4.2 K Operation of InAlAs/InGaAs Heterojunction Bipolar Transistor

Low temperature operation of heterojunction bipolar transistor (HBT) is a key requirement for a HBT large scaled integrated circuit with low power dissipation and high speed switching, according to HBT scaling principle. Graded and uniform base InAlAs/InGaAs HBTs were fabricated and characteristics were measured from 300 K down to 4.2 K. Excellent HBT operations at 4.2 K were obtained for the first time, in both graded and uniform gap base structure. Comparing current gain of 1500 Å graded (7 kV/cm) base HBT with uniform base HBT, the former is 1.5 times higher than the latter at 300 K, and enhanced 4.5 times at 4.2 K. In the large built-in field case (21 kV/cm) of 500 Å base, current gain as high as 180 was obtained at 4.2 K. Graded base structure is necessary for high speed HBT operation at low temperatures.