(Invited) Application-Specific SiGe HBT Optimization

State-of-the-art SiGe BiCMOS processes today show impressive high-frequency performance, with f_T and f_MAX exceeding 500GHz [1], [2]. However, there is also a vast application space that does not require these very high frequencies, but that can still benefit from the performance improvements made in the HBT technology. QUBiC, NXP’s most advanced BiCMOS technology platform, is currently in mass production already in its 8^th generation [4]. QUBiC-gen8 is based on 0.25µm CMOS, with a SiGe HBT having f_T/f_MAX=200/180GHz and a thick backend stack for elite passive components. The main products in this technology are amplifiers (LNA, MPA) for the mobile segment, meaning application frequencies below 6GHz. The noise performance of an LNA to a large extend depends on the minimum noise figure NF_MIN=10 log(F_MIN) of the SiGe HBT, which depends on a number of technology related device parameters, see Eq.(1), [3].

Experiments in which the intrinsic profile of the SiGe HBT was varied identified the dc current gain h_FE to be the dominant parameter at frequencies below ~10GHz [5]. By careful tuning, values of NF_MIN=0.55 and 0.35dB are achieved at 10 respectively 2GHz, see Figure 1.

For a Medium Power Amplifier (MPA) the maximum supply voltage of the high-speed device is generally not sufficient. For higher overall Power performance in a real-life (50µÙ) environment, for a typical MPA of ~100mW, a supply voltage of 5V would be preferred. By adding a deep N Well implant next to the regular Buried N collector layer, a device with a higher breakdown can be introduced [6]. The max V_SUPPLY can be traded-off against speed, resulting an optimal device with BV_CEO>5V while f_T=35GHz, see Figure 2. Since this device shares the same emitter-base construction, and low parasitics, of the high-speed device it still has an f_MAX>150GHz, making it an excellent PA.

At higher frequencies other parameters from Eq.(1) come into play. To meet the more demanding noise requirements in VSAT (Ku/Ka-band), up to 30GHz, the extrinsic (access) base resistance R_B is reduced. This is done by introducing a self-aligned emitter-base construction [7], which will be part of the next generation (gen9) of QUBiC, see Figure 3. By reducing R_B with as much as 20%, the measured NF_MIN of a typical Ku-band LNA reduces by 0.1dB, see Figure 4.

In conclusion, relatively low frequency applications can also benefit from the significant improvements made in recent SiGe BiCMOS technology, but it pays to fine-tune the process for the specific application requirements. The 8^th and 9^th generation QUBiC processes from NXP are suitable for a wide variety of applications, ranging from mobile at moderate frequency to high frequency satellite.

[1]     B. Heinemann et al., “SiGe HBT technology with f_T/f_max of 300GHz / 500GHz and 2.0 ps CML gate delay”, IEEE IEDM, p.30.5.1—30.5.4, 2010.

[2]     P. Chevalier et al., “Towards THz SiGe HBTs”, IEEE BCTM, p.57—65, 2011.

[3]     See e.g. chapter 3 in P.A.H. Hart, “Bipolar and Bipolar-MOS Integration”, Elsevier, ISBN 0-444-81510-4, 1994; or chapter 7 in J.D. Cressler and G. Niu, “Silicon-Germanium Heterojunction Bipolar Transistors”, Artech House, ISBN 1-58053-361-2, 2003.

[4]     W.D. van Noort et al., “BiCMOS Technology Improvements for Microwave Application”, IEEE BCTM, p.93—96, 2008.

[5]     P. H. C. Magnée et al., “SiGe:C profile optimization for low noise performance”, IEEE BCTM, p. 166—169, 2011.

[6]     H. Mertens et al., “Extended High Voltage HBTs in a High-Performance BiCMOS Process”, IEEE BCTM, p. 158—161, 2011.

[7]     H. Mertens et al., “Double-Polysilicon Self-Aligned SiGe HBT Architecture Based on Nonselective Epitaxy and Polysilicon Reflow”, IEEE BCTM, p. 60—63, 2012.