Conductor, Insulator, Semiconductor

Conductor:

In a conductor, the conduction and valence bands overlap as shown in Fig. 1. There is no forbidden energy band region. When an external electric field is applied, the electrons acquire additional energy and to higher energy level. These mobile electrons constitute a current. The overlapping of conduction and valence bands gives rise to high conductivity. The resistivity of conductors is of the order of 10^-7

Ω (ohm). Most metals are good conductors.

Fig:1: Conduction Band formation of Conductor

Insulator:

The energy band structure of insulators is shown in Fig. 2. A large forbidden band of several electron volts, exists between valence and conduction bands. The valence band is completely filled at absolute zero temperature. The valence electrons remain tightly bound to the nucleus. As the temperature of the insulator is increased, the added heat energy enables some valence electrons to jump the forbidden gap and occupy the unfilled level above. At room temperature the number of such electrons is negligible and conductivity is very small. The resistivity of an insulator is very high and is around 10¹⁰ to 10¹⁶ Ω (ohm).

Fig:2: Energy Band Diagram

Semiconductor:

In some substances the forbidden gap between conduction and valence bands is small (about 1 ev). These materials are known as semiconductors. Germanium and silicon are the two examples of semiconductor. At 0K temperature the forbidden gap in germanium is 0.785 ev and in silicon it is 1.21 ev. Even at room temperature some valence electrons have enough energy and can move into the conduction band. Therefore, these materials are slightly conducting. The forbidden energy gap decrease with increase in temperature. The resistivity of semiconductors is of the order of 1 ohm-m.

Formation Of Energy Band In Solids

Formation Of Energy Band In Solids:
When a large number of atoms are brought together to form a crystal, interactions occur the proper inter-atomic spacing for the crystal. The discrete energy levels of electrons  of an atom are modified by the presence of other nearby atoms. As the spacing between atoms is decreased, the energy level of each electron is increasingly influenced by the charge distribution of neighboring atoms. The magnitude of influence depend on the spacing between atoms as also the location of the electron in the group. Levels of outer shell electrons are changed consideration of the electrons are shared by more than one atom in the crystal. New energy levels of outer shell electrons can be determined by the principles of quantum mechanics. Pauli's exclusion principle dictates that no two electrons in a given interacting system may have the same quantum state. Thus there must be a splitting of the discrete energy levels of the discrete energy level of the isolated atoms into new levels belonging to the pair rather than individual atoms.

The Concept of Power Electronics

Power Electronics was started in 1900 with the Mercury Arc.Then the use of metallic tank rectifier, grid control vacuum tube rectifier, ignitron, pennotron and thyratron began.Until 1950 these devices were used to control the power electronics.

Power electronics evolved in 1948 with silicon transistor, which is invented by Scientist Bardeen, Brattain and Shockley in the Bell Telephone Laboratories.Currently, more modern electronic technologies are being discovered.After the SCR was invented by Bell Laboratories in 1956, the development of Power Electronics was rapid.In 1958, the second phase of electronics was evolving and commercially produced thyristors by the General Electronics Company. It introduces a new era of Power Electronics. Now a large number of devices and conversion methods are being discovered. The evolution of microelectronics facilitates rapid processing of large amounts of information. So it is possible to control a lot of energy in different means and quantities.

Solar Cell

The solar cell is a type of cell that transforms the light energy into electricity. The principle of solar cell is like photovoltaic action. This type of action occurs in all semiconductors, which are basically formed to absorb the light energies.

The basic solar cell consists of a P-type and an N-type semiconductor material (usually silicon or selenium), which consists of a PN joints/junction. The bottom layer is covered with a conducive contact's cover. The bottom layer is away from the light, i.e the light does not reach the bottom. Most of the upper surface area is open so that maximum light can fall and a small contact is placed on the top edge. The P-Type Surface layer is very thin (0.5mm) so that the light can reach the junction.

Although silicon is usually used to create solar cells. Apart from this, P-type junction is sometimes done by the type of p-type selenium. When the light is falling on the solar cell, then the PN nucleus of the solar cell is energized, the pair is formed in the electron hole, which is near the junction.Through the creation of positive and negative ions, an electric field is created near the PN junction, which develops a potential in the horizon of the junction. By the electric field many electron comes through the junction. If the intensity of light is increased, then the movement of carrier will carry more. The current flowing through the junction is proportional to the intensity of light.

Terminal voltage of cells is proportional to the intensity of direct light. Depending on external load, cell voltage may be up to a maximum of 0.6 volt. If many cells are arranged in an array, then more electric voltage can be achieved.When the cells are arranged in series or parallel, just like the battery works. If 10 cells are connected in series, each cell produces 0.5 volt in 150 mA, then the total will be 5 volt. Two parallel solar banks can provide 5 Volt at 300mA.

Solar Cell Uses:

• In Solar Energy Converter
• For the recharge of the battery of the board satellite
• Solar cells are used for driving
• Light measurements
• Light detection circuits.

DC Network

 1. Define electric current?

Ans: The valence electrons in different atoms of a material are free to move from one point to other. When an electron field is applied to a conductor the motion of electrons gets channelized. More electrons flow in one direction than in the other. The rate of flow of electrons is called current.

I= (dq/dt)

2. Define potential difference?

Ans: Absolute potential of a point is the work done in moving a unit positive charge from infinity to that point. The potential difference between two points is the work done in moving a unit positive charge from one point to the other. It measured in volts.

3. Define the word "Resistance"?

Ans: Electric current in a conductor consist of movement of electrons. When electrons flow through a material, they collide with other atomic particles and energy is lost in these collisions. The energy lost per unit charge is the voltage drop. The ratio of voltage drop to current is the resistance of material.

Uni-Junction Transistor (UJT)

A Uni-junction transistor (UJT) is a switching device.It switches from blocking to conducting state when the applied voltage reaches a critical value. It is a bar of high resistivity semiconductor with ohmic contacts at each end. The bar is usually of n-type material. A p-type emitter is alloyed at an intermediate position along the length. (In complementary UJTs the bar is p-type and the junction is n-type). When the junction is open or reverse biased, the resistance between base 1 and base 2 is a few kilo-ohms (fig. 1).

Fig: 1: The Symbol, Construction & Equivalent circuit of UJT 

When a positive voltage VBB is applied between B₂ and B₁ , the potential of point E will be ηVBB. The factor η is known as the intrinsic stand off ratio and depends on internal resistance R_B1 and R_B2 of the bar. If V_E is less than V_C , the p-n junction will be reverse biased, the emitter current will be negative and equal in magnitude to the small reverse saturation current. The V-I characteristics are shown in Fig. 2. When V_E = ηVBB + V_D ,where V_D is the forward voltage drop of the diode, the emitter base junction will become forward biased. The emitter current will become positive. The point A is known as peak point. The corresponding voltage and current are Vp and I_P respectively. The holes injected by the emitter drift towards B₁. Therefore, the number of charge carriers in the lower half region of the bar increases and the resistance R₁ decreases. Therefore, the potential of point C will decrease and the p-n junction will experience a higher forward bias. Therefore, the potential of point C will decrease and the p-n junction will experience a higher forward bias.

 Fig: 2: Characteristics Curve of UJT

Light Emitting Diode (LED)

Light energy can excite an electron into the conduction band. On the other band, when recombination takes place energy is released. In a semiconductor with an indirect band gap, e.g, silicon,germanium etc. recombination releases heat to the lattice. However, in some materials (e.g gallium arsenide), direct transitions occur between conduction band and transition band. In these materials, the radiation is in the visible and infrared regions. This effect, knows as injection electroluminescence, finds important application in digital watches, calculators etc. In a light emitting diode, an electric current causes the injection of minority carriers into regions of the crystal where they can recombine with majority carriers resulting in emission of light. The light output varies as (current)ⁿ , where n varies from 1.2 to 1.5 A wide range of photon energies extending from ultraviolet to infrared are available. The available energies can be increased still further by mixing the compounds. Commonly used displays are red, yellow,green and orange.
Figure-1 shows symbol for LED

Figure:01 LED Symbol

 Figure:02 LED circuit diagram

Figure-2 shows a simple circuit to regulate the current through an LED. The forward current I_f

 given by

I_f = (V-V_f  / R)

Where V_f is the forward voltages across the LED. The LEDs for different colours have different values of V_f and I_f. The diode D₁ is to protect the LED against reverse breakdown voltage. When reverse voltage exceeds a certain value, D₁ starts conducting and protect the LED.