Top1. Introduction
Low-dimensional semiconductors (LDSs) are semiconductor structures in which the carrier transport is restricted in 2-dimensions (quantum well) or 1-dimension (quantum wires) or even zero-dimension (quantum dots).
A quantum well is a thin layer which can confine particles (typically electrons or holes) in the dimension perpendicular to the layer surface, whereas the movement in the other dimensions is not restricted. Quantum wells are formed in semiconductors by having a material, like GaAs sandwiched between two layers of a material with a wider bandgap, like AlAs. These structures can be grown by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) with control of the layer thickness down to monolayers. Thin metal films can also support quantum well states. Because of their quasi-two dimensional (Q2D) nature, electrons in quantum wells have a density of states as a function of energy that has distinct steps. Additionally, the effective mass of holes in the valence band is more closely matching that of electrons in the conduction band. These factors lead to better performance of quantum wells in optical devices such as laser diodes. They are also used to make HEMTs (High Electron Mobility Transistors), which are used in RF low-noise electronics. Quantum well infrared photodetectors are also based on quantum wells, and are used for infrared imaging. By doping either the quantum well, or the barrier with donor impurities, a two-dimensional electron gas (2DEG) may be formed. Such a structure forms the conducting channel of a HEMT, and has interesting properties at low temperature. Nowadays, the layered LDS materials with high carrier mobility are highly desirable in emerging nanoelectronic devices. For instance, the monolayers of germanium monosulfide (GeS) and molybdenum disulphide (MoS2) are currently considered for the next generation of MOSFET transistors (Tomaneck et al, 2015).
Upon completion of this Chapter, the reader will be able to:
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Understand the notion of low-dimensional semiconductors (LDS) and define their main types.
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Differentiate between bulk (3-D) and low-dimensional structures (quantum wells Q2D, quantum wires Q1D and quantum dots Q0D).
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Explain the quantum band structure and density of states in LDS.
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Explain the charge carrier statistics in LDS.
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Explain the quantum transport mechanisms and models in LDS.
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Explain the concepts of a ballistic quantum transport and Landauer formulae.
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Explain the concepts of a quantum blockade and Kondo Effect.
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Describe the LDS-based devices, their properties and applications.
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Calculate the I-V characteristics of graphene FET transistor
Top2. Introduction To Low-Dimensional Semiconductors (Lds)
We have learned so far that conduction electrons in the bulk of a semiconductor can move freely, in all the three dimensions of the physical space. However, in certain semiconductor devices, free electrons are only permitted to move in one or two dimensions. Such semiconductors, are called low-dimensional semiconductors (LDS’s). For instance, the electrons filling the thin inversion layer in a conventional MOSFET (Metal-Oxide-semiconductor Field Effect Transistor) are only permitted to move freely in the 2 dimensions of the inversion layer. The motion of free electrons in the third dimension (which is normal to the thin layer plane) is confined and quantized.