Low temperature characterization of Si bipolar junction transistors and Si1-xGex heterojunction bipolar transistors

Hashim, Md. Roslan (1997) Low temperature characterization of Si bipolar junction transistors and Si1-xGex heterojunction bipolar transistors. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

This thesis investigates the low temperature characterization of Si bipolar junction transistors and Si_1-xGe_x heterojunction bipolar transistors. Device characteristics are measured between 300K and 200K, and a new analysis method is used for extracting the bandgap narrowing in the base from the temperature dependence of the collector current. Applying the analysis method to Si BJTs with mean base doping concentrations of 1.6x10¹⁸cm^-3 and 6.3x10¹⁸cm^-3 gives bandgap narrowing values of 37meV and 47meV due to heavy doping in the base. These values compare with the predicted values of 35meV and 54meV for the model of Klaassen et al. Applying the analysis method to a Si_0.88Ge_0.12 HBT and a Si_0.87Ge_0.13 HBT gives bandgap narrowing values of 119meV and 125meV due to the presence of the 12% and 13% Ge respectively. These values compare with the predicted values of 123meV and 128meV for the model of People.

One problem in npm SiGe HBTs is boron out-diffusion from the base. Boron dopant that out-diffuses into the emitter and the collector during SiGe growth or subsequent heat treatment results in the formation of parasitic energy barriers at the emitter/base and base/collector junctions. These barriers suppress the injection of electrons from the emitter to the collector which results in reduced collector current. Undoped SiGe spacers can be incorporated at the emitter/base and base/collector junction to contain boron diffusion within the SiGe base, thus eliminating the formation of the parasitic energy barriers. Investigations have been carried out on SiGe HBTs with 5, 10 and 15nm undoped SiGe spacers. The SiGe HBTs with 15nm spacers showed the highest collector current compared to those with 5 and 10nm spacers. This is explained by increasing amounts of boron out-diffusion as the base spacer thickness is reduced. It is also shown that boron out-diffusion is almost completely eliminated in devices with 15nm base spacers but this is not the case for devices with 5 and 10nm spacers. The size of the parasitic energy barrier at the emitter/base and base/collector junctions is quantified. The results show that boron out-diffusion is more critical at the base/collector junction than at the emitter/base junction due to the lower doping concentration in the collector (1x10¹⁶cm^-3) than in the emitter (1x10¹⁸cm^-3).

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