JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 16, NO. 6, JUNE 1998 1101 Performance Enhancement of GaInP/GaAs Heterojunction Bipolar Phototransistors Using DC Base Bias Ravi Sridhara, Student Member, IEEE, Steven M. Frimel, Student Member, IEEE, Kenneth P. Roenker, Member, IEEE, Noren Pan, Senior Member, IEEE, and James Elliott Abstract—GaInP/GaAs heterojunction bipolar phototransistors grown by metal organic vapor phase epitaxy (MOVPE) and operated with frontside optical injection through the emitter are reported with high optical gain ( 88) and record high frequency performance (28 GHz). Heteropassivation of the extrinsic base surface is employed using a depleted GaInP emitter layer between the nonself-aligned base contact and the emitter mesa. The photo- transistor’s performance is shown to improve with increasing dc base bias in agreement with predictions of a recently reported Gummel–Poon model. Experimental results are reported for devices with optical active areas of 10 10 m 2 , 20 20 m 2 , and 30 30 m 2 , with peak measured cutoff frequencies of 28.5, 23.1, and 18.5 GHz, respectively, obtained at collector current densities between 2 10 3 and 6 10 3 A/cm 2 . Index Terms— GaInP/GaAs bipolar transistor, heterojunction bipolar transistor, optical detector, optical receiver, phototran- sistor. I. INTRODUCTION F OR monolithically integrated optical receivers based on heterojunction bipolar transistors (HBT’s), p-i-n pho- todiodes have been widely used for optical detection be- cause of their high quantum efficiency and compatibility with the HBT [1]–[4]. Frequently, the HBT’s base, collector and subcollector layers are employed to implement the p- i-n photodiode to reduce the complexity of the epitaxial growth and fabrication process integration. To enhance the p- i-n’s optical performance, the HBT’s epitaxial layer structure is sometimes modified, e.g., the collector doping reduced and the collector thickness increased. However, the former can produce the onset of base pushout in the HBT at low biases while the latter can degrade the carrier transit time across the intrinsic region and so degrade the p-i-n’s response speed. In addition, the capacitance of the p-i-n can limit the high frequency performance of the optical receiver [4]. Metal-semiconductor-metal (MSM) photodetectors have also been employed in HBT-based receivers [5], [6]. While the MSM’s low capacitance enables higher frequency receiver Manuscript received November 17, 1997; revised March 2, 1998. This work was supported by the Space Electronics Division of the NASA Lewis Research Center. S. M. Frimel was supported by a Doctoral Fellowship from the Ohio Space Grant Consortium. R. Sridhara, S. M. Frimel, and K. P. Roenker are with the Department of Electrical and Computer Engineering and Computer Science, University of Cincinnati, Cincinnati, OH 45221 USA. N. Pan and J. Elliott are with Kopin Corporation, Taunton, MA 02780 USA. Publisher Item Identifier S 0733-8724(98)04082-1. performance, the device’s responsivity is low due to significant reflection losses due to the electrodes. Heterojunction bipolar phototransistors (HPT’s) provide an alternative which provide optical gain in addition to photode- tection and are inherently compatible with the HBT’s epitaxial structure and fabrication. In addition, the HPT’s operation is closely related to the HBT’s so that tradeoffs in epitaxial layer design for photodetection versus HBT optimization are not necessary. Previously, the operation of the device with the base terminal floating (two terminal configuration) has been extensively examined [7], [8]. Recently, however, there have been several reports of HPT’s operated with a dc base bias which have shown enhanced device performance [9], [10]. The advantageous effects of base bias on the device’s performance have also recently been analyzed by Frimel and Roenker [11], [12] in the development of a modified Gummel–Poon model for the HPT. For short wavelength applications, such as local area net- works and optical interconnects in multichip modules, there have been several reports of AlGaAs/GaAs HBT-based optical receivers with most employing p-i-n photodetectors [5], [10], [13], but with a few demonstrating integration of HPT’s with HBT’s [10]. As an alternative to AlGaAs/GaAs HBT’s, in recent years GaInP/GaAs HBT’s have been proposed and developed [14]–[17]. GaInP-based HBT’s exhibit a number of desirable attributes including a larger energy bandgap but smaller conduction band offset, lower interface and surface recombination, low noise, and a more reliable, aluminum-free emitter [18], [19]. While GaInP/GaAs HBT’s have been under development in recent years, there have been few reports of the related HPT in this material system [20], [21]. In this paper we report the performance results for a GaInP/GaAs phototransistor (HPT) that is suitable for integration with HBT’s for short wavelength optical receivers, i.e., utilizing the epitaxial structure of a conventional high performance HBT. We report here the demonstration of GaInP/GaAs HPT’s where frontside optical injection through the emitter was employed which exhibit optical gain as high as 88 and record microwave performance up to a cutoff frequency of 28.5 GHz. Enhancement of the device is shown when operated in the three terminal configuration with a dc bias applied to the base. Comparison of the measured results is made with a recently reported theoretical model for the HPT [11], [12] with excellent agreement found. 0733–8724/98$10.00  1998 IEEE