From 3D thermal simulation of HBT devices to their thermal model integration into circuit simulators via Ritz vectors reduction technique

As the size of the semiconductor devices is getting smaller and as the power density is getting higher with advanced technology, self-heating effects in power devices are becoming important. Electrothermal models of whole power devices are necessary for an accurate analysis of their performances. This paper deals with the integration of a reduced thermal model based on a three-dimensional finite element (FE) thermal simulation into circuit simulator for an accurate prediction of the electrothermal behavior of power devices. The reduced thermal model based on the Ritz vectors approach can be easily implemented in any kind of circuit simulator because it is described by a SPICE format subcircuit. The model has been successfully experimented with the Advanced Design Simulator (ADS). Electrical based thermal measurements of transient temperature response have successfully validated the approach. Coupled to a distributed electrical model, this electrothermal model has been used in order to simulate the instability phenomenon known as "the current collapse phenomenon" which can occur in multi-finger heterojunction bipolar transistors (HBTs).