IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 8, AUGUST 2005 1693 High-Performance InGaP/GaAs HBTs With Compositionally Graded Bases Grown by Solid-Source MBE Jin Hyoun Joe and Mohamed Missous, Senior Member, IEEE Abstract—N-p-n InGaP/GaAs heterojunction bipolar transis- tors (HBTs) with compositionally graded In Ga As (Be doped) bases have been successfully grown by solid-source molecular beam Epitaxy (SSMBE) using a gallium phosphide (GaP) de- composition source. In this paper, the dc and RF characteristics of HBTs with different indium mole fractions in the graded In Ga As base ( and ) are measured to investigate optimum-grading profiles. The measured average current gains, s of a control sample, a 10% graded-base sample and a 5% graded-base sample, are 162, 397 and 362, respectively. To our knowledge, these current gains are the highest values ever reported in compositionally graded-base InGaP/GaAs HBTs with a base sheet resistance of /sq establishing a new benchmark for InGaP/GaAs HBTs. Furthermore, these compositionally graded-base HBTs show higher unity current/gain cutoff frequency, and maximum oscillation frequency, . Compared to the control sample with the same base thickness, the base transit time of the graded sample is reduced by % to % by the induced built-in potential, resulting in an increase of from 16 to 18.5 GHz in a device with an emitter size of 10 10 m . Additionally, for the 5% graded-base sample, with a 5 5 m emitter region, and are 16.3 and 33.8 GHz, respectively, under low-level collector current. These results demonstrate that InGaP/GaAs HBTs with In Ga As graded-base layers have the potential for high-speed analogue to digital converters. Index Terms—Heterojunction bipolar transistor (HBT), induced built-in potential, InGaP/GaAs, In Ga As. I. INTRODUCTION T HE InGaP/GaAs material system has shown better per- formance compared to the AlGaAs/GaAs material system due to its favorable band lineup at the interface (conduction band discontinuity, eV [1] and valence band disconti- nuity, eV [2]), relatively inert surface, and fabrica- tion reproducibility due to the high etching selectivity between InGaP and GaAs. In addition, its low noise performance and its high frequency capabilities make it an ideal candidate for wire- less and high-speed devices [3], [4]. However, there exists a drawback in the InGaP/GaAs mate- rial system. For portable applications, the GaAs base layer is a limiting factor due to its large energy bandgap (1.424 eV) Manuscript received February 11, 2005; revised May 10, 2005. The review of this paper was arranged by Editor C.-P. Lee. The authors are with the Microelectronics and Nanostructures Group, School of Electrical and Electronic Engineering, The University of Manchester, Manchester M60 1QD, U.K. (e-mail: J.Joe@postgrad.manchester.ac.uk; m.missous@manchester.ac.uk). Digital Object Identifier 10.1109/TED.2005.852175 and Fermi-level pinning at the free surface, which increase the power dissipation. To overcome theses disadvantages, indium incorporation of up to a 10% grading in the base was applied in this paper. This led to a decrease of the native defects as well as the bandgap in the base. Other issues concern the realization of high-speed and high- gain devices. High-speed calls for high base doping but this must be managed to minimize the diffusion of p-dopants in the base during MBE growth and thus low temperature growth techniques (which still preserve crystal electronic quality) are essential. Finally, the important issues for both research and industrial production are safety and minimum contamination during epitaxial layer growth. To grow high-quality phosphorus-con- taining epitaxial layers such as InGaP and InP, it is essential to generate a P (dimer phosphorus) beam rather than a P (tetramer phosphorus) beam. This is due to its higher sticking coefficient and safety as a source for use in conventional molec- ular beam epitaxy (MBE) systems [5]. P can be generated by a phosphine PH source in metal–organic chemical vapor deposition, by a red phosphorus P source in MBE, or by a GaP decomposition source in MBE [6]. However, both PH and P sources are not ideal due to their high toxicity and contamination behavior. Additionally, P needs cracking at high temperature ( – C) in order to generate P . The GaP decomposition source was used in this work for growth at low temperature, to minimize base dopant diffusion while still generating high-quality material [5]. The GaP source generates a very pure beam of P , compared to phosphorus crackers which produce a mix of P and P . The very thin, compositionally graded-base layer (which was first proposed by Kroemer [2]) reduces the probability of recombination processes at the base surface and supplies a built-in potential in the base region that enhances the energy of injected carriers and changes the effective band discontinu- ities, resulting in a high-gain and high-speed device [7]. The In Ga As graded base has enormous advantages compared to Al Ga As graded base and conventional GaAs base [8], [9]. First, the mobility of carriers in In Ga As is higher than either that of GaAs or Al Ga As graded layers. Second, the In Ga As graded base does not change the valence band dis- continuity much at the interface in contrast with Al Ga As graded bases. Compared to the control sample with the same base thick- ness, the compositionally graded-base HBTs reported here have higher dc current gain by a factor of 2 and a 23% lower 0018-9383/$20.00 © 2005 IEEE