Evaluation of the radiation tolerance of several generations of SiGe heterojunction bipolar transistors under radiation exposure

For the potential use in future high luminosity applications in high energy physics (HEP) (e.g., the large hadron collider (LHC) upgrade), we evaluated the radiation tolerance of several candidate technologies for the front-end of the readout application-specific integrated circuit (ASIC) for silicon strip detectors. The devices investigated were first, second and third-generation silicon–germanium (SiGe) heterojunction bipolar transistors (HBTs).The DC current gain as a function of collector current was measured before and after irradiation with 24 GeV protons up to fluences of 10¹⁶ p/cm² and with a ⁶⁰Co gamma source up to 100 Mrad. The analog section of an amplifier for silicon strip detectors typically has a special front transistor, chosen carefully to minimize noise and usually requiring a larger current than the other transistors, and a large number of additional transistors used in shaping sections and for signal-level discrimination. We discuss the behavior of the three generations of transistors under proton and gamma exposure, with a particular focus on issues of noise, power and radiation limitations.