Network analyzer measurements and physically based analysis of amplitude and phase distortion in SiGeC HBTs
2005 (English)In: 2005 International Semiconductor Device Research Symposium, 2005, 74-75 p.Conference paper (Refereed)
We have investigated the linearity of advanced SiGeC bipolar transistors for different bias and input power conditions. The SiGeC HBTs were fabricated in an advanced process, with low base resistance, a minimum emitter area of 10 × 0.4 μm2, and balanced fT/fMAX values of 40-50 GHz. The BVCEO is approximately 2 V. For RF integrated circuits the linearity, i.e. distortion is an important issue. Harmonic and intermodulation distortion in Si and SiGe bipolar transistors has been discussed by several authors from a modelling perspective. However, the experimental studies on high-performance SiGe devices are limited so far. In this work the RF harmonic distortion was characterized by a novel approach using a 2-port network analyzer with a frequency offset option for the receiver port. Both amplitude and phase distortion results could be obtained in a fast and efficient manner. At each port of the network analyzer the power and the phase of the signal (A/B) a reference (R1/R2) are available. When port 2 is tuned to the fundamental frequency the S-parameters are directly obtained and standard 12-term error correction techniques (SOLT/TRL) can be applied. In the case where the port 2 is tuned to another frequency only direct power measurements of A and B are possible and the response at the 2ND and 3RD harmonic frequency has to be adjusted for the attenuation between the DUT and the network analyzer ports using a thru-structure measurement. Figure 1. shows typical output vs. input power characteristics at a DC collector current I C close to peak RF-gain. Due to the relatively low VCE of 1 V saturation will influence both gain compression and linearity. Since the load and source impedance of 50 Ω is the same for the direct power and S-parameter measurements we can compare uncorrected gain and the gain calculated from the corrected S-parameters using |S21| 2. In Figure 2 it observed that the gain difference is 1 - 2 dB. The harmonic distortion vs. IC is shown in Fig. 3 for three different input power levels. Characteristic minima are observed in the 2ND harmonics at 4 mA and the third harmonic at 0.6 mA. Previous studies have indicated that this effect is caused by cancellation between different non-linear, bias dependent effects, such as the collector-base capacitance CBC and transit time τEC. The cancellation occurs due to the phase difference in the non-linear components. To examine this effect we have plotted the phase of the measured output signal at the fundamental and harmonic frequencies, as shown in Fig. 4. The phase of the fundamental shows an expected change at high current due to increased transit time caused by base push-out. The behaviour at the 2ND harmonic shows a drastic phase turn close to 4 mA, which corresponds neatly to the minimum in Fig 3. Since the phase is almost constant at lower IC this suggests that two separate effects dominate the 2ND harmonic at low and high IC respectively. The rapid phase turn is similar to a resonance and thus complete cancellation could occur for a specific current, given that no other non-linear effects were contributing. The minima in the power and the phase turn of the 3RD harmonic are mainly related to the derivative of the 2ND harmonic. The most important non-linear element in the transistor at high IC and VCE close to saturation is CBC, which can be extracted from the S-parameters after standard open-pad de-embedding. In Fig. 5 it is observed that CBC increases strongly for IC larger than 10 mA. As a result the non-linear current generated by CBC will change in both amplitude and phase. Finally, the phase difference between different non-linear effects in the transistor depends on the transit time τEC. The τEC of the SiGeC HBT can be obtained from the phase of S21. Figure 6 shows the change in τEC in for different input power levels. Under large signal conditions the minimum τEC is higher and starts to increase at a lower IC since the transistor is driven into saturation by the large output signal swing. In conclusion, the analysis, using a 2-port network analyzer, of the phase behaviour at harmonic frequencies is useful to find the origin of the distortion in the SiGeC bipolar device structure.
Place, publisher, year, edition, pages
2005. 74-75 p.
Electrical Engineering, Electronic Engineering, Information Engineering
IdentifiersURN: urn:nbn:se:kth:diva-84631DOI: 10.1109/ISDRS.2005.1595984ScopusID: 2-s2.0-33847204001ISBN: 978-142440084-3OAI: oai:DiVA.org:kth-84631DiVA: diva2:499435
2005 International Semiconductor Device Research Symposium; Bethesda, MD; 7 December 2005 through 9 December 2005
QC 201203022012-02-132012-02-132012-03-02Bibliographically approved