3.2.Physics of PN Junction Diode.
Whenever two dis-similar materials are brought in contact, a contact potential is developed. This property is used in Thermo-couple.
Similarly when P-Type and N-Type is brought in contact, a built-in barrier potential (Φ BO ) develops at the metallurgical junction of P-Type and N-Type semiconductor. As shown in Figure 3.2., diffusion of the majority carriers leads to the development of a dipole layer at the junction.
This dipole layer is called space-charge layer, transition layer or depletion layer. The development of dipole layer prevents further diffusion of majority carriers.
This built-in barrier potential is always an up-hill for the majority carriers and is always a down-hill for minority carriers.
The diffusion of majority carriers from high concentration to low concentration is diffusion current and drift of minority carriers down the potential hill is drift current.
At this point a detailed charge balance is achieved as follows:
3.2.2
A more detailed picture of the depletion region is given in Figure 3.3 under zero-bias condition. This gives the space-charge profile, electric-field profile and the potential profile along the longitudinal axis of the PN Junction diode.
Under ZERO-BIAS condition,as seen in Figure 3.3. Holes, majority carriers in P-Bulk, are diffusing from P-Region to N-Region. Holes, minority carriers in N-Bulk, which survive to reach the edge of the depletion region just flow down-hill across the potential gradient as shown in Graph 3 in Figure 3.3. φ
As also seen in Figure 3.3. Electrons, majority carriers in N-Bulk, are diffusing from N-Region to P-Region. Electrons, minority carriers in P-Bulk, which survive to reach the edge of the depletion region just flow down-hill across the potential gradient as shown in Graph 3 in Figure 3.3.
Under zero-bias, hole diffusion is exactly balanced by hole drift and electron diffusion is exactly balanced by electron drift. Hence by detailed charge balance net current through the Diode under zero-bias is zero.
Graph 1 gives the Space Charge Density (ρ) C/cc. In the bulk region there is no space charge. The positive and negative are exactly balanced hence net charge is zero. But in depletion region d 0 , there is uniform negative space charge density on the P-Side extending over d p0 cm and there is uniform positive space charge density on N-Side extending over d n0 cm. The negative space-charge is due to negatively ionized Boron Atoms on P-Side and positive space charge is due to positively ionized Phosphorous Atoms on N-Side.Since I am discussing One-Sided Step Junction or Abrupt Junction hence doping density is uniform as seen in Figure 3.1. This is the reason why Space-Charge Density in depletion region is uniform on both sides though of opposite polarities.
On P-Side, Acceptor Dopent Density N A is uniform from –d p0 to zero. On N-side , Donor Dopent Density is uniform from 0 to d n0 .
Since depletion region is a dipole layer with equal positive and negative charges on the two poles hence: