SEMICONDUCTOR

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  • A semiconductor is a type of material posses electrical conductivity between conductor and an insulator.
  • They are neither good conductor nor good insulators. The electrical resistivity of semiconductor generally decreases strongly with increasing temperature.
  • Semiconductor atoms have four valance electrons which take part in covalent bonds within the crystal structure.
  • The electrical conductivity of a semiconductor material increases with increase in temperature, which is a behavior opposite to that of a metal.
  • Current conduction in a semiconductor take place through the movement of free electrons and ‘holes’, collectively known as charge carriers. Adding impure atoms to a semiconducting material known as “doping”, greatly increases the number of charge carriers within it.
  • Conductivity is related to the number of electrons freely available to take part in current flow, and the valance e in the semiconductor materials are bound into the crystal structure sufficiently tightly to make them poor conductors but not well enough to make them insulators.

Types of semiconductor

N-type Semi-Conductors

N(for negative) type materials have enhanced conductivity as the result of  the addition of an impurity with five valance electrons instead of the four normal to semiconductor materials. Antimony, arsenic and phosphorus are elements which could be used to donate an extra free electron. The atoms of the impurity used with four of their valance electrons joining with the atoms of the parent material (Si, Gr) in covalent bonds and the fifth left free in the structure to behave as a current carrier.

P-type Semi-Conductors

P type semiconductor are made by adding impurities such as Al, borium or indium. The impurities have only three valance electrons and can only participate in covalent pair bonds with three, not four, of the surrounding semi-conductor atoms. Each of these (Al, B, I) is able to produce the apparent effect of removing negative electrons, so leaving gaps or +ve holes which transmit charge. The P-type impurities are sometimes termed accept as for this reason.

Current flow through P-type semi-conductor material involves the movement of holes from the +ve terminal of the power supply towards the –ve terminal.

Thermistors

Conductivity of a semiconductor material increases with temperature rise and falls as temperature drops opposite to that found metals, whose resistance increases with rise in temperature.

The property of negative temperature coefficient of resistance is used in devices for temperature measurement, thermal compensation and thermal relays for protection against overheating. The devices are called thermistors.

They may be produced from material other than silicon or germanium because the properties of these are sensitive to impurities. Preferred materials are mixture of curtain pure oxides such as those of nickel, manganese and cobalt, which are sintered with binding compound to produce a small bead with a negative coefficient of resistance.

Application

  • Digital thermometers.
  • Any application that requires heating or cooling protection circuit for safe operation(oven, refrigerators, detectors)

Dielectric

A dielectric material is a type of electrical insulator which should be polarized by an applied electric field. When a dielectric is put in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, +ve charges are displaced toward the field and –ve charges shift in the opposite direction. This creates an internal electric field that decreases the overall field within the dielectric itself.

Diode

A diode is a specialized electronic component (Semiconductor device) with two terminals/electrode called the anode and the cathode, typically allowing the flow of current in one direction only.

The most common type of semiconductor today, is a crystalline piece of semiconductor material with a p-n junction connected to two electrical terminals.

Generally diodes are used to rectify waveforms, and can be used within power supplies or within radio detectors(unidirection current property)most diodes are made from semiconductors such as Si and Gr

Types

  1. Light emitting Diode(LED)
  2. Avalanche Diode
  3. Vaccum diodes
  4. Zener diodes

Application

  • Waveform Clipper(Peak Clipper)
  • Voltage shifter
  • Voltage regulator

Thyristors

Thyristors are solid state switches (with four layers of alternating N and P type material) which are turned on by application of a low level signal voltage through a trigger connection known as a gate electrode.

A solid state or static switch has no moving parts to wear, or contacts which can be damaged by arcing.

There are a no. of electronic devices which are classed as thyristors. The most commonly known are silicon controlled rectifiers (SCRS) and triacs. SCR are four layer devices and triacs have a greater number.

Despite their small size, thyristors can be used to control current greater than 100 amps and voltages in excess of 1000 volts. They can therefore replace large conventional switches.

It also acts for bistable switch conducts current when the gate receives a trigger current, and continue to conduct when the voltage across the device is not reversed.