ON THE OPTIMIZATION OF SIGE-BASE BIPOLAR-TRANSISTORS

Advanced epitaxial growth of strained SiGe into a Si substrate enhance s the freedom for designing high speed bipolar transistors. Devices ca n be designed by altering the Ge percentage, a procedure known as band gap engineering. An optimization study on NPN SiGe-base bipolar transi stors has been performed using computer simulations focusing on the ef fect of the Ge profile on the electrical characteristics. In this stud y it is shown that the base Gummel number is of major importance on th e maximum cutoff frequency and the Ge-grading itself, which induces a quasielectric field, is of minor importance. Because of the outdiffusi on of the boron dope in the base and the relatively thin critical laye r thickness of approximately 600 Angstrom it appears that a box-like G e profile with the leading edge approximately in the middle of the bas e is optimal. The predicted maximum cutoff frequency is 45 GHz, a shee t resistance of 8.5 k Ohm/square and current gain of 80. The optimized device was fabricated and measurements were performed showing good ag reement with the simulations.