Details

Abstrakt in Englisch

Silicon-Germanium (SiGe) heterojunction bipolar transistors (HBTs) are excellent candidates for radio frequency (RF) high power density applications due to their speed and drive capabilities. However, RF performance improvement of state-of-the-art high-speed SiGe HBTs inherently results in decreasing static breakdown voltages. This development raises concerns regarding safe operating limits and long-term reliability. In this context, the open-base collector-emitter breakdown voltage BV CEO is a static figure of merit often specified by foundries in process design kits (PDKs) as an upper output voltage limit. This thesis addresses the limits of long-term RF operation and the characterization, analysis, and modeling of SiGe HBT degradation.

An automated load-pull setup is used to run RF stress tests under varying load conditions, and to periodically characterize DC and admittance parameters. Long-term RF stress tests at elevated RF output power density, implying large RF voltage and current density swings, demonstrate the reliable operability of SiGe HBTs far beyond conventional DC operating limits. Furthermore, the industry standard HBT compact model HICUM/L2 is shown to be an excellent vehicle for the exploration of extended operating regions of SiGe HBTs. RF operating limits are investigated. The physical cause of device degradation resulting from RF operation beyond these limits is analyzed. The understanding of extended RF operating limits is used and verified by the design, characterization, and stress tests of single-stage RF bipolar power amplifiers.

SiGe HBT degradation, which goes beyond a conventional IB increase, causes an increase of the collector current I C . This type of degradation is measurable only in RF operation at extremely elevated RF output power density. DC stress tests reveal that similar degradation does not result from strongly reverse-biased p-n junctions at low current densities, or elevated DC power densities at varying collector current densities. Degradation caused by RF stress is analyzed and modeled by means of the transfer saturation current. This enables the proposal of a modeling approach for the estimation of SiGe HBT degradation during RF operation. Numerical device simulation suggests that the IC increase can be attributed to relaxation of compressive lattice strain in the base and its impact on the effective densities of states.

A comparative RF reliability study comprising a state-of-the-art SiGe HBT, an InP HBT and MOSFET technology is discussed. It reveals in particular that the BV CEO of SiGe HBTs, which is lower than that of InP HBTs with similar RF performance, does not result in reduced RF reliability.