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Properties of Semiconductors

Properties of Semiconductors

Materials are classified on the basis of conductivity and resistivity. Semiconductors are materials having electrical conductivity greater than that of an insulator but significantly lower than that of a conductor. Germanium and Silicon are most widely used semiconductors in device manufacturing applications.

These materials behave as insulator at low temperatures and as conductors at high temperatures. Moreover, these materials have two types of charge carriers, i.e., electrons and holes

Definition Based on Electrical Resistance

The resistivity of a semiconductor is lesser than an insulator but more than that of a conductor. It is in the order of 10-4 to 0.5 ohm metre.

Definition Based on Energy Band

A semiconductor has nearly empty conduction band and almost filled valance band with a very small energy gap ≈ 1ev.

General Properties of Semiconductors

♦ They  are formed by covalent bonds.

♦ They have empty condution band at 0K.

♦ They have almost filled valance band.

♦ They have small energy gap.

♦ They posses crystalline structure.

♦ They have negative temperature co-efficient of resistance.

♦ At 0K, they behave as insulators.

♦ Its resistivity is 10-4 Ωm to 0.5 Ωm.

♦ If impurities are added to a semiconductor, its electrical conductivity increases.

♦ If the temperature of a semiconductor is increased, its electrical conductivity increases. Large number of charge carriers are produced due to the breaking of covalent bonds.

♦ In semiconductors both electrons and holes are charge carriers and they also take part in conduction.

                   Total conductivity σ = σe + σh

   Where      σ = σe is conductivity due to electrons.

                    σh is conductivity due to holes.

♦ Semiconductor shows a photo-voltage or a charge in resistance upon irradiation by light.

9 comments:

  1. unforgiven_anil@yahoo.comJuly 22, 2015 at 11:56 PM

    Semiconductor is a material that has a resistivity value between that of a conductor and an insulator. The conductivity of a semiconductor material can be varied under an external electrical field. Devices made from semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, and many other devices. Semiconductor devices include the transistor, many kinds of diodes including the light-emitting diode, the silicon controlled rectifier, and digital and analog integrated circuits. Solar photovoltaic panels are large semiconductor devices that directly convert light energy into electrical energy. In a metallic conductor, current is carried by the flow of electrons. In semiconductors, current can be carried either by the flow of electrons or by the flow of positively-charged "holes" in the electron structure of the material.

    Silicon is used to create most semiconductors commercially. Dozens of other materials are used, including germanium, gallium arsenide, and silicon carbide. A pure semiconductor is often called an “intrinsic” semiconductor. The conductivity, or ability to conduct, of semiconductor material can be drastically changed by adding other elements, called “impurities” to the melted intrinsic material and then allowing the melt to solidify into a new and different crystal. This process is called "doping"

    ReplyDelete
  2. The electrical materials are classified into insulators, semiconductors and conductors according to their ability to conduct electricity. Semiconductors have advanced the electronics industry in a fantastic manner in the last few decades. What are the unique semiconductor properties which enabled and are still enabling such huge a evolution?

    ReplyDelete
    Replies
    1. There are several properties of semiconductors, which has been explored and currently being explored for several applications. Some of these properties are:
      1. Electrical conductivity: Easy control on electrical conductivity by doping. By changing the dopant the same semiconductors can conduct both the electrons and holes. High level of doping can cause degeneracy in semiconductor and material can behave like a metal. If one forms an amorphous film of semiconductor, it will start behaving like insulator. These properties had led to several electronic devices such as diodes, transistors etc.
      2. Optical properties: Semiconductors can have direct or indirect band gap. Direct band gap semiconductors can emit photon upon excitation by required wavelength. This property had led towards the development of lasers, LEDs and several other applications. Also the indirect band gap semiconductors can be made direct band gap by alloying.
      3. Photo conductivity: A p-n junction, when illuminated, can produce a electron-hole pair. This electron-hole pair can be collected in at the electrodes, thereby, producing a current. This property has been used to develop solar cells and photo detectors.

      There are several other properties also. There are several classes of semiconductors which are currently being used. One of these are organic semiconductors, which can be deposited over flexible substrates and thereby increasing possibilities towards the realization of flexible electronics and opto electronics.

      Delete
  3. Maybe that they allow to make "electrically controlled resistors"?

    ReplyDelete
  4. Electrically controlled resistors are used to control the current in field effect transistors: the junction field effect transistor and the metal insulator field effect transistor.But this is only one of the main features and there are remaining other features.
    Thank you Cyril for your important contribution.

    ReplyDelete
  5. I guess it could be seen in two ways; The mechanical and chemical properties of common semiconductors such as Silicon allows them to be manipulated and fabricated at a small scale, using mass processing techniques like photo-lithography. Under a completely different perspective, the electronic properties of semiconductors have enabled all of modern electronics. A range of semiconductors with specific properties exist because there are unique applications for them, and unique materials can behave like semiconductors under given conditions, such as CNTs.

    In terms of semiconductor junctions, important properties for continued development could be the recombination time of electrons and holes and the density of electrons and holes..

    ReplyDelete
  6. Don't you think a BJT is an "electrically controlled resistor" as well? Non-linear but still a resistor?

    ReplyDelete
  7. Yes indeed! Transfer+Resistor=Transistor. A BJT is known to have a low input impedance (B-E junction) and a high output impedance (C-B junction). In the common-base configuration, nearly the same current flows through the emitter and the collector to produce voltage amplification.

    ReplyDelete
  8. There are several properties of semiconductors, which has been explored and currently being explored for several applications. Some of these properties are:
    1. Electrical conductivity: Easy control on electrical conductivity by doping. By changing the dopant the same semiconductors can conduct both the electrons and holes. High level of doping can cause degeneracy in semiconductor and material can behave like a metal. If one forms an amorphous film of semiconductor, it will start behaving like insulator. These properties had led to several electronic devices such as diodes, transistors etc.
    2. Optical properties: Semiconductors can have direct or indirect band gap. Direct band gap semiconductors can emit photon upon excitation by required wavelength. This property had led towards the development of lasers, LEDs and several other applications. Also the indirect band gap semiconductors can be made direct band gap by alloying.
    3. Photo conductivity: A p-n junction, when illuminated, can produce a electron-hole pair. This electron-hole pair can be collected in at the electrodes, thereby, producing a current. This property has been used to develop solar cells and photo detectors.

    There are several other properties also. There are several classes of semiconductors which are currently being used. One of these are organic semiconductors, which can be deposited over flexible substrates and thereby increasing possibilities towards the realization of flexible electronics and opto electronics.

    ReplyDelete

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