SiGe HBT CML Ring Oscillator With 2.3-ps Gate Delay at Cryogenic Temperatures

We present a measured current-mode logic ring oscillator gate delay of 2.3 ps, a record for digital circuits in silicon-based technologies. This result was achieved in a silicon–germanium (SiGe) heterojunction bipolar transistor (HBT) technology operating at 25 K. In addition to higher cutoff frequency and lower collector-base capacitance, lower base resistance is also responsible for the improved switching speed at cryogenic temperatures. The self-heating characteristics of these SiGe HBT circuits are also investigated across temperatures.