Structure with spacer adjacent collector and related methods

A structure comprising:

• a bipolar transistor (BT) including a subcollector, a collector on the subcollector, an intrinsic base on the collector, an extrinsic base on a first portion of the intrinsic base, and an emitter on a second portion of the intrinsic base;
• and
• an isolation stack adjacent the BT, the isolation stack including:
• a first isolation layer on the subcollector, wherein a first air gap is between the first isolation layer and the collector, and wherein a portion of the first air gap is above a portion of the collector;
• a second isolation layer on the first isolation layer and adjacent the intrinsic base of the BT;
• and
• a third isolation layer on the second isolation layer, vertically between the second isolation layer and the extrinsic base of the BT, wherein a second air gap is adjacent the third isolation layer and below the extrinsic base, wherein the second isolation layer defines a physical boundary between the first air gap and the second air gap.

A method comprising:

• forming a first isolation layer on a subcollector, wherein a first air gap is between the first isolation layer and a collector of a bipolar transistor (BT);
• forming a second isolation layer on the first isolation layer and adjacent an intrinsic base of the BT;
• and
• forming a third isolation layer on the second isolation layer, vertically between the second isolation layer and an extrinsic base of the BT, wherein a second air gap is adjacent the third isolation layer and below the extrinsic base, wherein forming the second isolation layer defines a physical boundary between the first air gap and the second air gap.