Large signal electro-thermal LDMOSFET modeling and the thermal memory effects in RF power amplifiers
In this dissertation an analytical large signal electro-thermal LDMOSFET model – Agere Electro-Thermal (AET) model – is presented. Composed by three parts: a die level device model, an equivalent circuit for the package portion and a thermal network, AET model was implemented in Agilent EESOF's RF circuit design software – Advanced Design System (ADS). The methodology developed could be applied to other high power device models development as well. The LDMOSFET's distributed and dynamic thermal responses are computed by a modified image method. Thermal memory effects are studied with the aid of the newly developed distributed and dynamic thermal models. The package model in such discrete devices are extracted from geometry estimation, and S-parameter measurements. Loadpull design technique was implemented in simulation level, and the contours of output power and efficiency agree with those obtained in loadpull measurement. The model exhibits a good accuracy in predicting P1dB, gain, PAE, IMD3 and IMD. Pros and cons between analytical model and table based model are compared by using AET and a formally developed BSpline table model – OSUFET. A distributed electro-thermal model was developed to investigate the impact of a non-uniformly distributed temperature profile on the model accuracy. A 3D image method was used to compute the device's thermal resistance matrix. The complexity of the distributed electro-thermal model was further reduced by using its symmetry. Temperature distribution is reproduced in this model and it is found to have no significant impacts on electrical performance. The image method was further extended to compute the 3D transient temperature step responses, from which multiple thermal time constants can be extracted and applied to electro-thermal models. This improved transient thermal model is found to have a strong impact on the thermal memory effects in RF power amplifiers. With the aid of several electro-thermal models with different thermal transient accuracy, thermal memory effects and electrical memory effects can be characterized separately. Thermal memory effects are found to be stronger in amplifiers where predistortion technique is present, and is most significant for envelop frequency below 1 MHz.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:ldmos ldmosfet modeling electro thermal large signal rf power amplifier design linearization predistortion memory effects image methods
Date of Publication:01/01/2004