Abstract: In a field effect transistor having a quantum well is provided by a primary conduction channel, at least one secondary conduction channel immediately adjacent and in contact with the primary channel has an effect bandgap greater than the effective bandgap of the primary channel, and the modulus of the difference between the impact ionisation threshold IIT of the primary channel and the effective conduction band offset (the height of the step) between the primary and secondary channels being no more than 0.5 Eg (effective), or (alternatively) no more than 0.4 eV. Higher energy carriers which might otherwise cause impact ionization leading to runaway are thus diverted into the secondary channel allowing the device to run faster at increased voltages and/or to exhibit much greater resistance to runaway. The primary channel is prefereably of low bandgap material, for example InSb, InAs, InAs1-y, In1-xGaxAs.

Abstract: A bipolar transistor with vertical geometry comprises a base region (1) provided with a base contact (21), emitter and collector regions (2,3) arranged to extract minority carriers from the base region, and an excluding structure for counteracting entry of minority carriers into the base region via the base contact, wherein the base region has a bandgap of greater than 0.5 eV and a doping level greater than 1017 cm?3. As shown the base includes an excluding heterojunction (4) preventing entry of carriers from the base contact (21), but alternatively the base region could comprise a “high-low” doping homojunction. The construction shows improved resistance to thermal runaway even in multi-finger transistors. It is particularly useful for high power, high frequency transistors, e.g. base on gallium indium arsenide. The collector region preferably has a heterostructure.

Abstract: Liquid crystal compounds have been synthesized of general formula (I) and may be mixed with other liquid crystal compounds to give useful liquid crystal mixtures which may then be used in liquid crystal devices. Such devices include linear and non-linear electrical, optical and electro-optical devices, magneto-optical devices and devices providing responses to stimuli such as temperature changes and total or partial pressure changes. Formula (I) where each Z.sub.1-6 is given by formula (II), where Y for each of Z.sub.1-6 is independently COO, OCO, CH.sub.2 O and OCH.sub.2, m defines the number of substituents on each of Z.sub.1-6 and is independently 0-5,and X is independently for each substituent on each of Z.sub.1-6 alkyl (straight chain, branched and chiral), alkoxy (straight chain, branched or chiral), alkanoyl (straight chain, branched or chiral), alkenyl (straight chain, branched or chiral), halogen, halogenoalkyl (straight chain, branched or chiral) and CN, provided that at least one of Z.sub.