A study of the current transport in AlGaAs/GaAs HBTs with graded and setback layers

The combined effects of graded and setback layers (W/sub G/ and W/sub I/) on the AlGaAs/GaAs heterojunction bipolar transistor (HBT) d.c. performance are investigated, and an analytical model which can describe the behavior of such HBTs is presented. The HBT base and collector currents accounting for the variation of the conduction and valence bands due to the presence of W/sub G/ and W/sub I/ are also calculated. It is shown that including W/sub G/ and W/sub I/ actually degrades the HBT current gain at low current levels. The current gain at high current levels, on the other hand, can be enhanced if W/sub I/=150 /spl Aring/ and 0/spl les/W/sub G//spl les/300 /spl Aring/ or W/sub I/=0 and 150 /spl Aring//spl les/W/sub G//spl les/300 /spl Aring/ are used. The model predictions compare favorably with results calculated from a numerical model.