Thermal Stability Analysis of RF Power HBT and Ballasting Resistor Optimum for the Self-heating Effect Compensation

In order to improve the thermal stability of RF power HBT and eliminate the self-heating effect which degenerates the transistor's electrical characteristics,the physical significance of thermal stability factor S is presented in detail based on the thermal-electric feedback network analysis.Furthermore,the expression of the minimum ballasting resistance R_C of HBT to compensate the self-heating effect(S=0) is presented by taking into account of the temperature dependence of emitter current,the valence-band discontinuity at emitter junction(△E_v),the bandgap narrowing due to heavy doping(△E_g),additional ballasting resistance in emitter and base.It is found that the higher the temperature T is,the smaller the minimum ballasting resistance R_C to compensate the self-heating effect is under the condition that(△E_v+△E_g)>2KT.Owing to the reducing of ballasting resistance,RF power HBT will provide higher output power,power gain,and poweradded efficiency(PAE) in an amplifier block.