Graduation Semester and Year

2012

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering

Department

Electrical Engineering

First Advisor

Ronald L Carter

Abstract

Heterojunction Bipolar Junction Transistors (HBTs) are used in various high frequency applications in modern day technology. These devices produce high trans-conductance which is need for high frequency application. Their gain is significantly larger than complimentary metal-oxide-semiconductor (CMOS) devices which also imply there is high current within the devices. Since HBTs are very compact structure, the thermal heating generated within the devices needs to be characterized. This thesis explores the thermal effects from self heating in the static and time domains of the devices using 3D dimensional simulation and mathematical modeling using heat flow equation. The study concentrates on the thermal modeling aspects of the heterojunction bipolar transistors using the TCAD 3-dimensional thermal simulation. Die concentration and operational speed of transistors are rapidly increasing because of high market a demand, which can lead to thermal runaway complication and current crowding effect. The susceptibility of transistors to temperature change requires a more sensitive and accurate modeling for the thermal effects of the device. The heat source is the junction between base and lightly doped collector. This heat gets trapped within the device because of the presence of isolation oxide sidewalls and bottom oxide layer. The thermal impedance depends on the position of the heat source, its separation from the sidewalls and bottom oxide, as well as the thickness of the oxide wall and bottom. The thickness of the wafer also changes the thermal heating effect. The spreading resistance from the heat source to the sidewalls and bottom oxide, to the wafer, and then to ambient temperature has been calculated using the heat flow equation. The results are then compared to TCAD simulation. The mathematical model was within 10% when compared to the 3D TCAD simulation. The model presented in this work is based on an extension of the constant angle heat spreading, resulting in closed form expressions which can be used for practical applications. The electrical analogy developed from the thermal analysis can be used in VBIC, HICUM and MEXTRAM compact models, which are used to model the behavior of HBTs. Solder bump packaging results to the formation of a thermal equivalent transmission line for the heat flow from the heat source to the ambient temperature at the bumps. This paper analyses the effect of thermal heating when devices are connected using solder bumps. TCAD simulation is performed and mathematical model is developed to support the 3-D simulation result. The thermal impedance depends on the length of line from the heat source to the solder bumps, which is modeled by using electrical transmission line analogy. Closer the solder bump is to the device smaller is be thermal resistance, other tactics are discussed which can reduce the thermal resistance. An infinitely long line results in a static characteristic impedance for the line. The equivalent electrical analogy for the thermal transmission line is modeled which can be implemented in standard device modeling s like HICUM, VBIC or METRAM.

Disciplines

Electrical and Computer Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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