Extrinsic 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 N_D or the acceptor atom concentration N_A should exceed the intrinsic carrier concentration n_i = p_i. For example, in the case of silicon which has n_i = p_i = 10¹⁰cm^-3 at room temperature, N_D and N_A 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:

p_pn_p = 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,

σ_n = n_neµ_n and σ_p = p_peµ_p . (1.5)

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