The evolution of the microelectronic bipolar junction transistor

In this review paper, the evolution of the microelectronic bipolar transistor is tracked. Since its invention in 1947, the BJT has undergone vital enhancement of its electrical performance thanks to intensive research efforts through the years. The main task was to increase the speed of the transistor while keeping its current gain high enough for satisfactory circuit performance. This has been achieved by transistor scaling. The lateral dimensions shrunk thanks to the advancement in lithography and new isolation techniques, especially trench isolation, as well as development of self-aligned structures. The vertical dimensions shrunk by the introduction of controllable doping technologies such as ion implantation and low temperature epitaxy. Further scaling could be possible by using SiGe-based and AlGaAs/GaAs HBTs. Bipolar transistors with an f_T of greater than 140 GHz are routinely fabricated now. Especially GaAs HBTs can be further scaled to achieve more speed. These high speed transistors are required for main-frame computers and modem communication circuits.