P-type semiconductor

The extrinsic p Type Semiconductor is formed, when a trivalent impurity is added to a pure silicon or germanium atom in small amount and as result large number of holes are created in it.

A trivalent impurity like boron, aluminium, nitrogen, gallium, indium.

have having three valence electrons but silicon or germanium have four valance electrons. so, it forms only three covalent bonds. In the fourth covalent bond, only the germanium atom contributes one valence electron, while boron atom has no valence bonds. Hence, the fourth covalent bond is incomplete, having one electron short. This missing electron is known as a Hole. Thus, each boron atom provides one hole in the germanium crystal.

As an extremely small amount of boron impurity has a large number of atoms, therefore, it provides millions of holes in the semiconductor.

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BJT (bipolar junction transistor)

A bipolar junction transistor is a three terminal semiconductor current controlled device with two P-N junctions. The three terminals are emitter(E), base(B) and collector(C). the emitter junction is heavily doped, base is less doped and made very thin and collector terminal is moderately doped. Collector has grater size than emitter and base terminal is thinner than both. (The thinner the base, the stronger the E-C electric field, and the larger the impact of a small current injected into the base. Explained clearly in active mode operation below) emitter terminal is moderate in size. A BJT has two types of transistors: NPN transistor PNP transistor NPN transistor : In an NPN transistor a p-type material is sandwiched between two n-type materials. [gallery ids="979,978" type="rectangular"] PNP transistor : In a PNP transistor a n-type material is sandwiched between two p-type materials. [gallery ids="989,990" type="rectangular"] Oper...

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photodiode

Introduction : A photo diode is a semi-conductor device, with a p-n junction and an intrinsic layer between p and n layers. It can be used as a light detector, which involves the conversion of light into current or voltage depending on mode of operation. [gallery ids="867,866" type="rectangular"] construction : Working : When a photon of sufficient energy strikes the diode, it creates an electron-hole pair. This mechanism is also known as the inner photoelectric effect. If it occurs at the junction these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photo current is produced. The total current through the photo diode is the sum of the dark current (current that is generated in the absence of light) and the photo current, so the dark current must be minimized to maximize the sensitivity of the device. Mainly it is operated in two modes 1. Photo ...

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