Rf Power Hbt Self-heating Compensation Method

With the downscaling of the chip, the introduction of the silicon-on-insulator (SOI)technology, and the application of the trench-eteching technology, the thermal resistanceincrease continually. The self-heating effect and the electrothermal coupling effectbecome more and more serious. The self-heating effect degenerate the transistor'selectrics characteristics (such as current gain, power gain, fT and so on), cause the shift inDC bias point in transistor and the circuit, and even lead to the thermal runaway oftransistor and the circuit. In this thesis, thermal stability of the RF power HBT isinvestigated.First of all, we analysis the E-B negative slope of the power transistor, from whichwe could see that IC-VBE characterstis was affected seriously by the self-heating effect.Effects of various factors, such as thermal resistance (Rth), the voltage ofcollector-emitter (VCE), common emitter current gain (β) and the valence-banddiscontinuity at emitter junction (?EV), on the I-V characteristics of the HBT areinvestigated. Furthermore, the improvement of thermal stability with ballast resistanceused for compensating self-heating effect is studied. Results show that REmin of theheterojunction bipolar transistor (HBT) is smaller than that of the bipolar junctiontransistor (BJT). Therefore, HBT will provide higher output power, power gain, andpower-added efficiency (PAE) in an amplifier block. For HBT, the smaller β, Rth, VCE,and ?EV are, the better thermal stability of HBT is, and the smaller the REmin is.Secondly, we express the thermal stability of the transistor based on the thermalstability factor S. We explain the physical signification of the thermal stability factor S indetail. Taking into account of the temperature dependence of emitter current, thevalence-band discontinuity at emitter junction (?EV), the bandgap narrowing due toheavy doping (?Eg), additional ballasting resistance in emitter and base (RB and RE), anew analytical expression of the thermal stability factor S of power heterojunctionbipolar transistor (HBT) have been derived completely based on the analysis ofthermoelectric feedback network. This expression can be used to analyze the thermalstability of power HBT conveniently and explicitly. The thermal stability of the transistorcould be improved obviously by adding either base or emitter ballast resistance. Theresult shows that the stability of power HBT is superior to that of BJT. By selecting RBand RE properly, S could be equal to zero, and therefore, self-heating effect induced bythe internal power dissipation of the device itself could be compensated completely. As aresult, the electrical characteristic of device could be kept, and don't shift withself-heating effect. This couldn't be achieved in BJT. At the same time, the smaller β, Rth,VCE and ?EV are, the better thermal stability of HBT is. Compare with BJT, the ballastresistance of HBT is smaller than that of BJT under the same thermal stability factor S,because of the existence of the heterojunction (?E). Hence, HBT will provide higheroutput power, power gain, and power-added efficiency (PAE) in an amplifier block.In multi-finger HBT, besides the self-heating effect in each finger, there are alsoelectrothermal coupling effects among emitter fingers. The power multi-finger HBTheat-conduction model in three dimensions is present in this thesis. By usingself-consistency iterative method, we get the steady thermal distribution of the chip. Theresult simulated by Matlab indicates that the space-adjusted and length-adjusted deviceshave better thermal stability than that of the traditional devices. Therefore, the outputpower is enhanced effectively by utilizing the space-adjusted and length-adjustedstructure, at the same time, the power-handing capability can also be increased.According to the technology of institute of microelectronics, Tsinghua University,we design the device structure, mask plates and budget fabrication processes to fabricateRF power SiGe HBT.The design of the device structure (including space-adjusted and length-adjusteddesign) and the utilizing the character of heterojunction (?E) is present to compensate theself-heating effect and electrothermal coupling effect in the device, which makessignificant sense in both theory and practice to design and fabricate RF power SiGe HBTwith higher output power, power gain, and power-added efficiency (PAE) in an amplifierblock.