Designing microwave circuits today means relying on numerical circuit simulation. While not a substitute for one's own skills, knowledge, and experience, a designer must be able to count on the adequacy of circuit simulation tools to accurately simulate the circuit performance. Circuit simulators themselves are generally up to the challenge. However, there is a perpetual quest for good transistor models to use with the simulator, because models are usually the limiting factor in the accuracy of a simulated design. This is due to the continuous evolution of transistor technology, requiring the models to keep up, and also to the increasing demands placed on the models to perform with respect to wider classes of signals, operating conditions (e.g., temperature), and statistical variation. Circuit designers therefore often face the challenge of adapting the models that are provided with simulators to better describe the actual transistor that is being used in the design. This is achieved by characterizing the transistor, mainly by measurement, but also by electromagnetic and/or thermal simulation. Finally, model parameter values must be extracted from this data before the model can be used at all in a design.

As transistor modeling is a key to circuit design, many publications are available on the models for any type of transistor, ranging from model documentation in simulator products, to application notes and scientific papers in technical conferences and journals; but it seems that much less is published on how the respective model parameters can be determined.

If one is about to design a circuit, one certainly relies on a circuit simulation tool that provides us with the capability to determine circuit performance with high accuracy without even fabricating a prototype. We expect the simulation to provide us with the numerical algorithm that is capable of accurately calculating the relevant variables, such as currents, voltages, noise, distortion products, etc. At least as important is the description of the components that will be used, since ultimately the simulation can never be more accurate than the models of the components used. Component models commonly are provided as drag-and-drop components in modern circuit simulators. At least for established technologies, accurate models are available for passive and active components. All problems solved?

Unfortunately not. The models, especially compact transistor models, are parametrized. It is a big step from the general-purpose model that is capable of describing, say, SiGe heterojunction bipolar transistors (HBTs) in general to the specific model for a specific transistor of a specific size, from a specific foundry, that one plans to use in the actual design.

But why bother? One would expect the foundry selling the transistor also to provide us with a valid model.

In reality the situation is more like the following:

1. Some vendors simply do not provide their customers with appropriate models. Either one gets just plain data sheets providing some figures of merit and printed S-parameters. Or quite often, even for the most advanced transistors, only very basic SPICE-type model parameters are provided. While these models are available in literally all circuit simulators, their accuracy is often quite limited, since these models only describe the very basic transistor behavior.

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Introduction

Microwave power transistors commonly internally consist of a number of smaller transistor cells combined together in order to reach the desired performance. The individual transistor cells are positioned side by side, sometimes repeated in two dimensions. But often, only one single line of parallel cells is used, since power splitting and combining is less challenging compared to other configurations. Relying on an array of small transistors instead of only one large power transistor allows higher power at high frequencies to be realized. Reaching high frequencies calls for small and fast transistors. Inherently, reducing the physical size of a transistor will reduce the power-handling capabilities. Increasing the size of a single transistor with just one emitter or drain connection, on the other hand, is no option in the microwave regime, since unequal current or heat distribution within the device will rapidly degrade performance. Proper combination of many small transistors within a package to get one power device is therefore the only option. In addition to the advantages regarding electrical behavior, thermal management of the power transistor can be significantly simplified.

Various configurations of transistors in packages have emerged in recent years. Common to most of these solutions is the arrangement in bars, as single or multiline (i.e., 2D). However, 2D configurations of bars restrict the power transistor to lower frequencies where line lengths in general (e.g., bondwires) are small compared to the wavelength.