TYPES OF EXTRINSIC SEMICONDUCTORS

 EXTRINSIC SEMICONDUCTORS 

                            The intrinsic semiconductor has little current conduction capability at room temperature. For getting good conducting properties, some suitable impurity or doping agent is added. Such type of semiconductors is called extrinsic or impure semiconductors. The process of adding impurity to a semiconductor is called doping. The purpose of adding impurity is to increase either the number of free electrons or holes in a semiconductor crystal. Depending upon the type of impurity added, extrinsic semiconductors are classified into. 

                                        ( a ) n - type semiconductor

                                        ( b ) p - type semiconductor 

 N-TYPE SEMICONDUCTOR 

                            When a small amount of a pentavalent impurity is added to a pure semiconductor, it is known as an n-type semiconductor. Examples of pentavalent impurities are 5th group elements i.e .. Arsenic, Antimonic etc., As shown in Fig each antimony atom fons covalent bonds with the surrounding four silicon atoms with the help of four of its five electrons. The fifth electron is superfluous and is loosely bound to the antimony atom. Hence it is easily excited from the valence band to the conduction band by the application of an electric field or increase in its thermal energy. Thus practically every antimony atom introduced in to the silicon lattice contributes one conduction electron without creating a positive hole. This type of semiconductor is called n-type semiconductor. Antimony is called donor impurity because they donate free electrons to the semiconductor crystal. After the donation of one electron, the antimony becomes a positive donor ion .

In addition to the free electrons, some electron-hole pairs are generated with the application of electric field. In an n-type semiconductor the number of electrons are more than holes. The electrons constitute the majority carriers while the hole constitutes the minority carriers. The n - types of semiconductor is electrically neutral. 

P-TYPE SEMICONDUCTOR

 When a small amount of trivalent impurity is added to a pure semiconductor, it is known as a p - type semiconductor. Examples of trivalent impurities are 3rd group elements i.e., Boron, Gallium etc. As shown in Fig   in case of the three valence electrons of the boron atom form covalent bonds with four surrounding silicon atoms but one bond is left incomplete and gives rise a hole 

Thus boron which is called an acceptor impurity causes as many positive holes in a silicon crystal as there are boron atoms This type of semiconductor is called p - type semiconductor Boron is called an acceptor impurity. ( After acceptance of an electron it becomes a negative acceptor ion ) 

. In addition to holes, some electron-hole pairs are generated with the application of an electric field. In this type of semiconductor, conduction is by means of holes in the valence band. Accordingly, holes form the majority carriers where electrons constitute minority carriers. The p-type semiconductor is electrically neutral