the energy gap of semiconductor is very small,for Si Eg=1.1 eV and for Ge Egis =0.72 eV, then why a semiconductor is not provided with this small energy to make it behave like a metal?Then there should be no need of doping.

 

It could be done in both ways, doped or it could be excited be photons (enough energy) so it will conduct...

 

the energy gap of semiconductor is very small,for Si Eg=1.1 eV and for Ge Egis =0.72 eV, then why a semiconductor is not provided with this small energy to make it behave like a metal?Then there should be no need of doping.

Without doping you can't create junctions.
Without junctions you can't create diodes and transistors.
Without diodes and transistors you can't create solid-state circuits.
Tubes anyone?

 

if you look up silicon and germanium, they are both metals. it is the addition of the dopants, which are mostly metals that semiconductors are made. even copper, as copper oxide and zink can be made into semiconductors.

 

the energy gap of semiconductor is very small,for Si Eg=1.1 eV and for Ge Egis =0.72 eV, then why a semiconductor is not provided with this small energy to make it behave like a metal?Then there should be no need of doping.

The whole point of using semiconductors is that we can easily create and control with precision doping the conduction, carrier mobility and ultimately the QM energy movement properties of the material atomic lattice structure when making electronic devices. You only need the lattice structure on one side of the interface with a free-electron metal on the other side to make conventional band-gap junction device. Because the mechanics of the creation and operation of the band-gap is not classical EM it's also possible to make devices with a free-electron metal oxides and insulators if the barrier is so thin that QM effects like tunneling dominate its operation.
https://en.wikipedia.org/wiki/Metal-insulator-metal

 

Then acc to yo, why are semiconductors designed? Actually in almost every article of our daily use, SC( semiconductors) are used. Their basic need is to provide a controlled energy supply.. Either in terms of voltages or currents..... Which a metal dice can't provide. And to enhance this property, doping is done.

the energy gap of semiconductor is very small,for Si Eg=1.1 eV and for Ge Egis =0.72 eV, then why a semiconductor is not provided with this small energy to make it behave like a metal?Then there should be no need of doping.

 

the energy gap of semiconductor is very small,for Si Eg=1.1 eV and for Ge Egis =0.72 eV, then why a semiconductor is not provided with this small energy to make it behave like a metal?Then there should be no need of doping.

I would like to edit my question. I want to ask the energy gap of Si or Ge is so small.Why a small electric field ,say 5 eV can make the valence electrons move to conduction band?Why is it that once the electrons are in conduction band only then electric field can play the role?

 

What you are asking is a very complex question with no simple answers.
A simple mental picture (without the complications of holes) of the gap is a up-hill slope Fermi function f(E) with balls (electrons) traveling up that slope (Energy band Gap) at different angles from the base (from the valence band where they are trapped) of that slope to the top (to the conduction band where they are free to move). If the slope angle is high for the (Fermi) speed of the ball only the ones directly pointed at the top make it so we have limited electrons for (Intrinsic) current flow in an external field. When we dope the semiconductor we are placing the starting position (donor level) of the ball up the slope so the same energy level can now reach the top (conduction band) easily at many different travel angles increasing the number of free electrons for possible current flow. When we apply an external electric field to the free electrons they can move at the electron drift velocity as an electrical current in the material.

Study this first:

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/fermi.html
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/band.html
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/dsem.html

A very good link: http://www.doitpoms.ac.uk/tlplib/semiconductors/intro.php