HIGH-FREQUENCY CHARACTERISTICS OF LOW-TEMPERATURE PSEUDOHETEROJUNCTION BIPOLAR-TRANSISTORS

The high-frequency characteristics of a pseudoheterojunction bipolar transistor (pseudo-HBT), which operates like a heterojunction bipolar transistor despite a metallurgical homojunction utilizing bandbgap narrowing effect, are analyzed both theoretically and experimentally. Several design issues are discussed on achieving a high cutoff frequency at low temperatures. They include 1) a low-impurity-concentration graft base to reduce useless electron injection into the graft base, 2) an abrupt base profile to obtain a large effective-bandgap difference between the base and the emitter, and 3) an inversely graded base profile, in which the impurity concentration increases from the emitter side to the collector side, to effectively reduce the base transit time. The fabricated pseudo-HBT with a low-concentration graft base shows a higher cutoff frequency below 100 K than at room temperature. The low-concentration graft base reduces the useless electron injection into the graft base, resulting in a higher cutoff frequency. These design issues are also appropriate for conventional bipolar transistors operating at low temperatures.