Tài liệu INTRODUCTION TO ELECTRONIC ENGINEERING- P2 pptx

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Introduction to Electronic Engineering

21

Semiconductor Devices

–

+

+ + +

+ + +

– – –

– – –

Fig. 1.2

–

+

+ + +

+ + +

– – –

–

+

+ + +

– – –

– – –

Fig. 1.3

n-type p-type

First of them are n-type semiconductors with a pentavalent (phosphorus) impurity where the n stands

for negative (Fig. 1.3) because their conduction is due to a transfer of excess electrons. A pentavalent

atom, the one that has five valence electrons is called a donor. Each donor produces one free electron

in a silicon crystal. In an n-type semiconductor, the free electrons are the majority carriers, while the

holes are the minority carriers because the free electrons outnumber the holes.

Another type of semiconductors with a trivalent (boron) impurity has the hole type of conduction or

deficit conduction by transfer from atom to atom of electrons into available holes. A semiconductor in

which the conduction is due to holes referred to as a p-type semiconductor. Here, p stands for positive

because of the carriers acting like positive charges, for the hole travels in a direction opposite to that of

the electrons filling it. A trivalent atom, the one that has three valence electrons is called an acceptor

or recipient. Each acceptor produces one hole in a silicon crystal. In a p-type semiconductor, the holes

are the majority carriers, while the free electrons are the minority carriers because of the holes

outnumber the free electrons.

Summary. Semiconductor crystals are very stable thanks to the covalent bond. However, unlike the

metals their free carriers’ density can be changed by many orders. Moreover, semiconductors exhibit a

growth of resistance as the temperature falls, that is a bulk or a negative resistance. Because of thermal

ionization, any temperature or light rise will result in significant motion of atoms that dislodges

electrons from their valence orbits. The departure of the electron leaves the holes that carry the current

together with electrons by the join recombination. This process speeds up when the voltage is applied.

Doping additionally increases the conductivity of semiconductors. By doping, two types of

semiconductors are produced − p-type with extra holes and n-type with excess electrons.

1.1.3 pn Junction

When a manufacturer dopes a crystal so that one half of it is p-type and the other half is n-type,

something new occurs. The area between p-type and n-type is called a pn junction. To form the pn

junction of semiconductor, an n-type region of the silicon crystal must be adjacent to or abuts a p-type

region in the same crystal. The pn junction is characterized by the changing of doping from p-type

to n-type.

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Introduction to Electronic Engineering

22

Semiconductor Devices

Depletion layer. When the two substances are placed in contact, the free electrons of both come into

equilibrium, both their number and the forces that bind them being unequal. Therefore, a transfer of

electrons occurs, which continues until the charge accumulated is large enough to repel a further

transfer of electrons. The accumulation of the charge at the interface acts as a barrier layer, called so

due to its interfering with the passage of current.

As shown in Fig. 1.4, the pn junction is the border where the p-type and the n-type regions meet. Each

circled plus sign represents a pentavalent atom, and each minus sign is the free electron. Similarly,

each circled minus sign is the trivalent atom and each plus sign is the hole. Each piece of a

semiconductor is electrically neutral, i.e., the number of pluses and minuses is equal.

+

–

Fig. 1.4

–

+

– –

– –

+ +

+ +

+

–

p

n

depletion

layer

Fig.1.5

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