Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Junction Properties
Equillibrium condition : The Contact Potential
In this we wish to develop both a useful mathematical description of the p-n junction and a strong qualitative understanding of its properties.
Contact potential:
- Let us consider separate regions of p- and n-type semiconductor material, brought together to form a junction.
- This is not a practical way of forming a device, but this "thought experiment" does allow us to discover the requirements of equilibrium at a junction.
- Before they are joined, the n material has a targe concentration of electrons and few holes, whereas the converse is true for the p material. Upon joining the two regions,
- we expect diffusion of carriers to take place because of the large carrier concentration gradients at the junction.
- Thus holes diffuse from the p side into the n side, and electrons diffuse from n to p. The resulting diffusion current cannot build up indefinitely, however, because an opposing electric field is created at the junction (Fig b).
- If the two regions were boxes of red air molecules and green molecules eventually there would be a homogeneous mixture of the two after the boxes were joined.
- This cannot occur in the case of the charged particles in a p-n junction because of the development of space charge and the electric field ξ. If we consider that electrons diffusing from n to p leave behind uncompensated donor ions (Nd ) in the n material, and holes leaving the p region leave behind uncompensated acceptors (Na-).
- it is easy to visualize the development of a region of positive space charge near the n side of the junction and negative charge near the p side.
- The resulting electric field is directed from the positive charge toward the negative charge.
- Thus ξ is in the direction opposite to that of diffusion current for each type of carrier (recall electron current is opposite to the direction of electron flow).
- Therefore, the field creates a drift component of current from n to p, opposing the diffusion current (Fig c).