# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 1 Introduction  
The exponential growth in the information volume on networks including the Internet, with fiber-to-the-home or the fiber-to-the-premises, increasingly demands compact, power-efficient, and fast data processing. This has led to the continuous development of modern electronic devices with large data capacities, high functionalities, and high-speed operations. However, the current technologies of semiconductor devices and electronically integrated circuits are rapidly approaching their fundamental limits, in terms of both speed and data transmission bandwidth [1; 2; 3]. In addition, an increase in the transmission capacity requires greater power consumption, resulting in an energy-cost issue. To address this issue, optical technology is a promising solution, offering several intrinsic advantages. For example, light as signal carriers can deliver information with the speed of light, which allows ultrafast device operation. In addition, the optical network enables wavelength division multiplexing (WDM), which is suitable for high-capacity transmission [4]. Moreover, optical transmission is expected to consume less power per bit than electrical transmission. Therefore, tremendous research efforts have been directed toward the development of essential photonic elements and their integration, as well as miniaturization, for ultracompact photonic integrated circuits. Among the various photonic elements, the most essential one is a semiconductor light-emitting device such as light-emitting diodes (LEDs) and lasers. These devices generate high-quality beams of light and serve as a robust light source in an integrated circuit. The light-emitting devices generally employ various shapes of micro-sized or sub-micrometer-sized optical cavities, not only because the devices with a small active region can operate in a power-efficient manner, but also because the small physical footprint enables an increase in integration density [4]. For decades, researchers have proposed several micro- and nano-scale semiconductor cavities, investigated their optical properties, and characterized unique features using the optical excitation scheme [5; 6; 7]. However, optical pumping inevitably exhibits several inherent issues, the most critical being impracticality. To drive a photonic circuit that includes an optically pumped internal light source, there must be an external bulk light source outside the circuit, and a separate optical setup to deliver the excitation energy to the internalsource. In addition, the pump efficiency is impractically low for small and/or asymmetrical optical cavities (e.g., one-dimensional (1D) photonic crystals (PhCs), nanowires (NWs), nanotubes (NTs), and waveguide cavities). Although there are some advanced techniques to spatially modulate the shape of an excitation beam for controlled and adaptive pumping, it requires another complicated optical setup [8; 9; 10]. Furthermore, the simultaneous excitation of multiple light sources over a large area in a single chip is inherently impossible, which can seriously limit the key functions of integrated circuits (e.g., WDM). Therefore, the realization of electrically pumped, power-efficient, ultra-small light sources is essential for the practical applications of compact photonic integrated circuits. In this paper, we review the recent development in electrically driven micro- and nano-scale semiconductor light sources (i.e., lasers and LEDs), mainly focusing on various designs of optical cavities and corresponding carrier injection schemes. In addition, we explain how critically the cavity design and carrier injection schemes are related by describing several important and key parameters of cavity-based light sources such as quality factor, mode volume, and threshold current. In each section we describe the details of mechanisms for the simultaneous confinement of both electrical carriers and generated photons in the vicinity of the considered optical cavities.