Electrical Engineering ⇒ Topic : P-N Junction Diode

P-N JUNCTION DIODE

If a layer of P-type germanium is placed with a layer of N-type germanium such that the atoms of N-type combine with the atoms of P-type, a diode is obtained. This diode is called P-N junction diode [see Figure (a)]

In practice, P-N junction diode can be obtained in two ways:

(1) Grown junction type

A single crystal in this process is obtained from a melt containing impurity either P-type or N-type at first. In the middle of the growth process, opposite kind impurities are added to the melt so the remainder of the crystal grows into the opposite type.

(2) Fused junction type

In this process, a small dot of indium is pressed on a thin wafer of N-type germanium. Heat treatment is continued for a few minutes so that indium gets fused into the surface of germanium and P-type germanium is produced just below the surface of contact.

                                              figure (a)  P-N junction

When such junctions are formed, the following phenomena occur:

(i) Electrons from the N-side diffuse into the P-side where they combine with the holes.The reason of this diffusion is that there is a concentration gradient of electrons, i.e., more electrons in N-side than P-side.

(2) Holes from the P-side diffuse into the N-side, where they combine with the electrons.

(3) The diffusion of electrons and holes across the junction gives diffusion current.Recombination process occurs as soon as the charges cross the junction and combinewith the opposite charges. During the diffusion process, electrons are removed from N-side and holes are added to N-side. Similarly, the P-side loses holes and acquires electrons. The process of diffusion continues till the equilibrium is developed at the junction. The region between the two-arrows is called depletion layer or transition region. The thickness of this region is of the order of 6 x 10^-6 cm. The electrostatic potential difference across the P-N junction at this stage is known as the contact potential (or junction barrier or barrier field). The symbol of P-N junction diode is shown in Figure (2) The arrow head represents the P-section of the crystal and shows the direction of flow of holes or conventional current

                                                   figure (2)