A Novel Approach for Room-Temperature Intersubband Transition in GaN HEMT for Terahertz Applications

Terahertz (THz) technology has attracted tremendous attention recently due to its promising applications in various domains such as medical, biological, industrial imaging, broadband, safety, communication, radar, space science, and so on. Due to non-availability of powerful sources and highly sensitive and efficient detectors, the so-called THz gap remains largely unfilled. Despite seamless efforts from electronics and photonics technology researchers, the desired level of technology development to fill the THz gap still remains a challenge. GaN-based HEMT structures have been investigated as potential THz sources and detectors by a number of researchers. This chapter presents a very new and versatile mechanism for electrical tuning of intersubband transitions (ISBT) GaN high electron mobility transition (HEMT) devices. ISBT phenomena are usually demonstrated in photonic devices like a quantum cascade laser (QCL). Here we explore ISBT in an electronic GaN HEMT device. Conventional photonic devices like a QCL are operated at cryogenic temperature to minimize thermal effect. Tuning the conduction band through external gate bias is an advantage of an HEMT device for room temperature (RT) THz applications. This chapter demonstrates the theoretical and experimental novel ISBT phenomenon in GaN HEMT is for potential ambient applications in the THz range.

Absorption and Emission of Light in Quantum Structures

This chapter shows how to calculate the absorption coefficient, optical gain, and radiative recombination rates in quantum wells and superlattices. A detailed treatment of both interband and intersubband transitions is presented, and their differences and similarities are considered in detail. The optical properties of wurtzite quantum wells and zinc-blende quantum wires and dots are also discussed. Finally, the interaction of excitonic transitions with incident light in quantum wells is considered as a model for other two-dimensional materials.

Solar Cells, Thermophotovoltaics, and Nonlinear Devices Based on Quantum Wells

This chapter describes the basic principles behind the solar-cell operation using both an empirical picture and fundamental thermodynamic relationships. It considers how semiconductor materials are selected for use in solar cells and why materials with different gaps need to be stacked to improve the conversion efficiency. It also discusses advanced solar-cell concepts such as quantum-well, intermediate-band, and hot-carrier solar cells. Thermophotovoltaic devices that are similar to solar cells, but designed for emission peaks at much lower effective temperatures than the surface of the sun (and narrower gaps), are also discussed, and multistage thermophotovoltaic devices are described in detail. The chapter concludes by presenting the basic nonlinear physics of intersubband transitions in quantum wells, and how to take advantage of these physical principles for second-harmonic generation and difference-frequency mixing. The important application of generating THz emission from mid-IR quantum cascade lasers using difference-frequency mixing is emphasized.