Study of Cutoff Frequency at High Collector Current Density in SiGe Single-Heterojunction Bipolar Transistor

Abstract

The cutoff frequency performance of an NPN Si/SiGe/SiGe Single-heterojunction bipolar transistor (SiGe SHBT) at high collector current densities has been analyzed using a 2-dimensional MEDICI device simulator. A conventional NPN Si/SiGe/Si Double-heterojunction bipolar transistor (SiGe DHBT) having uniform 14%Ge in the base region has been investigated for comparison. The analysis shows the formation of a retarding potential barrier for minority carrier electrons at the base collector heterojunction of the DHBT structure. Whereas, the base-collector homojunction of the SiGe SHBT structure, having a uniform 14%Ge profile in its base and collector, inhibits the formation of such a retarding potential barrier. The SHBT structure with a base-collector homojunction shows an improved cutoff frequency at a high collector current density in comparison with conventional SiGe DHBT, which makes it more promising for high speed, scaled down, field-specific applications.

Silicon-germanium Single-heterojunction Bipolar Transistor

“…However, the conventional NPN Si/SiGe/Si DHBT structures exhibit sharp fall in cutoff frequency at high collector current densities, due to the formation of retarding potential barrier for minority electrons at base-collector heterojunction 5 . An alternate NPN SiGe HBT structure with linearly graded germanium atomic per cent (at %) has been proposed in collector to smooth-out this retarding potential barrier at base-collector hetrojunction for improving the current gain and cutoff frequencies at high collector current densities 6 .…”