ABSTRACT
This chapter reports improved electrical properties in molecular beam epitaxy (MBE)-grown Ge layers for collector-top n-GaAs/p-Ge/n-Ge heterojunction bipolar transistors (c-top GaAs/Ge HBTs). Both the electron-beam-gun (E-gun) and the Knudsen-cell (K-cell) were tested for use in Ge evaporation. E-gun grown Ge pn-diodes had the problem of large leakage current in reverse bias conditions. Leakage current was sufficiently low with K-cell grown diodes, and Ga diffusion into the Ge layer from the GaAs layer was reduced by a factor of 2 as compared to E-gun growth. n-GaAs/p-Ge diodes produced by E-gun evaporation exhibited as nearly ideal I-V characteristics as did those produced with K-cell evaporation. HBTs fabricated on K-cell grown wafers had a base sheet resistance of 90 ohms/square, and a current gain was as high as 80. Wafers grown by K-cell are unaffected by ionized Ge atoms. In order to avoid the decomposition of PBN crucible, however, the growth rate is limited to 0.2 um/hour.
