Effect of band alignment and density of states on the collector current in p-Si/n-Si/sub 1-y/C/sub y//p-Si HBTs

p-Si/n-Si/sub 1-y/C/sub y//p-Si heterojunction bipolar transistors with varying carbon fractions in the base were grown by rapid thermal chemical vapor deposition (RTCVD), to better understand the potential of Si/sub 1-y/C/sub y/ in enhancing the performance of Si-based bipolar technology. The band line-up issues which make Si/sub 1-y/C/sub y/ a desirable choice for forming the base region in a p-n-p HBT are discussed. Electrical measurements performed on the p-Si/n-Si/sub 1-y/C/sub y//p-Si HBTs (y=0.6, 0.8 at.%) are used to extract important information regarding the electronic properties of the Si/Si/sub 1-y/C/sub y/ material system, e.g., the bandgap reduction in Si/sub 1-y/C/sub y/ compared to Si and minority carrier recombination lifetime in Si/sub 1-y/C/sub y/. Temperature dependent measurements of the collector current were performed to extract the bandgap narrowing at the Si/Si/sub 1-y/C/sub y/ heterojunction. This paper includes a detailed analysis of the impact of heavy doping and reduced density of states in Si/sub 1-y/C/sub y/ compared to Si on the extraction of the energy bandgap offset, and on the collector current of p-n-p HBTs. The impact of the reduced density of states on the design of p-n-p Si/Si/sub 1-y/C/sub y/ HBTs is discussed. The measured value of the energy band offset is (65 meV/at.% C) very close to previously measured values of the conduction band offset at the Si/Si/sub 1-y/C/sub y/ heterojunction. The results are thus consistent with a band line-up at the Si/Si/sub 1-y/C/sub y/ interface that is dominated by a conduction band offset with little if any valence band offset.