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Extrinsic Conduction

Electrons in Solids | The Capacitances of a Semiconductor Diode | Semiconductor Diodes as Rectifiers | Structures of Semiconductor Diodes | The Tunnel and Inversed Diodes | Microwave Semiconductor Diodes | Physical Processes in a Transistor | The Basic Circuit Configurations of Transistors | Models of Transistors | Bias Supply and Temperature Compensation for Transistors |


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  1. Conduction of electricity
  2. Electrical Conduction
  3. Intrinsic Conduction

If a semiconductor contains other substances as impurities, it will display what may be called extrinsic conduction in addition to its intrinsic conduction. It depends on the type of impurity present, and may be electron or hole conduction.

Antimony (Sb), arsenic (As), phosphorus (P) and other ele­ments with five valence electrons are called donors because they donate electrons to the host crystal. On giving up their fifth valence electrons, the donor atoms become positively charged (Fig. l.5).

 

Fig.1.5. Mechanism of extrinsic electron conduction

Semiconductors showing a predominance of electron conduction are called n-type semicon­ductors.

Boron (B), indium (In), aluminium (Al), with three valence electrons are called acceptors because they take on electrons from the crystal instead of donating them as do antimony and arsenic. On capturing these electrons, the acceptor atoms of borom are charged negatively (Fig. 1.6).

 

Fig.1.6. Mechanism of extrinsic hole conduction

 

 

Semiconductor devices are mostly made of semiconductor materials containing donor or acceptor impurities, and they are called extrinsic semiconductors.

For extrinsic conduction to exceed intrinsic conduction, the donor atom concentration ND or the acceptor atom concentration NA should exceed the intrinsic carrier concentration ni = pi. For example, in the case of silicon which has ni = pi = 1010cm-3 at room temperature, ND and NA may range anywhere between 1013 and l015cm-3.

Conduction in n -type silicon at ordinary temperature is by means of electrons supplied by the donor impurity. These excess electrons are spoken of as majority carriers. Holes are thus in the minority, and in this situation they are called the minority carriers.

It has been found that n -type extrin­sic semiconductors always satisfy the following equality:

n n p n = n i p i = = . (1.4)

In our example, 1013×107 = (1010)2 = 1020 .

For p -type semiconductors, the equality:

ppnp = n i p i = =

also holds always.

The electric conductivity of extrinsic semiconductors is determined in the same way as for intrinsic semiconductors. If we neglect the con­ductivity due to the minority carriers, then we may write for n -type and p -type semiconductors,

σn = nnn and σp = ppp . (1.5)

 


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