the Optimization of e-Base Bipolar Transistors

Advanced epitaxial growth of strained SiGe into a Si substrate enhances the freedom for designing high speed bipolar transistors. Devices can be designed by altering the Ge percentage, a procedure known as bandgap engineering. An optimization study on NPN SiGe-base bipolar transistors has been performed using computer simulations focussing on the effect of the Ge profile on the electrical characteristics. In this study it is shown that the base Gummel number is of major importance on the maximum cutoff frequency and the Ge-grading itself, which induces a quasielectric field, is of minor importance. Because of the outdiffusion of the boron dope in the base and the relatively thin critical layer thickness of approximately 600 A it appears that a box-like Ge profile with the leading edge approximately in the middle of the base is optimal. The predicted maximum cutoff frequency is 45 GHz, a sheet resistance of 8.5 kR/O and current gain of 80. The optimized device was fabricated and measurements were performed showing good agreement with the simulations. The simulated results are compared with devices containing a graded Ge profile showing small differences in the AC characteristics. From these calculations we found that the graded Ge profile has only small influence as long as the base Gummel number is kept constant. Similar box-like Ge profiles were used in a recent publication (4) on selectively grown SiGe transistors. Fabricating SiGe HBT's with high figures of merit can hardly be accomplished without extensive simulations. Per- forming simulations is necessary to optimize or predict the device characteristics. When the material parameters in the device simulators are calibrated by measurements, simulations introduce several advantages: Fewer processing iterations are necessary to accomplish 0 Simulations give a good insight into the device physics the optimization