Effects of Mechanical Uniaxial Stress on SiGe HBT Characteristics

The impact of mechanical uniaxial stress on the characteristics of SiGe heterojunction bipolar transistors is reported. A four-point bending apparatus was used to apply a uniaxial stress in the range of -200 MPa to +200 MPa. Due to the strain-induced modulation of carrier mobility and energy bandgap, the changes in the collector current (IC), base current (IB), current gain (Β), and breakdown voltage (B VCEO) are related to the strain-polarity. The strain-polarity dependence of the collector current IC was found to oppose that of the breakdown voltage B VCEO, revealing a trade-off between uniaxial compressive and tensile stress. The response of the cutoff frequency under uniaxial stress was also investigated. Unlike the obvious changes of dc characteristics, a minor change in the maximum fT could be found for uniaxial stress levels below 200 MPa. Finally, various components of the transit time are analyzed to assess their contribution to the frequency response.