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 100-element 10-GHz grid amplifier has been developed. The active devices in the grid are chips with heterojunction-bipolar-transistor (HBT) differential pairs. The metal grid pattern was empirically designed to provide effective coupling between the HBTs and free space. Two independent measurements, one with focusing lenses and the other without, were used to characterize the grid. In each case, the peak gain was 10 dB at 10 GHz with a 3-dB bandwidth of 1 GHz. The input and output return losses were better than 15 dB at 10 GHz. The maximum output power was 450 mW, and the minimum noise figure was 7 dB. By varying the bias, a signal could be amplitude modulated with a modulation index as large as 0.65. Tests show that the grid was quite tolerant of failures-the output power dropped by only 1 dB when 10% of the inputs were detuned. The grid amplifier is a multimode device that amplifies beams of different shapes and angles. Beams with incidence angles up to 30 degrees were amplified with less than a 3-dB drop in gain. >

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A 100-element HBT grid amplifier

A complete DC model for the heterojunction bipolar transistor (HBT) is presented. The DC characteristics of the HBT are compared with the Ebers-Moll (EM) model used by conventional bipolar junction transistors (BJTs) and implemented in simulation and modeling programs. It is shown that although the details of HBT operation can differ markedly from those of a BJT, a model and a parameter extraction technique can be developed which have physical meaning and are exactly compatible with the EM models widely used for BJTs. Device I-V measurements at 77 and 300 K are used to analyze the HBT physical device performance in the context of an EM model. A technique is developed to extract the device base, emitter, and collector series resistances directly from the measured I-V data without requiring an ideal exp(qV/sub be//kT) base current as reference. Accuracies of the extracted series resistances are assessed. AC parameters of HBT are calculated numerically from the physical device structure. For modeling purposes, these parameters are shown to be comparable with those of conventional BJTs. >

The DC characteristics of GaAs/AlGaAs heterojunction bipolar transistors with application to device modeling

One undesirable phenomenon observed when AlGaAs/GaAs heterojunction bipolar transistors (HBT's) are operated under high power density is the collapse (of current gain). The collapse manifests itself by a distinct abrupt decrease of collector current in the transistor common-emitter current-voltage (I-V) characteristics. In this investigation, we study the substrate temperature dependence of the collapse. A unified equation is introduced to relate the collapse instability criterion with other thermal instability criteria proposed for silicon bipolar transistors. The effects of the thermal instability on the collapse behavior of 2-finger and 1-finger HBT's are examined. We also present a numerical model to adequately describe the collapse in multi-finger HBT's having arbitrary geometry. The I-V characteristics and regression plots of both ballasted and unballasted HBT's are compared. >

The collapse of current gain in multi-finger heterojunction bipolar transistors: its substrate temperature dependence, instability criteria, and modeling

We have studied the photoreflectance spectra at 300 K from a number of GaAs/Ga1−x AlxAs heterojunction bipolar transistor (HBT) structures grown by molecular beam epitaxy and metalorganic chemical vapor deposition. From the observed Franz–Keldysh oscillations we have been able to evaluate the built‐in dc electric fields Fdc in the Ga1−x Alx As emitter as well as the n−‐GaAs collector region. In addition, the Ga1−x Alx As band gap (and hence Al composition) has been determined. The obtained values of Fdc are in good agreement with numerically computed values for the analyzed HBT structures, thus making it possible to deduce doping levels in these sections.

Characterization of GaAs/Ga1−xAlxAs heterojunction bipolar transistor structures using photoreflectance

Issues important for the manufacturing of GaAlAs/GaAs heterojunction bipolar transistors (HBTs) and their prospects for application in various areas are discussed. The microwave and digital performance status of HBTs is reviewed. Extrapolated values of maximum frequency of oscillation above 200 GHz and frequency divider operation at 26.9 GHz are reported. Key prospects for further device development are highlighted. >

G. J. Sullivan

Papers

GaAlAs/GaAs heterojunction bipolar transistors. Issues and prospects for application

GaAlAs/GaAs Heterojunction Bipolar Transistors: Issues and Prospects for Application