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. >

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

A new theory is developed in this paper to explain the collapse of current gain in multi-finger power AlGaAs/GaAs Heterojunction Bipolar Transistors (HBT's). The reasons behind this unwanted phenomenon are fully clarified using a simple model to investigate the thermo-electrical interaction between the fingers. The existence of multi-value equilibrium points in model's constitutive equations is shown to be the necessary condition for the collapse of current gain to appear. For a N-finger device, N different patterns of collapse exist. The criterion to select the global stable pattern is given. The method has been used to predict the collapse in AlGaAs/GaAs HBT's and the agreement is excellent. The method also predicts that the collapse can happen far earlier than is normally expected in multi-finger high-power devices. The influence of ballasting resistance and thermal resistance is also investigated.

Analysis of thermal instability in multi-finger power AlGaAs/GaAs HBT's

Accurate modelling of the microwave large-signal characteristics of AlGaAs/GaAs Heterojunction Bipolar Transistors (HBT's) is extremely useful for microwave power applications of the device. This paper presents a new type of HBT large-signal model which is valid for dc, small-signal and large-signal ac modes of operation. The model may be used over a wide range of operating conditions and includes allowance for self-heating effects which are very important for HBT's. Through the use of several novel features, the proposed approach is differentiated from the traditional Ebers-Moll or Gummel-Poon BJT representations. The new model is accompanied by a very simple parameter extraction process requiring only a series of conventional dc and multi-bias point small-signal S-parameter measurements. Finally, the model is validated by independent power sweep measurements on HBT's from two different manufacturers. >

A new large-signal AlGaAs/GaAs HBT model including self-heating effects, with corresponding parameter-extraction procedure

High-bit-rate optical communication links require high performance circuits. Electrical time division multiplex (ETDM) single channel bit-rate of 40 Gb/s is at hand, due to recent progress in both technology and design methodology. Multilevel modulation format can be envisaged for ETDM transmission. An InP double heterojunction bipolar transistor technology is presented in this paper. The methodology used and tools developed with optical communications in mind are also discussed. Fabricated circuits are reported: 40 Gb/s multiplexer and demultiplexer, a 20 Gb/s driver, a 30 Gb/s selector-driver, a 22 Gb/s decision circuit, and a decision-decoding circuit for multilevel transmissions.

InP DHBT technology and design methodology for high-bit-rate optical communications circuits

An analytical description is developed which highlights the important physical parameters influencing the temperature dependence of the current gain in heterojunction bipolar transistors (HBTs). Each of the possible base current components is discussed and its relative importance to temperature dependence is assessed. The manifold nature of the contributions explains the widely different conditions under which negative differential resistance has been previously reported. Space charge region recombination is found to be the main contribution to temperature variation of current gain at low current densities. Factors affecting modern highly doped base devices at high current densities and elevated temperatures are reverse hole injection and base bulk recombination.

Analysis of the temperature dependence of current gain in heterojunction bipolar transistors

A DC thermal-electrical heterojunction bipolar transistor (HBT) model is presented. Only three parameters were needed to simulate completely an HBT with the self-heating effect. It can be very easily implemented in any CAD software which uses the SPICE bipolar junction transistor model. Parameter extraction has been carried out on measured data and good fits were obtained over a wide temperature range. This model can be used to design high-power heterojunction bipolar transistors and circuits with analysis of their thermal effects. >

Temperature dependence of DC currents in HBT

An interactive program for the extraction of transistor parameters has been developed using a SPICE-like circuit simulator instead of an explicit set of model equations. Consequently, many simulation capabilities (i.e. a variety of elements and their models, analyses and simulation modes) are available at the extractor level. Several optimisation methods are integrated into the program to provide robust as well as efficient fitting of device characteristics. Analysis of heterojunction bipolar transistor self-heating effects is presented in greater detail as an example application. >

C. Algani

Papers

Temperature dependence of DC currents in HBT

Simulation-based parameter extraction, its implementation and some applications