Simulation Of Microwave Power SiGe HBTs For Improving Linearity

The SiGe (silicon-germanium) technology is used widely in the fields ofmicrowave power devices and RF integrated circuits because of its easily integratedfunction and excellent high frequency characteristics. And making high linearitymicrowave power SiGe HBTs (heterojunction bipolar transistors) has been a projectwith high practical meaning. The linearity problem of SiGe HBTs used on RFintegrated circuits or microwave power devices have been analyzed by someexperiments through studying the Ge-Profile of SiGe HBTs and building devicesmodel, and the IP3 of existing SiGe HBTs has reached about 25dBm. Butcomprehensive methods that can improve the linearity of SiGe HBTs from the anglesof devices design are still to be researched. To analyze the high frequency linearitycharacteristics of SiGe HBTs, a nonlinear model is built. And this model is also usedto deduce the formulae of harmonic currents which are induced by several kinds ofnonlinear factors, such as nonlinear base resistor RB, nonlinear base-collectorcapacitor CBC and nonlinear transconductance gm. Combing the Volterra series andCMNA (compacted modified nodal analysis) method to calculate the harmonicvoltages of the nodes of the model, and use the result to explain the nonlinearitycancellation phenomenon and to find the ways that can improve the linearity of themicrowave power SiGe HBTs. Designing reasonable simulation structures accords existing SiGe fabricationtechnologies. Calculate parameters of high frequency characteristics of the devicesthrough small signal AC simulation and transient simulation: the ? and ? T maxreachhigher than 20Ghz and 100Ghz respectively when the VCE of the device is 2.5V; thehighest IP3 (third-order intercept point) can reach 28dBm when the working frequencyis 1Ghz. Through a great deal of simulations and comparisons of IP3 among SiGeHBTs with different size or different doping concentration the followings can befound. 1) Augmenting emitter resistance RE can increase the minus feedback of thecircuit and reduce the nonlinearity of transcondutance gm, and then improve thelinearity of the device (the IP3 of the device whose RE is 0.7? is 1~1.5dBm higherthan the device whose RE is 0.2?). 2) The third-order harmonic current induced bybase resistance RB is in proportion to the high-order value of RB, so the IP3 isincreased by decreasing the doping concentration or by decreasing the base width. 3)When the electron density injecting into collector could not be ignored compared with - II -摘要the doping concentration, the CBC becomes the main factor of the nonlinearity of thedevice. At this time the space charge region will change with the change of currentdensity, the more the boundary of space charge region changes the worse the linearity.Through the outcome of simulations it is found that the IP3 of SiGe HBT whosecollector doping concentration is 5×1016cm-3 is 2~3dBm higher than the SiGe HBTwhose collector doping concentration is 4×1016cm-3. 4) Increasing the width ofcollector will make the current spreading effect more obvious, and then extend thespace charge region and make the linearity characteristic of the devices worse. 5) Theway that can decrease the Cbcx (extrinsic base-collector collector) by making trenchcan improve the linearity of the device when the current density is small. But whenthe current density increases, the linearity characteristic will decline because thecurrent in collector could not spread transversely. The simulations in this paper are combined with theory analysis all the time. Thedata of simulations and the analysis result will be helpful in designing microwavepower SiGe HBTs with high linearity used in modern wireless communication.