The present disclosure provides an HBT that includes (i) a semiconductor support layer; at least four wall structures side-by-side on the support layer; (iii) a semiconductor collector-material ridge structure disposed on the support layer between two adjacent wall structures of the at least four wall structures; (iv) a semiconductor base-material layer, wherein a first part of the base-material layer is disposed on a first region of the ridge structure and a second part of the base-material layer is disposed across the wall structures, wherein the base-material layer is supported by the wall structures; (v) a semiconductor emitter-material layer disposed on the first part of the base-material layer; (vi) a base contact layer disposed on the second part of the base-material layer; an emitter contact layer disposed on the emitter-material layer; and (viii) a collector contact layer disposed on a second region of the ridge structure.