![]() The ability of the layer structure to simultaneously give high β and high V BCEO is described by the figure of merit β⋅ V BCEO. The breakdown voltages (V BCEO ) for strained Si HBT, SiGe HBT and Si BJT were found to be 2.5, 2.7 and 4.5 V, respectively. 4 shows that strained Si HBTs exhibit off-state I C comparable with SiGe HBT and Si BJT devices (< 20 fA). In all cases the ideality factor for the collector current (I C ) was confirmed to be 1. ![]() The ideality factor for the base current (I B ) is 1.4, 1.2, and 1.4 for the strained Si HBT, the SiGe HBT and the Si BJT, respectively, indicating that the incorporation of a SiGe base and virtual substrate did not increase recombination in the base for strained Si HBTs compared with Si BJTs. In this case, the bandgap in the base is 1.17 eV for the Si BJT, 1.04 eV for the SiGe HBT and 0.98 eV for the strained Si HBT. The ratio of electron injection into the base to hole injection into the emitter increases with decreasing bandgap in the base, therefore, larger values of β are achieved with a higher Ge content in the SiGe base. The increase in β is mostly attributed to the bandgap difference between the emitter and the base. The values of β in the strained Si HBTs were found to be larger than those suggested by previous simulations, however, the relative β improvements compared with the SiGe HBT ( β ε Si / β SiGe ~ 11) and the Si BJT ( β ε Si / β Si ~ 27) are in good agreement 6. SiGe HBTs exhibit values of β comparable with those commonly reported for SiGe HBTs having similar Ge content in the base 2-5,8. 3 demonstrates that the maximum values of β for the strained Si HBTs is 3700, about an order of magnitude higher than the SiGe HBT (~ 334) and Si BJT (~ 135) controls. Raman spectroscopy confirmed that strain in the emitter of the strained Si HBT was fully maintained following processing. 2 shows the Ge composition and doping profiles for each epitaxial structure measured by secondary ion mass spectroscopy (SIMS) following device fabrication (emitter drive-in data not shown). The process concluded with the deposition of Al contacts with a TiW barrier layer. An emitter drive-in step was subsequently carried out at 900oC for 10 s, followed by poly etch and a second TEOS isolation step. TEOS was used to isolate the structure and contact windows were opened at the emitter and collector for heavily doped n + poly contacts (P, 5 × 10 19 cm -3 ). Phosphorous collector link and boron extrinsic base were subsequently implanted at 1 × 10 15 cm -2 (35 keV) and 5 × 10 15 cm -2 (20 keV), respectively. First, a mesa structure for the transistor was formed by etching the surrounding material down to the collector layer. A simplified process flow illustrating the main fabrication stages is shown in Fig. the base for the Si BJT and the emitter for the Si BJT, SiGe HBT and strained Si HBT) was 750oC. Finally, the growth temperature for all Si capping layers (i.e. For the control material, the Si subcollector and collector were grown at 750oC and 1080oC, respectively. For the strained Si and pseudomorphic SiGe HBTs, the SiGe base layers were grown at 650oC, having Ge compositions of 30% and 15%, respectively. The relaxed SiGe virtual substrate for the strained Si HBT was grown at 850oC using terrace grading with an average rate of 10%-Ge μ m -1, topped with a 1.2 μ m thick constant composition Si 0.85 Ge 0.15 layer. ![]() The material for the Si BJT and both types of HBT was grown epitaxially on (100) Si wafers in an ASM Epsilon 2000E RP- CVD reactor using in-situ doping. Pseudomorphic SiGe HBTs having a Si 0.85 Ge 0.15 base (so that y - x is the same as in the strained Si HBT) and Si control BJTs were also manufactured for comparison. Strained Si HBTs were fabricated on a Si 0.85 Ge 0.15 virtual substrate having a compressive Si 0.7 Ge 0.3 layer for the base and capped with a strained Si layer for the emitter. The layer structure and doping levels for the devices are given in Table 1. The strained Si HBTs exhibited satisfactory breakdown voltage (2.5 V) compared with SiGe HBTs (2.7 V) and Si BJTs (4.5 V) and excellent control of collector off-state leakage ( x > 0 and the emitter is strained Si, for the pseudomorphic HBTs y > x = 0 and for the control Si bipolar junction transistor (BJT) x = y = 0. This represents 10x and 27x larger gain compared with pseudomorphic SiGe HBTs and Si control BJTs which were manufactured in parallel and had current gains of 334 and 135, respectively. Si HBTs have been demonstrated for the first time with a maximum current gain ( β ) of 3700 using a relaxed Si 0.85 Ge 0.15 virtual substrate, Si 0.7 Ge 0.3 base and strained Si emitter. ![]()
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