Instabilities associated with hot electrons in semiconductors have been investigated from the beginning of transistor physics in the 194Os. The study of NDR and impact ionization in bulk material led to devices like the Gunn diode and the avalanche-photo-diode. In layered semiconductors domain formation in HEMTs can lead to excess gate leakage and to excess noise. The studies of hot electron transport parallel to the layers in heterostructures, single and multiple, have shown abundant evidence of electrical instability and there has been no shortage of suggestions concerning novel NDR mechanisms, such as real space transfer, scattering induced NDR, inter-sub band transfer, percolation effects etc. Real space transfer has been exploited in negative-resistance PETs (NERFETs) and in the charge-injection transistor (CHINT) and in light emitting logic devices, but far too little is known and understood about other NDR mechanisms with which quantum well material appears to be particularly well-endowed, for these to be similarly exploited. The aim of this book is therefore to collate what is known and what is not known about NDR instabilities, and to identify promising approaches and techniques which will increase our understanding of the origin of these instabilities which have been observed during the last decade of investigations into high-field longitudinal transport in layered semiconductors. The book covers the fundamental properties of hot carrier transport and the associated instabilities and light emission in 2-dimensional semiconductors dealing with both theory and experiment.