Electronic Fundamental Muhammad Zahid BS Regular 18 February, 17 Semiconductor materials Lecture - 2 Electronic Fundamental Muhammad Zahid

N-Type and P-Type Semiconductors

Doping To increase the conductivity of semiconductor controlled impurities are added to the intrinsic (pure) semiconductive material. This process is called doping. Doping increases the number of current carriers (electrons or holes). There are two categories of impurities, n-type and p-type.

N-Type Semiconductor To increase the number of conduction-band electrons in intrinsic silicon, pentavalent impurity atoms are added. These are atoms with five valence electrons such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb).

N-Type Semiconductor each pentavalent atom (antimony, in this case) forms covalent bonds with four adjacent silicon atoms. Four of the antimony atom’s valence electrons are used to form the covalent bonds with silicon atoms, leaving one extra electron. This extra electron becomes a conduction electron because it is not involved in bonding. Because the pentavalent atom gives up an electron, it is often called a donor atom.

Majority & Minority Carriers in N-Type Semiconductor Most of the current carriers are electrons in N type semiconductor. The electrons are called the majority carriers in n-type material. There are also a few holes that are created when electron-hole pairs are thermally generated. These holes are not produced by the addition of the pentavalent impurity atoms. Holes in an n-type material are called minority carriers.

Majority & Minority Carriers in N-Type Semiconductor Most of the current carriers are electrons in N type semiconductor. The electrons are called the majority carriers in n-type material. There are also a few holes that are created when electron-hole pairs are thermally generated. These holes are not produced by the addition of the pentavalent impurity atoms. Holes in an n-type material are called minority carriers.

P-Type Semiconductor To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. These are atoms with three valence electrons such as boron (B), indium (In), and gallium (Ga).

P-Type Semiconductor Each trivalent atom forms covalent bonds with four adjacent silicon atoms. All three of the trivalent atom’s valence electrons are used in the covalent bonds. since one electrons is left from bonding, a hole is created. Because the trivalent atom can take an electron, it is often referred to as an acceptor atom.

Majority & Minority Carriers in P-Type Semiconductor Since most of the current carriers are holes in p-type semiconductor. The holes are the majority carriers in p-type material. There are also a few conduction-band electrons that are created when electron-hole pairs are thermally generated. These conduction-band electrons are not produced by the addition of the trivalent impurity atoms. Conduction-band electrons in p-type material are the minority carriers.

The PN Junction If a piece of intrinsic silicon is doped so that part is n-type and the other part is p-type, a PN junction forms at the boundary between the two regions and a diode is created.

The PN Junction The free electrons in the n region are randomly drifting in all directions. At the instant of the PN junction formation, the free electrons near the junction in the n region begin to diffuse across the junction into the p region where they combine with holes near the junction.

The PN Junction The positive ion are formed at the boundary in the N region by loosing an electron. The electron from the n side enters in p side and becomes minority carrier, loose energy and fall into valance band. The number of electrons exceed than proton and become positive ion.

The PN Junction The positive ion are formed at the boundary in the n region by loosing an electron. The electron from the n side enters in p side and becomes minority carrier, loose energy and fall into valance band. The no of electron exceed than proton and become positive ion.

The PN Junction A layer of positive ion is developed on the n side and a layer of negative charges are developed on the p side. The process continues until no more electron is allowed to pass toward p region by negative layer of ion.

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