Semi conductive materials do not conduct current well and are of limited value in their intrinsic state. This is because of the limited number of free electrons in the conduction band and holes in the valence band. Intrinsic silicon or germanium must be modified by increasing the number of free electrons or holes to increase its conductivity and make it useful in electronics devices. This is done by adding immaturity to the intrinsic materials. Two types of extrinsic semi conductive materials n-type and p-type are the key building blocks for most types of electronic devices. To build these semi conductors there is a method due to which we can get pure semiconductors , this process is called Doping. So, Lets know about doping firs…
The conductivity of silicon and germanium can be drastically increased by the controlled addition of impurities to the intrinsic (pure) semi conductive material. This process, called doping, increase the number of current carriers (electrons or holes). In this process silicon or germanium is mixed in simple words, and so we get n or p type semiconductors which have high ratio of free electrons or holes, due to these elements best and required conducting takes place. Both silicon and germanium have different properties , e.g. different conducting levels.
Now we see more about n – type and p – type semiconductors in brief.
N – Type Semiconductors:
To increase the number of conduction – band electrons in intrinsic silicon,
pentavalent impurity atoms are aded. These are atoms with five valence electrons such as arsenic (As), Phosphorus (P) , Bismuth (B) , and Antimony (Sb).
Each pentavalent atom forms covalent bonds with four adjacent silicon atoms. Four of the Phosphorus atoms’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 ti is not attached to any atom. Because the pentavalent atom gives up an electron, it is often called a donor atom. The number of conduction electrons can be carefully controlled by the number of impurity atoms added to the silicon. A conduction electron created by this doping process does not leave a hole in the valence band because it is in excess of the number required to fill the valence band. While studying about semiconductors a term which we should keep in mind is majority and minority carriers. So, before we start p – type materials, first look at a glance of Majority and Minority Carriers.
Majority and Minority Carriers:
Since most of the current carriers are electrons, silicon for germanium doped with pentavalent atoms is an n-type semiconductor (the n stands for the negative charge o an electron), . The electrons are called the majority carriers in n-type material.
Although the majority of current carriers in n-type material are electrons, there are also a few holes that are created when electron – hole pairs are thermally generated. The 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 get P – type semiconductors we increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. These are those atoms with three valence electrons such as Boron (B), indium (in), and gallium (Ga). Each trivalent atom forms covalent bonds with four adjacent silicon atoms. All three of the boron atom’s valence electrons are used in the covalent bonds; and, since four electrons are required, a hole results when each trivalent atom is added. Because the trivalent atom can taken an electron, it is often referred to as an acceptor tom. The number of holes can be carefully controlled by the number of trivalent impurity atoms added to the silicon. A hole created by this doping process in not accompanied by a conduction free electron.