Multi-finger Heterojunction Bipolar Transistors Research Articles

Analytical model for the AlGaAs/GaAs multiemitter finger HBT including self-heating and thermal coupling effects

An analytical model which can be used to predict the thermal as well as electronic behaviour of the multiple emitter heterojunction bipolar transistor (HBT) is presented. The model is developed from a knowledge of device make-up (doping concentrations, layer thicknesses etc.), and relevant physics (such as the effects of graded heterojunction, self-heating, thermal coupling and ballast emitter resistance) is included in a unified manner. Thermal runaway phenomenon observed in the multifinger HBT at high current levels has been successfully described. Experimental evidence obtained from six-finger and four-finger HBTs are included in support of the model. The thermal runaway phenomenon is caused by the uneven increase of the base and collector currents at elevated temperatures due to the thermal effect.

Transient Response of Junction Temperature in Power Heterojunction Bipolar Transistors

We present a theoretical analysis for calculating the transient device junction temperature as a function of position and time upon a stepwise increase of power dissipation. The derivation applies in particular to multifinger heterojunction bipolar transistors designed for microwave power application. It is determined that for a typical substrate thickness of 100 µm, the rise time of the junction temperature is roughly 3×10-7 s.

Theoretical calculations of temperature and current profiles in multi-finger heterojunction bipolar transistors

A theoretical analysis is presented to determine both the temperature profile and current distribution among the fingers for a multi-finger heterojunction bipolar transistor designed for microwave power applications. This analysis allows for arbitrary transistor geometries and non-uniform emitter ballast resistors in the fingers. The effects of varying ballast resistances, emitter width, emitter length, and spacing between the fingers will be discussed.

Current gain collapse in microwave multifinger heterojunction bipolar transistors operated at very high power densities

The rapid development of heterojunction bipolar transistor (HBT) technologies has led to the demonstration of high power single-chip microwave amplifiers. Because HBTs are operated at high power densities, the ultimate limits on the performance of HBTs are imposed by thermal considerations. The authors address a thermal phenomenon observed when a multifinger power HBT is operating at high power densities. This phenomenon, referred to as the collapse (of current gain), occurs when suddenly one finger of the HBT draws most of the collector current, leading to an abrupt decrease of current gain. A quantitative model and the condition separating the normal operation region and the collapse are presented. Critical difference of the collapse in the constant l/sub b/ and constant V/sub be/ modes of operation is discussed for the common-emitter l-V characteristics. The collapse in the common-base l-V characteristics and its relationship with avalanche breakdown are also described. A solution to eliminate the collapse is experimentally verified. >