N-TYPE SEMICONDUCTORS/ P-TYPE SEMICONDUCTORS

N-TYPE SEMICONDUCTORS/ P-TYPE SEMICONDUCTORS

                   N-Type Semiconductors

 Thus far we have seen that pure semiconductor crystals are very poor conductors. High temperatures can make them semi-conduct because thermal carriers are produced. For most applications, there is a better way to make them semiconduct. Doping is the process of adding another material called impurities to the silicon crystal to change its electrical characteristics. One such impurity material is arsenic. Arsenic is known as a donor impurity because each arsenic atom donates one free electron to the crystal. Arsenic is different from silicon in several ways, but the important difference is in the valence orbit. Arsenic has five valence electrons When an arsenic atom enters a silicon crystal, a free electron will result. The covalent bonds with neigh[1]boring silicon atoms will capture four of the arsenic atom’s valence electrons, just as if it were another silicon atom. This tightly locks the arsenic atom into the crystal. The fifth valence electron cannot form a bond. It is a free electron as far as the crystal is concerned. This makes the electron very easy to move. It can serve as a current carrier. Silicon with some arsenic atoms will semi-conduct even at room temperature. Doping lowers the resistance of the silicon crystal. When donor impurities with five valence electrons are added, free electrons are produced. Since electrons have a negative charge, we say that an N-type semiconductor material results.

 

        P-Type Semiconductors

 Doping can involve the use of other kinds of impurity materials Note that boron has only three valence electrons. If a boron atom enters the silicon crystal, another type of current carrier will result. t one of the covalent bonds with neighboring silicon atoms cannot be formed. This produces a hole or missing electron. The hole is assigned a positive charge since it is capable of attracting, or being filled by, an electron. Boron is known as an acceptor impurity. Each boron atom in the crystal will create a hole that is capable of accepting an electron. Holes serve as current carriers. In a conductor or an N-type semiconductor, the carriers are electrons. The free electrons are set in motion by an applied voltage, and they drift toward the positive terminal. But in a P-type semiconductor, the holes move toward the negative terminal of the voltage source. Hole current is equal to electron current but opposite in direction.