DO PHYSICS ONLINE. conduction band. ~ 6 ev. Fig. 1. Energy band diagram for diamond (insulator) and silicon (semiconductor).

Transcription

1 DO PHYSIS ONLINE FROM IDEAS TO IMPLEMENTATION ATOMS TO TRANSISTORS SEMIONDUTORS ENERGY BANDS Diamond is a very good insulator. The electronic configuration in the ground state is 1s 2 2s 2 2. It might appear that diamond is a conductor because it has only two electrons in the energy level and that the band is only partly filled. However, there are two distinct energy bands separated from each other by an energy gap of 6 ev. The lower band is completely filled. At room temperature, energy due to thermal motion is only about 0.03 ev, much small than the energy gap, so virtually no electrons will be found in the upper energy band. Silicon has a much smaller band gap and therefore less energy is required for electrons to be free and take part in conduction, therefore, silicon is classified as a. energy bands for diamond insulator Z = 6 1s 2 2s 2 2 forbidden band exists between two bands energy bands for silicon Si Z = 14 1s 2 2s 2 6 3s 2 3p 2 forbidden band exists between two 3p bands 3p 3p 3s 2s 1s ~ 6 ev 2s ~ 1 ev 1s Fig. 1. Energy band diagram for diamond (insulator) and silicon (). When an electric field is applied to a material, only if electrons can gain sufficient energy to move into the, will they move freely to establish a current. The greater the number of charged carriers in the, the better the conduction. 1

2 electron energy insulator conductor Fig. 2. Schematic diagram of energy bands for different materials. INTRINSI SEMIONDUTORS An intrinsic is a pure crystal, for example, silicon or germanium. Germanium was the first widely used because it was easier to purify than other materials. The first transistors were made from germanium. However, there are thermal problems with using germanium in devises. The small energy band gap in germanium means that electrons can be easily excited into the and so the conducting properties of germanium devices were sensitive to fluctuations in temperature. Today, silicon is the most widely used material. In metals, the charge carriers are the free electrons. However, in s, the charge carriers are electrons and s. When an electron is freed from being bonded to a particular atom, it leaves behind a vacancy called a in the electronic structure of the crystal. An electron requires little energy to move into the, but as it does so it leaves a new in its previous location. So s move like a positive charge carrier with the mass of an electron. Therefore, in a, s drift in the direction of the externally applied electric field and the free electrons move in the opposite direction. E (vacancy) propagates to the in the direction of applied electric field electron propagates in a direction opposite to the direction of the applied electric field as the electron moves into the vacancy and creating a new vacancy in its former position Fig. 3. Movement of electrons and s through an intrinsic under the action an applied electric field. 2

3 increasing energy electron energy Fig.4. Semiconductor energy band structure showing the movement of the positive s and negative electrons as the charge carriers for an electric current. EXTRINSI SEMIONDUTORS The conductivity of s is markedly affected by slight amounts of impurities. When a pure (intrinsic) has controlled amount of impurity atoms embedded within its crystal structure, it is called an extrinsic. Suppose we add arsenic atoms to a silicon crystal. Silicon like carbon and germanium has four electrons in its outer most shell, whereas an arsenic atom has five electrons in its outer shell. Since only four of the five electrons of an arsenic atom can be shared with four neighbouring silicon atoms in a covalent bond, the remaining electron needs little energy to be detached and move about freely within the crystal. In terms of an energy band explanation, the effect of the arsenic impurity is to create an energy levels just below the empty in which electrons must be present in for conduction to take place. These levels are called donor levels, and the material is called an n-type because electric current in it is carried by the motion of electrons (negatively charged). Si bonded electron pair - electrons need little energy to move from a donor level into donor levels As : silicon E g = 1.1 ev As extra electrons can move freely through the Si crystal Fig. 5. n-type (+5 valency impurity atoms).. Donor levels due to presence of arsenic atoms in silicon crystal. onduction is by means of excess electrons. 3

4 increasing energy Suppose we add gallium atoms to a silicon crystal. A gallium atom has three electrons in its outer shell, and their presence leaves vacancies called s in the electronic structure of the silicon crystal. An electron requires little energy to move into a, but as it does so it leaves a new in its previous location. When an electric field is applied to the crystal in which gallium is present as an impurity, electrons move towards the positive electrode by successively filling s. The current is best described in terms of the motion of s, which behave as though they are positive charges since they move towards the negative terminal. A material of this kind is called a p-type. In the energy band diagram, the effect of gallium as an impurity is to create energy levels called acceptor levels just above the valance band. The electrons that enter these levels leave behind unoccupied levels in the formerly filled band which makes possible the conduction of current. Si bonded electron pair Ga - : silicon E g = 1.1 ev electrons need little energy to move from valance band to an acceptor level acceptor levels Fig. 6. p-type (+3 valency impurity atoms). Acceptor levels due to presence gallium atoms in silicon crystal. onduction by means of s (+) in the valance band. Ga electrons can move freely through the Si crystal by filing vacancies creating a new 4

5 valency 3: p type impurities valency 4: s valency 5: 5 type impurities valance electrons: # electrons in outer shell Fig. 7. Section of periodic table showing the elements used for doping (adding controlled amounts of impurities) to create p-type and n-type s. 5