HBT

HBT is an acronym that may refer to several concepts in science and engineering. Two widely cited meanings are the Hanbury Brown–Twiss effect in quantum optics and astronomy, and the heterojunction bipolar transistor in electronics.

The Hanbury Brown–Twiss (HBT) effect describes correlations between intensity fluctuations detected at two separate photodetectors. Proposed by Robert Hanbury Brown and Richard Q. Twiss in 1956, it demonstrates photon bunching for chaotic light and enables intensity interferometry. In astronomy, HBT interferometry was used to measure the angular diameter of stars, exploiting the fact that the second-order coherence function g2(τ) reveals information about source size. In high-energy and nuclear physics, HBT techniques are extended to two-particle correlations of identical bosons, providing insights into the space-time structure of the particle-emitting source created in relativistic collisions.

The heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor that uses a heterojunction—typically at the emitter-base or base-collector junction—to improve performance. By combining materials with different band gaps, HBTs achieve higher speed, greater gain at high frequencies, and better isolation of base charge, making them suitable for RF and microwave amplification and high-speed digital circuits. Common implementations use III–V semiconductors such as GaAs/AlGaAs or InP/InGaAs.