Design of multi-finger HBTs with a thermal–electrical model
Introduction
Heterojunction bipolar transistors (HBTs) for power applications usually employ a multiple emitter finger structure to improve the current handling capability [1]. Another advantage of this design is to reduce the out-of phase delay along the direction of emitter finger length. However, thermal coupling effects among emitter fingers result in a higher temperature at the center fingers. Because the emitter current has a positive temperature coefficient, the center fingers then conduct more current and consequently generate even more heat, which eventually gives rise to a “local hot spot” and possibly permanent device damage.
To alleviate the thermal effects in multi-finger HBTs, several techniques can be used, including emitter ballasting resistors [2], base ballasting resistors [3], the use of silicon substrate [4], the use of via holes, and variable emitter finger widths [5]. In addition, the non-uniform finger spacing is also expected to improve the temperature distribution effectively [6]. Compared to the ballasting resistor technique, the non-uniform finger spacing provides a better temperature distribution without the adverse effect on device speed. Design of these structures requires a thermal–electrical model to predict the electrical device behavior at a high temperature. There are several models available. For examples, Prof. Liou developed a 2-D model for graded heterojunction [7], and Dr. Gao derived a 3-D model for abrupt heterojunction [8]. In this paper, we present a 3-D thermal–electrical model for graded junction and InGaP/GaAs HBTs, and use it to design the non-uniform spacing of multi-finger structures.
Section snippets
Theoretical model
The thermal–electrical model described by Gao [8] is based on the well-known steady-state heat flow equation:where T(x,y,z) is the temperature at a point (x,y,z). As shown in Fig. 1(a), assuming the chip sizes in the directions of x, y, z are L, W, t, respectively, the boundary conditions can be expressed as follows.
The heat source is assumed to be located on the top surface (z=0), and p(x,y) denotes the power density on the
Results and discussion
For a 6-finger HBT with a 2×10-μm2 finger area, a uniform spacing of 22.9 μm (center-to-center) is assumed, which results in a higher temperature at the center fingers. When an optimized emitter ballasting resistor [2] is used, the overall device temperature is lowered, but the center fingers are still hotter than others. Using the thermal–electrical model, we have designed non-uniform spacing (15–27–30.5–27–15 μm) with unchanged structure in each finger and the same total spacing. For these
Conclusion
We have used a three-dimensional thermal–electrical model to design non-uniform spacing for multi-finger HBTs. The model is derived for graded AlGaAs/GaAs heterojunction, and also for InGaP/GaAs junction. Different design approaches are used for different number of fingers (6, 12, and 26). For a six-finger HBT, non-uniform spacing is applied to the entire device. For a 12-finger HBT, non-uniform spacing is only applied to outside fingers, and inside fingers remain uniformly spaced. For a
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Structure optimization of multi-finger power SiGe HBTs for thermal stability improvement
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2008, Solid-State ElectronicsCitation Excerpt :As an alternative method, the technology of non-uniform finger spacing has been proposed to lower the peak temperature and improve the non-uniformity of the temperature profile. For example, Yang-Hua Chang et al. [8] proposed a design of multi-finger InGaP/GaAs HBTs with the non-uniform finger spacing, and calculated temperature profiles at one bias condition, but no experimental result of the temperature profile was provided. Lee et al. [9] simulated temperature profiles of the AlGaAs/GaAs HBT, and the experimentally thermal regression loci of the device at one bias condition was shown, but no further experimental result of the temperature profile is given.
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