A tunnel diode collector contact to InP based PNP heterojunction bipolar transistors (HBTs) is suggested and demonstrated. The additional heavily doped n-type contact layer replaces the thick p-type contact layer required in conventional structures. The thermal and electrical properties of the collector contact layer thus become similar to those of NPN HBTs. A secondary ion mass spectroscopy study explores the maximum tin doping level that can be obtained in the base. Finally, the temperature dependence of the current gain is presented and interpreted. (C) 2002 Elsevier Science Ltd. All rights reserved.