Research On The Design Of Ge Composition Profile And Non-Uniform Emitter Finger Spacing To Improve The Thermal Performance Of Multi-Finger SIGE HBT

Power Heterojunction bipolar transistors (HBT) are playing an important role inmicrowave and power applications. In order to obtain high power, power HBTsusually employ multiple emitter fingers. However, the self-heating effects caused bythe power dissipation of the device and the thermal coupling effects among emitterfingers will result in a higher temperature at the center fingers. Because of the positivetemperature coefficient of emitter current, the center fingers will conduct more currentand consequently generate more heat, which eventually gives rise to a “local hot spot”and possibly permanent burnout. Therefore, the research on thermal effects ofmulti-finger HBT is paid more and more attention.In this dissertation, the two technologies,“bandgap engineering”(Ge composition)and non-uniform emitter finger spacing, is studied to enhance the thermalperformance of multi-finger SiGe HBT. Then, a new optimized device structure tofurther improve the thermal performance of multi-finger device was proposed by thecombination of the two technologies above. The main work is as follows:Firstly, the structure model of multi-finger SiGe HBT is established by use of theprocess simulation module of semiconductor device simulator SILVACO/ATHENA.Then the effect of “bandgap engineering” on the thermal characteristic of multi-fingerSiGe HBT is studied using SILVACO/ATLAS module.Under the same total base Ge amount condition, the vertical temperaturedistribution, the current gain β and the cut-off frequency fTand temperaturedependence of them for SiGe HBTs with different Ge gradient were given for the firsttime. The results show, as Ge fraction gradient increases, the peak temperature ofdevices is lowered; uniformity of temperature distribution becomes better. At thesame time, the fTincreases, β deceases, and the effect of temperature on β and fTisweakened. Based on these results, a novel Ge composition profile with thecombination of the uniform and graded Ge composition was proposed so as to make atrade-off among uniform temperature profile, gain and frequency characteristics.Secondly, in order to calculate accurately surface temperature distribution ofmulti-finger HBT, the emitter fingers were divided into a number of heat units, andthe thermal conductivity and the temperature are iterated, which makes thetemperature distribution obtained by solving steady-state heat conduction equationmore real and reasonable.On this basis, the thermal performance of multi-finger HBT designed by thenon-uniform emitter finger spacing is studied. The results indicate that the maximumjunction temperature is decreased significantly and the non-uniformity of surfacetemperature distribution is also improved greatly compared with that of HBT with uniform emitter finger spacing. All the results have been verified experimentally.Finally, a new multi-finger SiGe HBT structure with the combination of the“bandgap engineering”(Ge composition) and non-uniform emitter finger spacingtechnology was proposed. It is able to further take the thermal and electricalcharacteristics of SiGe HBT into account, and was validated by the commercialsemiconductor device simulator——SILVACO. The results indicate that the proposednovel structure SiGe HBT has not only better temperature profile uniformity, but alsothe better gain and frequency characteristics with insensitivity to temperature. Thesenew results provide valuable reference for design and manufacture of SiGe HBT witha ability of wide temperature range operation, high microwave power handlingcapability and thermal stability under high-power.