This thesis describes experiments performed on three types of 3-5 semiconductor device and the deductions that can be formed from these experiments about the operation of the devices. The experiments concerned the effects on electrical conduction in the devices of low temperatures, magnetic fields and device parameters. The magnetophonon effect in gallium arsenide field effect transistors has been studied. The results obtained are strongly influenced by the geometry of the device, its orientation with respect to the magnetic field and its doping level. These parameters have been varied over a wide range. The field effect transistor has the advantage that the geometry of the conducting region can be varied by applying a gate voltage, and this has been used to study the effect of reducing one of the dimensions of the conducting region until it is comparable with a critical length, which is either the Debye length or the cyclotron radius, depending on the device. The triangular barrier switch is a device that was proposed only five years ago and its operation is not yet fully understood. It has two stable states, one of high impedance and one of low. It has been shown that the switching between these states is hysteretic, and new types of behaviour have been seen under the previously uninvestigated conditions of temperatures below 77 kelvin and of high magnetic fields. The best existing model of this device has been extended to account for these effects. The graded gap diode contains a potential barrier formed by sandwiching a layer of one type of semiconductor between regions of a different type. It is shown that depletion regions are about as important as the barrier in determining the current. The changes with temperature of the mechanism of conduction through this structure can be followed experimentally and have been used to measure properties of the barrier. A novel, unusually large, current instability has been seen at low temperatures. A study has been made of the statistics of this instability to identify its origin.