Recent developments in the physics and technology of III-V compound heterojunction bipolar transistors (HBTs) are reviewed. The technologies discussed are AlGaAs/GaAs, GaInP/GaAs, InP/InGaAs, and AlInAs/InGaAs based heterostructures. WIth current gain cut off frequencies over 100 GHz and maximum oscillation frequencies of about 200 GHz, the III-V compound semiconductor based HBTs have advanced to the point of commercialization. These developments also had the fortuitous effect of providing impetus and theoretical base to advance Si based HBT technologies, such as SiGe HBTs, to advance also. Recent SiGe HBTs, taking advantage of advanced Si processing technologies, have also recorded performances in excess of 100 GHz with applications envisioned in high speed analog-digital converters. While there remain some voids in the fundamental understanding of HBTs, the state-of-the-art of the GaAs HBT technology, concerning reproducibility and reliability, is at a point where problems related to production are at the forefront. The next few years are going to prove interesting with each technology recording improved performance.
