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.

Maybe that they allow to make "electrically controlled resistors"?

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.

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..

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

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.

Basically semiconductors with 2 kinds of current carriers, holes and electrons, together with the ability to inject impurities to make one carrier as a majority while the other one is a minority carrier, is what made this evolution possible. @AlDmour

Hello Dr Zekry,

I nearly agree to Cyril's view regarding the most basic feature of semiconductors. I think, the fact that their electrical conductivity (also conductivity type) can be controlled in a big way by impurity doping and geometrical shaping (two- or one-dimensional structures) is the key feature.

Dear Dr Mallik,

Thank you for your contribution Electrical resistance control is a general term. It is okay but my question is about the unique material properties that led to such useful electronic device such as transistors.

I would like to thank all other colleagues Dr Cyril, Dr Fry, and Dr Ismat. I am satisfied with your response on my question.

I wait for more key properties of the semiconductors.

Best regards

Dear Dr Zekry,

Yes, that's why I referred to electrical conductivity rather than electrical resistance (or conductance) in my response. For example, doping intrinsic silicon with even one part per million of boron or phosphorous atoms will change its conductivity and conductivity type enourmously. This involves only material changes and is independent of any applied voltage of current.

Are you asking why is it possible to change the properties of semiconductors in this way?

Dear Dr Mallik,

I mean that there is still other important features of semiconductors in addition to what you and the colleagues mentioned.

Thank you for your interest.

An interesting feature that is quite important is that they can convert energy between different forms, namely light and conduction electrons. By controlling the band gap of the semiconductor, it can be made sensitive to various wavelengths of light, this is the basis for semiconductor lasers and solar cells...

Fundamentally, it's the presence of the bandgap, which can itself be modified by doping, plus the mobility of the charge carriers. This allows the desirable response to electric fields, as well as the existence of barriers to currents, which again be modified by doping.

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.

Crystalline Structure and the number of Valence Electrons are the basic features of a Semi Conductor.

The PN Diode or PNP or NPN Transistors should be a singly grown Germanium or Silicon Crystal with the required amount of doping materials added . The Diodes or Transistors can be grown continuously and sliced for each piece.Or, the different materials may be in the form of substrates.

The number of Valence Electrons in the base and the Doping Materials decide the type of the semiconductor Device

In I.C.s a very large number of different components are incorporated in a single chip

P.S.

I am very pleased with the contribution of all of these bright minds in answering my question and wait for more answers..Thank you my colleagues.

Basic properties of semiconductors is presented in popular paper (see attachment).

The referred paper is almost a short treatise on Semiconductors covering various aspects.

P.S.