EXTRINSIC MATERIALS

EXTRINSIC MATERIALS-

n- AND p- TYPE

The characteristics of semiconductor materials can be altered significantly by the addition of certain impurity atoms into the relatively pure semiconductor material. These impurities, although only added to perhaps 1 part to  million, can alter the band structure sufficiently to totally charge the electrical properties of the material.

        A semiconductor material that has been subjected to the doping process is called an extrinsic material.

       There are two extrinsic materials of immeasurable importance to semiconductor device fabrication: n-type and p-type.

n-Type Material

The n-type is created by introducing those impurity elements that have five valence electrons, such as antimony,arsenic,and phosphorus.Note that the four co-valent bonds are still present.There is, however, an additional fifth electron due to the impurity atom, which is unassociated with any particular covalent bond.Th remaining electron, loosely bound to its parent atom, is relatively free to move within the newly formed n-type material.Since the inserted impurity atom has donated a relative  "free" electron to the structure.

fig. 1.1

   Diffused impurities with five valence electrons are called donor atoms.

p-type material

The p-type material is formed by doping a pure germanium or silicon crystal with impurity atoms gaving three valence electrons.The elements most frequency used for this purpose are boron, gallium and indium.The effect of one of these elements boron. on a base of silicon is indicated in fig. 1.2

                                                fig. 1.2

The resulting vacancy is called a hole and is represented by a small circle or positive sing due to the absence of a negative charge.Since the resulting vacancy will readily accept a "free" electron:

   

The diffused impurity with three valence electrons are called acceptor atoms.

The resulting p-type material is electrically neutral, for the same reasons described for the n-type material.