Germanium Profile, Graduality and Base Doping Level Influences in the Performance of SiGe HBT

Abstract

Emerging markets in high frequency applications such as automobile anti-collision radars, high-speed wireless Internet, optical communications, need high performance semiconductor devices. Owing to its low cost production and high reliability, SiGe HBT technology has proved to be an excellent option for these applications [1]. The transistor dynamic performance is usually characterized by the current gain transition frequency (f T ) and the maximum oscillation frequency (f MAX ). These two figures-of-merit are related to the transistor capacitances, resistances and carriers transit times. The differences in germanium content and base doping profiles between different devices directly impact their maximum f T and f MAX . In this paper, results are presented for eight samples. The investigated SiGe HBTs are compatible with the CMOS core process. They are based on a double poly-silicon technology (base and emitter), use a fully self aligned architecture (FSA emitter-base and collector-base junctions) and selective base epitaxy (SEG) with carbon incorporation [2]. Table 1 gives main technological and electrical parameters for these devices. The effective emitter-base junction area is 0.17 × 5.7 µm 2 . The base thickness (W B ) acts on the transit times and on the collector current density. Variations of the base doping profile simultaneously to the germanium content and to the graduality make difficult the splitting of the influence of W B from those led by the germanium content. CMOS process requires high thermal budget that increases the diffusion of base dopants. The small lateral size of the intrinsic base complicates the optimisation of the base epitaxy to achieve a specific profile and a thin highly doped base layer.An estimation of the effective W B is based on calculation with SIMS data, supposing a Gaussian shape profile of the boron doping. The results are summarized in table 1. Analytical formulations [3] are used to calculate the collector current and the emitter (τ E ), the base (τ B ) and the collector (τ C ) transit times according to the doping, the Ge content at the emitter-base junction and the Ge graduality. A relatively good agreement is obtained for the collector current at 295 K, as shown in figure 1 (V BE of 0.65 V) while the agreement is far to be good at cryogenic temperature (not shown). The electron mobilities within SiGe were taken from [4]. The measured and the calculated transit times are presented in figure 2 at two temperatures. The calculations give the same tendency as measurements, but one can notice that calculated values are underestimated, especially at 295 K. This is due to the lack of accurate mobility analytical model versus germanium content for minority carriers at very high doping level in SiGe. Analytical formulae are interdependent on W B , on the Ge content and on graduality. They help us to separate each contribution, but we do not attempt to compare directly calculation to measurement due to the assumption in formula simplifying the complex character of the minority ca...