Two- and multi-terminal CMOS/BiCMOS Si LEDÕs Monuko du Plessis * , Herzl Aharoni 1 , Lukas W. Snyman 2 Carl and Emily Fuchs Institute for Microelectronics (CEFIM), Department of Electrical, Electronic and Computer Engineering, University of Pretoria, Pretoria 0002, South Africa Available online 12 October 2004 Abstract Silicon is an indirect bandgap material, but light emission is observed from reverse biased pn junctions. Even though the quan- tum efficiency is low, it may still be advantageous to use these devices in all-silicon optoelectronic integrated circuits (OICs). In this paper new research results with regard to low-voltage field emission BiCMOS and CMOS two- and multi-terminal Si LEDs are pre- sented. The differences observed between avalanche and low-voltage field emission LED performance are presented. It is shown that the low-voltage devices exhibit a square-law light intensity vs. reverse current non-linearity at low-current levels, but a linear depen- dency at higher currents, compared to the linear behaviour of avalanche devices at all current levels. The detail spectral character- istics of the field emission devices are investigated, showing that in the non-linear region of operation, the shape of the emitted spectrum changes, with reduced short wavelength generation at lower current levels. Bipolar junction transistor (BJT) multi- terminal devices are also discussed, and the square-law behaviour of these devices is presented. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Light emission from reverse biased Si junctions was reported for the first time in 1955 [1]. Several mecha- nisms have been proposed regarding the origin of the light generation [2–5], and it is still being investigated in order to fully understand the origin of the emission processes [6,7]. In recent years several technologies uti- lizing single crystal silicon junctions as practical light emitting diodes (Si LEDs) have been proposed. How- ever, the majority of these technologies make use of non-standard silicon IC technology, for example quan- tum confinement and superlattices, erbium impurity in silicon, porous silicon and nanoparticles [8]. Si LEDs will only become practical if they can be fully integrated with existing silicon integrated circuits, using standard IC technology. Therefore, we have decided to investigate the design and properties of silicon light emitting devices fabricated by conventional CMOS/BiCMOS IC techno- logy [9–11]. Field emission and avalanching processes in single-crystal silicon are inherently fast, and dynamic operation of two-terminal Si LEDs at frequencies in ex- cess of 10 GHz has been reported [12]. It was theoreti- cally shown [12] that for the distribution of digital clock signals on a chip, Si optical interconnects (in this case Si LEDs were used as sources, and taking the relatively low quantum efficiency into account) will be better than metal interconnects in terms of power when the total metal clock line capacitance is in excess of 16.7 pF, and the optical interconnect line delay will be better if the metal lines are longer than 5000 lm. Once Si LEDs can be made more efficient, they are ideal can- didates to act as optical sources in all-silicon optical integrated circuits. Large area avalanching Si LEDs with uniform light emission across the whole active area were some of the first devices designed by us [13]. For example, a typical 0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.08.063 * Corresponding author. E-mail address: mplessis@postino.up.ac.za (M. du Plessis). 1 Visiting professor from the Department of Electrical and Com- puter Engineering, Ben Gurion University of the Negev, Beer Sheba, Israel. 2 Present address: Department of Electronic Engineering, Tshwane University of Technology, Pretoria, South Africa. www.elsevier.com/locate/optmat Optical Materials 27 (2005) 1059–1063