Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Basic Electronics
GUNN diode-The Transferred Electron Mechanism
The Transferred Electron Mechanism:
- We discussed the nonlinearity of mobility at high electric fields.
- In most semiconductors, the carriers reach a scattering-limited velocity, and the plot of velocity vs. field saturates at high fields.
- In some materials, however, the energy of electrons can be raised by an applied field to the point that they transfer from one region of the conduction band to another, higher-energy region.
- For some band structures, negative conductivity can result from this electron transfer.
- E(k) band structure for GaAs is shown in figure given below.
- Some of the detail has been omitted in this diagram to isolate the essential features of electron transfer between bands.
- In n-type GaAs,the valence band is filled and the central valley (or minimum) of the conductionband at T(k = 0) normally contains the conduction electrons.
- There is a set of subsidiary minima at L (sometimes called satellite valleys) at higher energy, but these minima are many kT above the central valley and are normally unoccupied.
- Therefore, the direct band gap at V and the energy bands centered at k = 0 are generally used to describe the conduction processes in GaAs.
- The presence of the satellite valleys at L is crucial to the Gunn effect, however.
- If the material is subjected to an electric field above some critical value (about 3000 V/cm), the electrons in the central T valley of gain more energy than the 0.30 eV separating the valleys.
- Therefore, there is considerable scattering of electrons into the higher-energy satellite valley at L.