Avalanche-mode silicon LEDs for monolithic optical coupling in CMOS technology
Complementary Metal-Oxide-Semiconductor (CMOS) integrated circuit (IC) technology is the most commercially successful platform in modern electronic and control systems. So called "smart power" technologies such as Bipolar CMOS DMOS (BCD), combine the computational power of CMOS with high voltage transistors (~20-100 V) to enable the monolithic integration of advanced smart power applications used in e.g. automotive (car) applications, digital audio amplifiers, and integrated analog-digital systems. Many of such systems require data communication or signal transfer with galvanic isolation, for safety and interference reasons or to interface between low voltage digital parts and high voltage (power) components on an IC. Optocouplers transfer signals optically across a galvanically isolated channel. They can be operated for a wide range of data rates (including DC), and are less prone to external electromagnetic interference. Monolithic integration of such optocouplers in CMOS ICs require research and development of suitable light emitters and light detectors for an energy efficient, high speed, and cost effective operation of the system.
This PhD thesis covers two broad aspects. Firstly, it deals with the physics, design, and analysis of efficient light emitting diodes (LEDs) in silicon CMOS technology. Silicon LEDs conventionally emit infrared light (~1100 nm), which is not compatible with the spectral detection efficiency of silicon photo-detectors. This is because silicon can efficiently detect light having wavelengths of less than ~1000 nm. Therefore the focus is on a specific design solution to this problem, where the LED is biased in "avalanche breakdown". In this situation, there exists a high electric field in the device, which is responsible for light being emitted at shorter wavelengths (400 nm-900 nm). Such an emission, if properly guided laterally across the CMOS IC, would be detected by a silicon photodiode with a relatively high quantum efficiency. Wide-spectrum Si LEDs are promising for the integration of opto-electronics in CMOS.
Secondly, this thesis analyzes the feasibility of realizing a monolithic optical link using silicon LEDs in a BCD silicon-on-insulator (SOI) CMOS technology, from a device physics viewpoint. The optical coupling is treated as a conversion process from electrons to photons (in the LED) and back again to electrons (in the detector). Analysis is done from the viewpoint of coupling efficiency, where also the effect of heating across such a link due to high power dissipation in the avalanche-mode LED has been considered. Optical propagation via built-in waveguides in SOI technologies is also studied using Finite difference time-domain simulations. The analysis of this link is aimed at integrating avalanche-mode LEDs, which have the potential to be driven at high speeds (~ GHz), with single-photon sensitive optical detectors (e.g. using SPADs).
