/collector (E/C) layer on the semiconductor layer, the E/C layer having a first doping type;
• a first base layer on the semiconductor layer and horizontally adjacent the E/C layer, the first base layer having a second doping type opposite the first doping type and including:
• an upper portion having a first horizontal width from the E/C layer,
• a lower portion under the upper portion and having a second horizontal width from the E/C layer less than the first horizontal width, and
• a sidewall adjacent the E/C layer including a concave surface;
• a spacer on the upper portion of the first base layer and adjacent the E/C layer, wherein an upper portion of the E/C layer is adjacent the spacer and a lower portion of the E/C layer is below the spacer;
• and
• a second base layer on the first base layer and adjacent the spacer, the second base layer having the first doping type, wherein the upper portion of the first base layer separates a lower surface of the second base layer from the E/C layer.

A method of forming a lateral bipolar transistor structure, the method comprising:

• forming a semiconductor layer on an insulator;
• forming an emitter/collector (E/C) layer on the semiconductor layer, the E/C layer having a first doping type;
• forming a first base layer on the semiconductor layer and horizontally adjacent the E/C layer, the first base layer having a second doping type opposite the first doping type and the first base layer including:
• an upper portion having a first horizontal width from the E/C layer,
• a lower portion under the upper portion and having a second horizontal width from the E/C layer less than the first horizontal width, and
• a sidewall adjacent the E/C layer including a concave surface, and
• forming a second base layer on the first base layer and adjacent a spacer, the second base layer having the first doping type, wherein the upper portion of the first base layer separates a lower surface of the second base layer from the E/C layer.