L.W. Snyman

An efficient low voltage, high frequency silicon CMOS light emitting device and electro-optical interface

A silicon light emitting device was designed and realized utilizing a standard 2-/spl mu/m industrial CMOS technology design and processing procedure. The device and its associated driving circuitry were integrated in a CMOS integrated circuit and can interface with a multimode optical fiber. The device delivers 8 nW of optical power (450-850 nm wavelength) per 20-/spl mu/m diameter of chip area at 4.0 V and 5 mA. The device emits light by means of a surface assisted Zener breakdown process that occurs laterally between concentrically arranged highly doped n/sup +/ rings and a p/sup +/ centroid, which are all coplanarly arranged with an optically transparent Si-SiO/sub 2/ interface. Theoretical and experimental determinations with capacitances and series resistances indicate that the device has an intrinsic high-frequency operating capability into the near gigahertz range.

Increased efficiency of silicon light-emitting diodes in a standard 1.2-μm silicon complementary metal oxide semiconductor technology

Scaled versions of a variety of silicon light-emitting diode elements (Si LEDs) have been realized using a standard 1.2-µm, doublepolysilicon, double-metal, n-well CMOS fabrication process. The devices operated with a n+p junction biased in the avalanche breakdown mode and were realized by using standard features of the ORBIT FORESIGHT design rules. The elements emit optical radiation in a broad band in the 450- to 850-nm range. An emitted intensity (radiant exitance) of up to 7.1 µW/cm2 (or about 8 nW per 60-µm-diam chip area) has been obtained with 5 mA of current at an operating voltage of 18.5 V. Excellent uniformity in emission intensity of better than 1% variation was obtained over areas as large as 100x500 µm. A best power conversion efficiency of 8.7x 10-8 and a quantum efficiency of 7.8x 10-7 were measured. All of these values are about one order of magnitude better than previously reported values for Si LED avalanche devices. Coupling between the elements as well as electro-optical coupling between an element and an optical fiber was realized.

Silicon LEDs fabricated in standard VLSI technology as components for all silicon monolithic integrated optoelectronic systems

It is shown that, by using conventional VLSI design rules and device processing, a variety of two terminal and multiterminal integrated silicon light-emitting devices (Si-LEDs) can be routinely fabricated without any adaptation to the process, enabling the production of all-silicon monolithic optoelectronic systems. Their specific performance can be tailored by their different geometries and structures, yielding, by design, area, line, and point light-emitting patterns. The light-generating mechanisms are based on carrier quantum transitions in Si pn junctions, operated in the field emission or avalanche modes. Field emission Si-LEDs can operate at supply voltages compatible with those of integrated circuits (5 V or less). Avalanche Si-LEDs require higher operating voltages, but yield higher light intensities. The two terminal Si-LEDs yield a linear relation between the emitted light intensity and the driving current. The multiterminal Si-LEDs exhibit a nonlinear relation between the light emission intensity and the controlling electrical signal, enabling signal processing operations, which can not be attained in two terminal Si-LEDs. Two basic structures of multi terminal Si-LEDs are presented, i.e MOS-like structures, or carrier injection based structures (BJT-like devices). They possess different input impedances and both their emitted light intensities and emitting area patterns can be controlled by the input electrical signal.

A silicon transconductance light emitting device (TRANSLED)

A novel multi-terminal silicon light emitting device (TRANSLED) is described where both the light intensity and spatial light pattern of the device are controlled by an insulated MOS gate voltage. This presents a major advantage over two terminals Si-LEDs, which require direct modulation of the relatively high avalanche current. It is found that, depending on the bias conditions, the light intensity is either a linear or a quadratic function of the applied gate voltage. The nonlinear relationship facilitates new applications such as the mixing of electrical input signals and modulating the optical output signal, which cannot readily be achieved with two terminal Si-LEDs, since they exhibit a linear relationship between diode avalanche current and light intensity. Furthermore, the control gate voltage can also modulate the emission pattern of the light emitting regions, for example, changing the TRANSLED from an optical line source to two point sources.

A dependency of quantum efficiency of silicon CMOS n/sup +/pp/sup +/ LEDs on current density

A dependency of quantum efficiency of nn/sup +/pp/sup +/ silicon complementary metal-oxide-semiconductor integrated light-emitting devices on the current density through the active device areas is demonstrated. It was observed that an increase in current density from 1.6/spl times/10/sup +2/ to 2.2/spl times/10/sup +4/ A/spl middot/cm/sup -2/ through the active regions of silicon n/sup +/pp/sup +/ light-emitting diodes results in an increase in the external quantum efficiency from 1.6/spl times/10/sup -7/ to 5.8/spl times/10/sup -6/ (approximately two orders of magnitude). The light intensity correspondingly increase from 10/sup -6/ to 10/sup -1/ W/spl middot/cm/sup -2//spl middot/mA (approximately five orders of magnitude). In our study, the highest efficiency device operate in the p-n junction reverse bias avalanche mode and utilize current density increase by means of vertical and lateral electrical field confinement at a wedge-shaped n/sup +/ tip placed in a region of lower doping density and opposite highly conductive p/sup +/ regions.

Spatial and intensity modulation of light emission from a silicon LED matrix

A novel experimental multiterminal silicon light emitting diode matrix is described, where both the emitted light intensity and the spatial light pattern of the device are controlled by insulated metal-oxide-semiconductor (MOS) gate voltages. It is found that the light intensity is a nonlinear quadratic function of the applied gate voltage. The nonlinear relationship enables, for example, the mixing of electrical input signals and modulation of the optical output signal, which can not readily be achieved with two terminal Si-light-emitting diodes, since they exhibit a linear relationship between diode avalanche current and light intensity.

Characterization of breakdown phenomena in light emitting silicon n+p diodes

A 10 kV, 10−10u2009A focused electron beam was used to map localized charge multiplication, and localized avalanche breakdown sites in the depletion region of light emitting silicon n+p junctions. It was observed that the localized avalanche breakdown sites led to the formation of large densities of extremely small current filaments in the junction. The dimension of each filament was well into the submicron range (100–150 nm in diameter) and the densities of the filaments varied laterally across the junction between 4×108 and 4×1010u2009cm−2. Accurate and high resolution maps of the current filaments could be obtained. The formation of the individual current filaments (microavalanche sites) at a submicron level are apparently related to localized defect and materials effects. The distribution of the current filaments at the micron level relates to the strength and distribution of electric field at the junction interface. Direct evidence has been obtained that the nonuniformities in the light emission patterns on ...

Optical sources, integrated optical detectors and optical waveguides in standard silicon CMOS integrated circuitry

A series of light emitting devices were designed and realized with a standard 2 micron CMOS technology, 1.2 micron CMOS technology and 0.8 micron Bi-CMOS integrated circuit fabrication technology. The devices operated in the reverse breakdown avalanche mode, at voltage levels of 8 - 20 V and in the current range 80 (mu) A - 10 mA. The devices emit visible light in the 450 - 750 nm wavelength region at intensity levels of up to 1 nWmicrometers -2 (10 mW.cm-2). A series of optimized optical detectors were developed using the same technologies in order to detect lateral and glancing incidence visible and infrared radiation optimally. A series of waveguiding structures of up to 100 micron in length were designed and realized with CMOS technologies by utilizing the field oxide, the inter- metallic oxides and the aluminum metal layers as construction elements. Signal levels ranging from 60 nA to 1 micro-amperes could be detected at the detectors of waveguiding structures of up to 100 micron in length. Finally, a complete optoelectronic integrated circuit was designed and simulated with 0.8 micron Bi-CMOS technology with some of the developed light sources, detectors, waveguiding structures and added driving and amplification circuitry. In particular a very powerful high gain wide- bandwidth MOSFET signal amplifiers was developed that could be successfully integrated in the optoelectronic integrated circuit. The developed technologies show potential for application of optoelectronic circuits in next generation silicon CMOS integrated circuits.

Silicon light emitting devices in standard CMOS technology

Photon emission from reverse biased silicon pn junctions was reported for the first time in 1955. However, Si-LEDs will only find applications if they can be fully integrated with standard silicon integrated circuits, and several attempts have been made in this regard. This paper discusses the characteristics and design of silicon light emitting devices in standard CMOS technology with no process modifications.

Practical Si LED's with standard CMOS technology

Multi-junction silicon light emitting devices (Si LEDs) were designed and realised by using standard 1.2 micron and 2 micron CMOS processes with a bipolar capability and with no modifications to the processes. The designs were optimised to increase the power conversion efficiency, quantum conversion efficiency, intensity of emission and also the uniformity of emission. The devices emit light of several nW per 5 to 10 mA at 4-30 V in the 450 to 850 nm wavelength range. All the devices operated with at least one pn junction in the field emission or avalanche breakdown mode. Quantum conversion efficiencies of up to 1.5/spl times/10/sup -8/ have been measured which is two and a half orders to three orders of magnitude higher than previously published values for light emission from Si p-n avalanching junctions. Some directional light emission characteristics were also observed. The developed devices are viable for on-chip electro-optical applications and also for high speed chip-to-environment electro-optical applications.