P - N JUNCTION

When a P -type material is placed in contact with an N - type material the contact surface is called P - N junction. As soon as the junction is produced free electrons near the junction in the N - type material move across the junction in the P - type material , where they fill the holes as the result a positive charge is appears near the junction in the N - type material and the negative charge appears in the near the P - type material. At the same time holes diffuse across the junction from the P -type to the N - type material. The exchange of the charge soon stops because the positive charge on the on the N - type side opposes the further flow of holes and the negative charge on the P -type side opposes the further flow of electrons. The region on the either side of the junction becomes fairly free of majority charge carriers and is called depletion layer. The with of depletion layer is of order of 1 micrometer. As the depletion layer builds up a difference of potential appears across the junction because of the positive charge on the N - type side and negative charge on the P -type side. This difference of the potential is called junction voltage or barrier p. d. . At the room temperature the junction voltage is approximately 0.7 V for silicon P- N junction and 0.3 V for Germanium P - N junction.