Current gain-Early voltage products in graded-base Si/Si/sub 1-x/Ge/sub x//Si heterojunction bipolar transistors

An examination is made of the current-gain-Early-voltage ( beta -V/sub A/) tradeoff for graded base Si/Si/sub 1-x/Ge/sub x//Si heterojunction bipolar transistors (HBTs). A two-layer stepped-based structure was used to achieve a beta V/sub A/ product greater than 100000 V at room temperature for a device predicted to have a cutoff frequency in excess of 10 GHz. This is an improvement of this figure of merit by a factor greater than 20. It is shown that the Early voltage in a bipolar transistor can be improved dramatically by inserting a thin heavily doped p/sup +/-SiGe layer at the collector side of the base, without any reduction in gain.