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| Author: | Olavsbråten, Morten |
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| Title: | Practical Thermal and Electrical Parameter Extraction Methods
for Modelling HBT´s and their Applications in Power Amplifiers |
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| Degree: | Dr. Ing. |
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| Defence: | 03.04.2003 |
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| Faculty: | ||||||
| Year: | 2003 |
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| Press
release |
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| Fulltext: PDF (10521kB). Requires
Acrobat Reader |
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| List of papers: | ||||||
| 1. M. Olavsbråten, ”A Simple Practical
Technique for Estimating the Junction Temperature and the Thermal Resistance
of a GaAs HBT”, 2002 IEEE MTT-S International Microwave Symposium Digest,
vol. 2, pp. 1005-1008, Seattle 2002 |
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| 2. M. Olavsbråten, “ A Practical Method of Parameter Extraction for the VBIC Model used on a GaAs HBT”, GAAS 2000 Conference Proceedings, pp. 56-59, Paris 2000 | ||||||
| 3. M. Olavsbråten, “Parameter Extraction and Evaluation of the Bias Dependence of Tf, for the VBIC Model used on a GaAs HBT”, GAAS 2001 Conference Proceedings, pp. 315-318, London 2001 | ||||||
| 4. M. Olavsbråten, L. Indresæter,
“A Low Cost Temperature-Controlled Chuck for a Cascade Probestation”, Proposed
for the 2003 IEEE MTT-S International Microwave Symposium |
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| Abstract: In Chapter 3, a new practical technique for estimating the junction temperature and the thermal resistance of an HBT was developed. This technique has been published by the author in 2002 [5], and can be found in Appendix A.1. The new technique estimates an interval for the junction temperature. The main assumption in the new technique is that the junction temperature can be calculated from three separate phenomena: • Thermal conduction of the substrate.The new technique shows good accuracy, when applied to several InGaP/GaAs HBT’s from Caswell Technology, and compared to the results from other methods in [3, 4]. All the results presented in [3, 4] fall well within the estimated junction temperature intervals from the new technique, except for the 4x40C transistor evaluated at 85 °C, where the result is just outside the estimated interval. As an example of results from the new technique, the estimated junction temperature increase - relative to the ambient temperature - for the P4x40C is calculated to be Tj = [62.9, 81.3] °C. In [3, 4] the junction temperature increase is found using two different techniques which give Tj = 71.8 °C and Tj = 80.3 °C. These results are all found for a dissipated power of 0.4 W at an ambient temperature of 25 °C. A simple method for estimating the thermal capacitance was also developed. This method was applied to the 1x40C transistor giving an CTH = 1.4⋅10-9 J/K. Using the thermal resistance RTH = 380 K/W from Table 3.6, the thermal time constant becomes τTH = 0.53 µs. Caswell Technology reports the thermal time constant to be somewhat smaller than 1 µs. From this the method can be assumed to have good accuracy. In Chapter 4, an overview of the modelling concepts is given. Based on several requirements the VBIC model was chosen for all the practical modelling. An overview of all the parameters associated with VBIC model is given together with an explanation on how to find values for these parameters. From this, a practical parameter extraction method for the VBIC model was developed. This method is the first published practical parameter extraction technique for the VBIC model used on an InGaP/GaAs HBT, as far as the author knows. The method was published by the author in 2000 [6], and can be found in Appendix A.2. The main features of the extraction method are: • Only a few common measurements are needed (forward Gummel, reverse Gummel,The method includes the extraction of the bias dependent forward transit time. The parameter extraction and an evaluation of the transit time implementation in the VBIC model were published by the author in 2001 [7], and can be found in Appendix A.3. The extraction method was evaluated on a single finger, 1x40, InGaP/GaAs HBT from Caswell Technology. Only four iterations were required to fit the measurements very well. There is less than 1 % error in both the IC-VCE and VBE-VCE plots in Figure 4.12 and 4.13. At Ic = 20 mA and Vce = [2, 3, 5] V, S21 and S12 have the largest error. The maximum magnitude and phase error in the whole frequency range up to 40 GHz are less than 1.5 dB and 15 degrees. This chapter also includes measurements and modelling of SiGe HBT’s. Models for a single finger and a 8-finger transistor were extracted. All the dc characteristics of the modelled transistors have less than 3.5 % error. Some amplitude and phase errors are observed in the s-parameters, especially for the single finger. The errors are caused by uncertainties in the calibration due to a worn calibration substrate, high temperature drift during the measurements, and uncertainties in the physical dimensions/properties caused by lack of information from the foundry. Overall, the extracted models fit the measurements quite well. In Chapter 5 a very linear class A power amplifier has been designed using the InGaP/GaAs HBT’s from Caswell technology. The thermal junction estimation technique developed in Chapter 3 has been used to make a very good thermal layout of the power amplifier. The estimated average junction temperature is 98.6 °C above the ambient temperature, calculated with a total dissipated power of 6.4 W at an ambient temperature of 45 °C. The maximum junction temperature difference between the transistor fingers is less than 11 °C. The estimated thermal resistance of each finger was used in the VBIC model extracted in Chapter 4. The PA was constructed with a ‘bus bar’ power combinder at both input and output, and optimized for maximum gain with a 10 % bandwidth. The PA had a small signal transducer power gain of 15.3 dB and a maximum output power of 34.8 dBm. The 1 dB compression point was simulated to be 34.5 dBm, and the third order intercept point was 49.9 dBm. At 33 dBm output power the PAE was 27.2 %, the gain compression was 0.285 dB and the phase error -0.728 degrees. The PA was unconditionally stable for all frequencies above 100 MHz. The bias circuits must be included for a complete stability analysis at low frequencies. In a complete stability analysis of a PA, the ‘bus bar’ should be analysed in a 2.5D simulator. This has not been performed here. |
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