Question

Draw the constructions of Gunn diode and explain its working.

Answer 1

Construction:

When a dc bias of value equal or more than the threshold field (of about 3.3kV/cm) is applied to an n-type GaAs sample, the charge densities and electric field within the sample become non-uniform creating domains i.e. electrons in some region of the sample will be first to experience the valley transfer than the rest of the sample. The electric field inside the dipole domain will be greater than the fields on either side of the dipole. So the electrons in that region or domain will move to U – valley and hence will have reduced mobility. This creates a slight deficiency of electrons in the region immediately ahead. This region of excess and deficient electrons forms a dipole layer. As the dipole drifts along, more electrons in the vicinity will get transferred to the U – valley until the electric field outside the dipole region is depressed below the threshold electric field. This dipole continues towards the anode until it is collected. Upon collection, the field in the sample jumps immediately to its original value and the next domain formation begins as soon as the field value exceeds the threshold value and this process is repeated cyclically. The time taken by the dipole domain to travel from cathode to anode is the transit time of the device. The fundamental frequency in MHz is given by:  

drift velocity and L=device length in µm.