An investigation of silicon heterojunction bipolar transistors

Shafi, Zia Alan (1992) An investigation of silicon heterojunction bipolar transistors. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

An important means of improving the high-speed performance of silicon bipolar circuits, is to scale the transistor dimensions in the vertical direction. For conventional silicon bipolar transistor technology, the extent by which vertical scaling can be practically achieved is limited by high base resistance, high emitter/base capacitance, forward biased tunnelling, and punch-through. A novel transistor structure which features a thin, high doped base, and a low doped emitter, is proposed to overcome the vertical scaling limitations of conventional transistors. The novel structure is termed the pseudo-heterojunction bipolar transistor. In addition, a true heterojunction bipolar transistor implemented in silicon is identified as a means for further improving the performance of high-speed silicon circuits. In this work, both transistor structures are theoretically and experimentally investigated. An investigation of the expected high-speed performance of ECL circuits incorporating pseudo-heterojunction and Si/Si_1-xGe_x heterojunction bipolar transistors is undertaken, and considerably improved performance predicted for both, compared with circuits incorporating conventional bipolar transistor structures. ECL propagation delays of 11.6, 7.2, and 6.1 ps, using 1.0, 0.5 and 0.2 μm line-width geometries respectively, are predicted for circuits incorporating Si/Si_1-xGe_x heterojunction bipolar transistors which have 0.02 μm wide bases doped to 10²⁰ cm^-3, and emitters doped to 10¹⁸ cm^-3. These figures represent at least a factor of two improvement over predictions for conventional silicon homojunction technology. The predicted results for the pseudo-heterojunction circuits lie between those predicted for Si/Si_1-xGe_x heterojunction, and conventional homojunction circuits. A process is developed to fabricate pseudo-heterojunction bipolar transistors. The completed devices exhibit ideal collector and base current characteristics over five orders of magnitude. Detailed electrical measurement and device modelling results are presented to determine the accuracy of the most widely accepted heavy doping models, particularly applied to heavily doped p-type silicon, with concentrations in excess of 10¹⁹ cm^-3. A process is developed to fabricate Si/Si_1-xGe_x heterojunction bipolar transistors. The completed devices exhibit ideal collector current characteristics over five orders of magnitude. Collector current enhancement equal to that predicted theoretically is demonstrated. However, the current gain is compromised by an increase in base current. Detailed electrical measurements, device modelling, and material analysis demonstrate that the dominant base current mechanisms are recombination in the emitter/base depletion region for devices which have strain relaxed, meta-stable Si_1-xGe_x base layers. In contrast, recombination in the neutral base is shown to be the dominant base current mechanism for devices with strained, stable Si_1-xGe_x base layers. An electron lifetime of 63 ps is fitted for the strained Si_1-x base, which is a factor of 15x lower than for bulk silicon.