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...