Bipolar transistor structures with semiconductor film and related methods

A structure comprising:

• a subcollector on a semiconductor substrate;
• a collector on the subcollector, the collector having a first doping type;
• an intrinsic base on the collector, the intrinsic base having a second doping type;
• a semiconductor film on the intrinsic base and horizontally surrounding the intrinsic base;
• a dielectric layer on the subcollector and horizontally distal to the collector, wherein an air gap is between the collector and the dielectric layer;
• an emitter having the first doping type on a first portion of the semiconductor film, wherein the first portion of the semiconductor film includes the first doping type and the second doping type;
• and
• an extrinsic base having the second doping type on a second portion of the semiconductor film, wherein the second portion of the semiconductor film includes only the second doping type.

A method comprising:

• forming an intrinsic base on a collector, wherein the collector has a first doping type;
• forming a semiconductor film on the intrinsic base and horizontally surrounding the intrinsic base;
• forming an emitter having the first doping type on a first portion of the semiconductor film;
• and
• forming an extrinsic base having a second doping type on a second portion of the semiconductor film.