Characterisation of NPN and PNP SiGe Heterojunction Bipolar Transistors formed by Ge' implantation

This work compares NPN and PNP SiGe HBTs fabricated simultaneously using Ge' implantation in the base to achieve an average Ge concentration of 4at.%. Electrical measurements are presented and discussed with TEM and SIMS analysis. The results show that the PNP HBTs give superior performance to the NPNs, as they have more ideal collector characteristics. The NPN HBTs exhibit collector-emitter leakage. Both types of Ge' implanted device have non-ideal base currents. TEM images show that both NPN and PNP devices have a high density of defects in the Ge' implanted area. The electrical results are explained by the opposing effect of the Ge' implant on the diffusion coefficients of boron and arsenic, as seen in the SIMS profiles. The increased emitter difhsion in the NPN HBTs creates a narrower base region, which is more prone to collector-emitter leakage. The hairpin dislocations in the base are thought to be the cause of the base current ideality deterioration. Introduction Although SiGe HBTs have been successfully fabricated by MBE' and CVD2, Ge' implantation offers an alternative processing technique which is fully compatible with existing VLSI, self-aligned technology3. It also offers advantages such as a graded profile and control over the location and dose of the Ge. Device simulations show that the graded profile produced across the base of the HBTs should peak at the collector-base junction for optimum performance, as shown in figure 14 . In contrast to MBE and CVD, Ion Beam Synthesis (IBS) involves the epitaxial regrowth of amorphous SiGe layers. Previously NPN SiGe HBTs using IBS have shown improved performance when compared with Si BJTs'. The aim of this work is to fabricate SiGe HBTs using Ge' implantation within a standard complementary bipolar process so that the performance of NPN and PNP devices made simultaneously can be compared. Theory One advantage of SiGe HBTs is the collector current enhancement provided by the bandgap narrowing in the base. Doping levels greater than 10'7cm" will cause further bandgap narrowing to provide increased collector current6. However, this high doping will increase the base Gummel number, Gb, which acts to reduce the collector current as shown in equation (1). To take account of these factors, expression (2) can be used as an approximation to predict the expected collector current enhancement from the dopant profiles. In this work, an average Ge content is assumed by integrating over the basewidth, and this value is used to obtain the ratios of the density of states (NcNv)~ and the diffusion coefficient of the minority carriers in the base (Db)'. 0-7803-5298-W99/$10.00