Development of a New Electro-thermal Simulation Tool for RF circuits

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8 1.2. The limitations of present simulation codes Figure 1.9: Electro-thermal simulation scheme based on thermal resistance. Figure 1.10: Electro-thermal simulation scheme performed by a circuit simulator. tion performed by a circuit simulator is shown in Fig. 1.10. The thermal information is extracted from a layout, and the electrical data is saved in a schematic. After the electro-thermal simulation, the results are plotted in Current-Voltage graphs or temperature maps. Such a simulation can be performed using a modern circuit simulator like e.g. ADS. Some simulation tools are unable to perform this task, due to certain limitations within their structure. These limitations are briefly described in the Sec. 1.2. 1.2. The limitations of present simulation codes Engineers involved with the design and development of solid-state devices and circuits have to consider thermal aspects as having the same importance as purely electrical ones. In principle, designers can resort to electro-thermal simulation tools in order to accomplish this complex task. Unfortunately, traditional programs such as the widely used simulator SPICE were realized for the analysis of integrated circuits, and are unsuitable for this purpose, since they do not account for self-heating (the temperature of any active device is assumed to be constant, that is, independent of dissipated power) and temperature gradients (the temperature of the circuit is specified by the user, and is
Chapter 1. The relevance of the electro-thermal phenomena 9 Figure 1.11: SPICE macro-modelling approach for two parallel transistors. The electro-thermal feedback is created via CCVS and VCVS. uniform in the entire circuit under analysis). A commonly adopted approach to extend the SPICE capability toward the electro-thermal simulation of devices and circuits is the so-called macromodeling technique. The structural macromodeling method starts from the intrinsic SPICE model as a main core and describes some specific nonlinear device effects and the electro-thermal feedback by adding supplementary standard SPICE elements (i.e., resistors, capacitors, inductors, diodes, and transistors). The basic advantage of the structural macro-models is their manage-ability in all SPICE versions. However, they often require long analysis time, are associated to complicate parameter extraction techniques and it is difficult to model the hard nonlinearities. As an alternative, it is possible to employ the Analog Behavioural Modeling (ABM)-based macromodeling technique presented in Fig. 1.11, which makes almost entirely use of voltage-controlled voltage/current sources that enable the direct "in line" implementation of any kind of nonlinear expression. This allows tackling most of the problems arising with the structural macromodeling. Unfortunately, the computational time nonlinearly rises with the number of ABM sources; as a consequence, it is necessary to keep this number as small as possible; this might be solved by not accounting for certain effects that have no influence or no interest for a given simulation. Nevertheless, it is evident that such SPICE-like tools can be computationally viable only for circuits where the number of active devices is relatively low. To overcome all the above shortcomings a novel simulation code is hereby presented based on the commercially available Advanced System Design (ADS) software from Agilent. Contrarily to SPICE, tools like ADS incorporate recently developed bipolar transistor models that include the possibil-
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APPENDIX A Design of efficient opti
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Appendix A. Design of efficient opt
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List of publications ⋆ F. M. De P
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Bibliography 139 [8] B. Jagannathan
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Bibliography 141 ii. a novel analys
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Bibliography 143 [50] C. C. McAndre
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Development of a New Electro-thermal Simulation Tool for RF circuits

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