Heterojunction Bipolar Transistors
Heterojunction Bipolar Transistors (HBTs) are an advanced development of the Bipolar Junction Transistors (BJTs). The basic principles of operation of bipolar transistors are explained in detail elsewhere, e.g. in [305, 496, 570].
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5.1 General Considerations Heterojunction Bipolar Transistors (HBTs) are an advanced development of the Bipolar Junction Transistors (BJTs). The basic principles of operation of bipolar transistors are explained in detail elsewhere, e.g. in [305, 496, 570]. Typically HBTs employ a heterojunction (a junction between two different semiconductor materials) for the emitter-base junction. Using a wider bandgap emitter material or lower bandgap base material introduces an energy barrier which prevents the injection of minority carriers in the emitter while preserving high injection of majority carriers from the emitter into the base. Thus a high current gain can also be maintained at a higher base doping concentration, which means a reduced base resistance and improved high-frequency performance [69]. Some devices, which are often referred to as Double Heterojunction Bipolar Transistors (DHBTs), employ a second heterojunction for the base-collector junction. For Si HBTs this is inevitable because of the lack of an emitter material with a wider bandgap than that of Si. Therefore a lower bandgap material (SiGe) is used in the base, while Si is used in the emitter and the collector. In III-V HBTs the second heterojunction is usually introduced on purpose to improve the breakdown performance, as demonstrated later in this chapter. The heterojunctions can be either graded or abrupt, depending on whether a gradual change of the material composition or abrupt change of the material is introduced. The former is not a 'true' heterojunction and is governed by diffusion processes. In the latter, both the current flow and the carrier heat flow are dominated by tunneling-supported thermionic field-emission processes. This chapter provides industrially relevant examples of AlGaAs/GaAs, InGaP / GaAs, and InAlAs/InGaAs/lnP SHBTs, as well as Si/SiGe/Si and InP /InGaAs/ InP DHBTs. All HBTs are fabricated as npn-structures because of the better high-frequency performance and thus wider popularity. The simulations have been performed mostly with the device simulator MINIMOS-NT indexMINIMOSNT and have been compared to available measurements or, in some cases, to results from a commercial device simulator [227] . Benchmark structures with exact layer specifications are given in Appendix A and can be used for comparison and calibration of different device simulators.
V. Palankovski et al., Analysis and Simulation of Heterostructure Devices © Springer-Verlag Wien 2004
5.2 SiGe HBTs
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5.2 SiGe HBTs SiGe HBTs are currently undergoing a rapid development challenging the highspeed domain, which has been restricted to III-V-based semiconductor devices so far. The devices employ strained SiGe layers in the base to achieve better transport properties in the base and thus better performance. 5.2.1 Device Fabrication There are various techniques for epitaxial growth of strained SiGe layers. The very first SiGe HBT [229] was produced by Molecular Beam Epitaxy (MBE). This technique allows for very accurate control of layer thicknesses down to a few
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