Abstract: Taking into account of the temperature dependenee of base-emitter voltage and the additional ballasting resistor in emitter.an improved three.dimensional thermal.electrical model is presented to study the temperature distribution in power heteroiunction bipolar transistor(HBT).It is found that multi-finger HBT with uniform emitter ballasting resistor exhibits a higher temperature at the center of the device，which reduces the power.handing capability of HBT.Therefore，non-uniform emitter ballasting resistor design of multi-finger power HBT is presented to improve the power-handing capability of HBT. Taking 12-finger Si0.8 Ge0.2 HBT for example，the design procedure of non-uniform emitter ballasting resistor is described in detail.Compared with the uniform design，the center finger temperature reduces obviously and the device temperature distri- bution is uniform with the design of non.uniform emitter ballasting resistor under the condition that the total emitter ballast- ing resistance is the same. Furthermore,it is found that the temperature distribution is more uniform with the exponential variation of emitter ballasting resistance. As a result, the power-handing capability of HBT is improved obviously。which is beneficial to the design of power HBT.

Key words: heterojunction bipolar transistor, thermal·electrical model, emitter ballasting resistor