Design of an a-Si:H(n)/GaAs(p)/GaAs(n) high-gain heterojunction bipolar transistor with 10 GHz cut-off frequency F.G. Della Corte a,b, * , F. Pezzimenti b a Institute of Research on Electromagnetics and Electronic Devices – National Research Council (IRECE-CNR) Via Diocleziano 328, Naples I-80124, Italy b DIMET – University of Reggio Calabria, Loc. Feo di Vito, Reggio Calabria I-89100, Italy Abstract A simulation study aimed at designing a GaAs-based heterojunction bipolar transistor employing a wide gap emitter made of hydrogenated amorphous silicon (a-Si:H) with optimal high frequency characteristics is performed. The analysis indicates that the emitter thickness has a strong influence on the maximum device current, while the base thickness weakly affects the cut-off frequency. The role played by the electronic properties of the thin film amorphous emitter is discussed, leading to the conclusion that the device’s ac characteristics are principally limited by the poor carrier mobility typical of a-Si:H. However, a cut-off frequency close to 10 GHz can be predicted for an optimised device with standard values of the material electronic parameters. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 73.40.Kp; 73.61.Jc; 85.30.Pq 1. Introduction GaAsintegratedcircuits(ICs)arewidelyutilised in analog and digital devices for wireless commu- nication, optoelectronics and high speed comput- ing. Presently, the fabrication of these ICs involves either molecular beam epitaxy (MBE) or metal organic chemical vapour deposition (MOCVD), two outstanding epitaxial technologies in this field which, however, require expensive equipment. The AlGaAs(n)–GaAs(p)–GaAs(n) heterojunction bi- polar transistor (HBT) is a well known example of an application of these technologies [1], where the wide gap AlGaAs emitter permits the fabrication of devices with a high gain, which is in turn traded for larger bandwidths. Recently the possibility of exploiting hydrogenated amorphous silicon (a- Si:H), or other related materials, for the realisation of wide gap emitter regions in silicon-based bipolar junction transistors (BJT) has been demonstrated experimentally [2,3]. In the present work the characteristics of a new GaAs HBT with an a-Si:H emitter are investigated by means of numerical simulations. For a careful tuning of the electronic Journal of Non-Crystalline Solids 299–302 (2002) 1365–1369 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +39-081 570 7999; fax: +39-081 570 5734. E-mail addresses: dellacor@irece.na.cnr.it, f.dellacorte@ ieee.org (F.G. Della Corte). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(01)01101-2