Magnetotransport characterization of surface-treated InP/InGaAs heterojunction bipolar transistors Richard K. Oxland a, * , Andrew R. Long b , Faiz Rahman a a Department of Electronics and Electrical Engineering, University of Glasgow, Rankine Building, Oakfield Avenue, Glasgow G12 8LT, United Kingdom b Department of Physics and Astronomy, University of Glasgow, Kelvin Building, University Avenue, Glasgow G12 8QQ, United Kingdom article info Article history: Received 12 January 2009 Received in revised form 15 April 2009 Accepted 8 May 2009 Available online 18 May 2009 Keywords: Heterojunction bipolar transistor Magnetotransport measurements Transistor characteristics Surface passivation Device encapsulation Semiconductor sensor abstract The results of surface modification induced effects on InP/InGaAs single heterojunction bipolar transis- tors, as revealed by magnetotransport experiments, are described here. The surface treatments included both sulphur-based surface passivation and ion bombardment-induced surface damage. The former is known to improve device characteristics and the latter to degrade device operation. In this work the aim was to assess these techniques for tailoring device performance for surface sensing applications. Device characteristics were found to be sensitive to surface preparation prior to measurements. Measure- ments revealed that surface treatments that improve device performance also reduce sensitivity to exter- nal magnetic fields while treatments that degrade performance make devices more sensitive to externally applied magnetic fields. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Heterojunction bipolar transistors (HBTs) made from silicon– germanium alloys, gallium arsenide alloy systems and indium phosphide alloy systems have been much investigated over the past several years and continue to be a very vigorous field of semi- conductor device research [1,2]. The original impetus behind this research interest was their potential as very high speed transistors capable of operating in the tens to hundreds of GHz range. Indeed, during recent years these devices have been very successful in microwave communication systems employed in high frequency and wide bandwidth communication applications. Their outstand- ing characteristics such as the large heterostructure valence band offset at the emitter-base junction allows negligible carrier back- flow to the current injecting emitter region. This allows higher doping levels in the minority carrier-injected base region which, in turn, reduces the intrinsic base resistance and increases the maximum frequency for current gain (f T ) and the maximum switching frequency (f max ) figures. Because of the formation of the device layers through epitaxial growth processes such as molecular beam epitaxy (MBE) and metal organic chemical vapour deposition (MOCVD) the fabrication process of HBTs differs radi- cally from that of more traditional homojunction bipolar silicon transistors. Thus, whereas the latter posses a buried structure de- rived from successive ion implantation processes, HBTs almost invariable feature a stepped mesa structure where pre-grown doped layers are selectively isolated by wet or dry etching pro- cesses. As a consequence, the overall transistor structure is not strictly planar and some topographic relief develops at the site of the transistor. The inevitable result of this structure is that a con- siderable amount of doped region and junction periphery adjacent to the space charge region remains exposed to the ambient in these transistors. The exposed area is then subject to surface conduction effects and other non-idealities that would not appear in a buried junction device. The most prominent effect of exposed surface is the reduction of current gain seen in HBTs. Various solutions have been proposed to reduce the deleterious effects from exposed HBT surfaces. We have carried out detailed magnetotransport studies on InP/InGaAs HBTs with a liquid phase passivation treatment and with plasma-based surface modification. In this work we have paid particular attention to the post-passivation and post-plasma performance of HBTs and their potential as a sensor element on ac- count of the sensitivity of charge transport in these devices to pre- vailing surface conditions. The conduct and results of our experiments are described here. 2. Device structure The epitaxial structure of our InP/In 0.53 Ga 0.47 As single hetero- junction bipolar transistor device is shown here in Fig. 1. The large-emitter (75  75 lm) HBTs were fabricated using 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.05.007 * Corresponding author. Tel.: +44 0141 330 8631. E-mail address: r.oxland@elec.gla.ac.uk (R.K. Oxland). Microelectronic Engineering 86 (2009) 2432–2436 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee