Simulation of Silicon Germanium HBTs Using ATLAS/BLAZE

C. K. Maiti and G. A. Armstrong
Department of Electrical & Electronic Engineering,
Ashby Building, The Queen's University of Belfast

 

The aim of this technical report is to study the suitability of ATLAS/Blaze for the simulation of state-of-the-art high-speed SiGe heterojunction bipolar transistors (HBTs) fabricated in close-to-conventional Si-technology. Simulation studies of advanced SiGe HBTs are presented and, where possible, verified with experimental results.

Material Parameters

The incorporation of germanium significantly changes the properties of the base region and the base-emitter and base-collector junctions in a Si/Si1-xGex/Si HBT. Addition of Ge reduces the bandgap of Si, leading to the narrow bandgap SiGe base of the HBT. The lattice constant of Si1-xGex alloy differs considerably from that of Si . Hence, the incorporation of Ge causes strain in the SiGe (compressive in-plane), modifying the energy band structure and density of states in the conduction and valence bands.

The quality of epitaxially grown SiGe/Si heterostructures has improved dramatically in recent years and reported results are very promising in terms of the possibilities offered by the SiGe technology. However, while silicon has been well-characterized over the past 40 years, not nearly as much is known about strained SiGe and many simplifying assumptions are made in the strained-SiGe/Si material parameters.

Simulation of Advanced SiGe HBTs

Since the introduction of SiGe into conventional Si technology, various research groups have demonstrated high performance SiGe base HBTs with different approach to forming the Ge profile in the base. While the IBM group [1] uses graded Ge profiles, the Daimler Benz group [2] focuses on SiGe HBTs with a uniform Ge profile. Recently Harame et al. [1] have developed a high-performance SiGe-base BiCMOS process.

The highest speed (fmax) SiGe transistor reported so far appears to be by Schuppen et al. [2]. They used a relatively thick (60 nm) base and heavy doping to reduce the intrinsic base resistance. The base transit time was reduced by a strong electric field with 0 to 15% Ge grading. Meister et al. [3] have reported a SiGe HBT with a 74 GHz fmax , resulting in a record CML gate delay of 11 ps.

The key issue in this study was to investigate HBT structures which offered both high fT and fmax. The simulation results have been compared with the latest reported experimental results [3] as summarized in Table 1. Figure 1 shows the simulated device structure where the Ge profile and doping profiles in various transistor regions are shown. The choice of this structure was made because of the availability of reliable experimental data for such a device.

 

Figure 1. Doping profile and Ge profile (graded base) of a SiGe HBT.
Based on Meister et. al. IEDM 95 p. 739.

 


Table 1. Comparison of simulated and measured data.

The Ge concentration in the base has been graded from 0% at the emitter-base junction to 12% at the base-cellector junction. Figure 2 shows the Gummel plot and it is seen that almost ideal base current characteristics are observed. The transistor has a simulated peak dc current gain of about 210, as shown in Figure 3, which compares well with the experimental value of 220. The simulated cutoff frequency fT is seen to be about 63 GHz while the calculated fmax has a value of more than 100 GHz, as shown in Figures 4 and 5 respectively.

Figure 2. Gummel plot of a SiGe (graded based) HBT.
Based on Meister et. al. IEDM 95 p. 739. [3].

Figure 3. Simulated DC current gain of a graded base SIGe HBT.
Based on Meister et. al. IEDM 95 p. 739.

Figure 4. Simulated Frequency (fT) of a graded base SiGe HBT.
Based on Meister et. al. IEDM 95 p. 739.

Figure 5. Simulated Frequency of Oscillation of a graded base SiGe HBT.
Based on Meister et. al. IEDM 95 p. 739.

In Figure 5 also shown are the small signal current gain bac extracted from h-parameters and the maximum available gain (MAG). From these curves a unilateral power gain of 22 dB at 10 GHz and an fmax of about 110 GHz (MAG) have been obtained at the reverse base-collector voltage of 2 V. In particular this high fmax value originates from the integration of the SiGe base, providing both high cutoff frequency and low intrinsic base resistance.

The effect of collector doping on the Early voltage VA obtained from the simulated output characteristics (Ib = 15 nA) is shown in Figure 6. It is seen that as the doping concentration increases in the collector, the Early voltage decreases. For a collector doping of 5x1016 cm-3, the predicted VA is 110 V, giving a high b.VA product exceeding 22000. A Ge fraction of 12% at the base-collector junction has provided the high Early voltage.

Figure 6. Typical simulated output characteristics of a
graded base SiGe HBT as a function of collector doping.
Based on Meister et. al. IEDM 95 p. 739.

Summary

Relatively simple material parameters for strained SiGe layers grown on Si(100) are incorporated in ATLAS/Blaze. The carrier mobility models in strained SiGe are very conservative and are similar to silicon. Mobility enhancements due to strain have not been modeled.

As more studies are performed on the electrical properties of SiGe strained layers, the program can easily be modified to include the latest findings. Simulation results for both silicon and SiGe bipolar transistors are in good agreement with reported experimental results available in the literature.

SiGe HBTs with a constant (flat) germanium concentration in the base have very large current gains but similar fT to silicon devices. Unity gain cutoff frequency of over 63 GHz was simulated by ATLAS for a graded base SiGe HBT. This is consistent with experimental value of 61 GHz reported by Meister et al. [3]. The projected fmax was reported to be about 74 GHz and our simulation results give rather a more optimistic value of over 100 GHz.

References

  1. D. Harame et al.,
    "SiGe HBT Technology: Device and Application Issues"
    IEEE IEDM Tech. Dig., pp. 731 - 734, 1995.
  2. A. Schuppen, U. Erben, A. Gruhle, H. Kibbel, H. Schumacher and U. Konig
    "Enhanced SiGe Heterojunction Bipolar Transistors with 160 GHz fmax"
    IEEE IEDM Tech. Dig., pp. 743 - 746, 1995.
  3. T.F. Meister, H. Schafer, M. Franosch, W. Molzer, K. Aufinger, U. Scheler, C. Walz, M. Stolz, S. Boguth and J. Bock
    "SiGe Base Bipolar Technology with with 74 GHz fmax and 11 pS Gate Delay"
    IEEE IEDM Tech. Dig., pp. 731 - 734, 1995.