Base doping effects and design of silicon/silicon-germanium/silicon heterojunction bipolar transistors

Recent advances in high speed performance of {dollar}rm Si/Sisb{lcub}1-x{rcub}Gesb{lcub}x{rcub}/Si{dollar} heterojunction bipolar transistors (HBT's) and the possibility of their integration into standard silicon bipolar technology have been the focus of attention among Si-based heterojunction devices. This thesis focuses on {dollar}rm Si/Sisb{lcub}1-x{rcub}Gesb{lcub}x{rcub}/Si{dollar} HBT's, specifically issues related to process integration, the design of these devices and empirical DC modeling.; The devices in this work were grown by Rapid Thermal Chemical Vapor Deposition (RTCVD). The quality of epitaxial material and interfaces was studied in a wide pressure range by x-ray reflectivity (XRR), photoluminescence, electrical performance of p-type resonant tunneling diodes, and x-ray diffraction (XRD) of a superlattice. An upper limit to interface roughness of below 5A is established by XRR and XRD.; Apart from high gain, low noise and high output resistance, {dollar}rm Si/Sisb{lcub}1-x{rcub}Gesb{lcub}x{rcub}/Si{dollar} HBT's offer low intrinsic device delays (high f{dollar}sb{lcub}rm T{rcub}{dollar}) due to germanium (bandgap) grading in the narrow epitaxial base and low parasitic base resistances due to heavy base doping, essential for high speed circuit performance. When integrated into Si technology, processing needs to be adjusted to reduced thermal cycles (below 800{dollar}spcirc{dollar}C) to prevent strain relaxation and to minimize base dopant diffusion. A heavily doped base in a bipolar transistor can lead to a p{dollar}sp+{dollar}-n{dollar}sp+{dollar} base-emitter junction. An upper limit to the doping on the lighter doped side of the junction of {dollar}5times 10sp{lcub}18{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar} is established before the onset of significant parasitic tunneling current. Hall and drift lateral hole mobilities are measured in a wide range of base p-type dopings and germanium concentrations. The first empirical model for effective bandgap narrowing for minority carrier transport in the p-{dollar}rm Sisb{lcub}1-x{rcub}Gesb{lcub}x{rcub}{dollar} base over a wide range of base dopings and Ge concentrations, extracted from the room temperature collector current measurements, is presented. The DC design trade-off between the base sheet resistance and gain is modeled. Minority carrier diffusion length is measured for the first time in p-type {dollar}rm Si{lcub}1-x{rcub}Gesb{lcub}x{rcub}{dollar} as a function of doping.; Finally, a new vertical transport device in the Si-based material system, a symmetric electron resonant tunneling diode, is demonstrated for the first time. The anomalous temperature behaviour of the lowest bias resonance is explained by a phonon-absorption-assisted model.