27-11-2012, 02:32 PM
Semiconductor
[attachment=41313]
Semiconductor
A semiconductor is a material with
Conduction ability between conductors and insulators
Four valence electrons
Si,Ge,Cu
Energy bands
e-s confined to the band
Shells represent bands of energy levels
e-s absorb energy become free e-s
What is energy gap?
Thermal ionization
Bonds broken by thermal ionization
Electron-hole pair generated
Charge of a hole=charge of an electron
At room temperature, some of the covalent bonds are broken by thermal ionization. Each broken bond gives rise to a free electron and a hole, both of which become available for current conduction.
Carrier motion in semiconductors
There are two mechanisms for carrier motion in semiconductors
Drift
Diffusion
Carrier drift occurs when an electric field is applied across a piece of Si
Diffusion of the particles occurs due to the difference of the concentration
Diffusion
Elements of diffusion:
A medium (Si crystal)
a gradient of particles (electrons and holes) inside the medium
collisions between particles and medium send particles
off in random directions:
→overall, particle movement down the gradient
→ Diffusion flux ∝ - concentration gradient
Where
Flux = number of particles crossing unit area per unit time [cm−2 · s−1]
Summary
So in semiconductors
Charged particles move → particle flux → electric current density
Particle flux: number of particles that cross surface of unit area placed normal to particle flow every unit time
Current density: amount of charge that crosses surface
of unit area placed normal to particle flow every unit time
Drift current: produced by electric field
Diffusion current: produced by concentration gradient
[attachment=41313]
Semiconductor
A semiconductor is a material with
Conduction ability between conductors and insulators
Four valence electrons
Si,Ge,Cu
Energy bands
e-s confined to the band
Shells represent bands of energy levels
e-s absorb energy become free e-s
What is energy gap?
Thermal ionization
Bonds broken by thermal ionization
Electron-hole pair generated
Charge of a hole=charge of an electron
At room temperature, some of the covalent bonds are broken by thermal ionization. Each broken bond gives rise to a free electron and a hole, both of which become available for current conduction.
Carrier motion in semiconductors
There are two mechanisms for carrier motion in semiconductors
Drift
Diffusion
Carrier drift occurs when an electric field is applied across a piece of Si
Diffusion of the particles occurs due to the difference of the concentration
Diffusion
Elements of diffusion:
A medium (Si crystal)
a gradient of particles (electrons and holes) inside the medium
collisions between particles and medium send particles
off in random directions:
→overall, particle movement down the gradient
→ Diffusion flux ∝ - concentration gradient
Where
Flux = number of particles crossing unit area per unit time [cm−2 · s−1]
Summary
So in semiconductors
Charged particles move → particle flux → electric current density
Particle flux: number of particles that cross surface of unit area placed normal to particle flow every unit time
Current density: amount of charge that crosses surface
of unit area placed normal to particle flow every unit time
Drift current: produced by electric field
Diffusion current: produced by concentration gradient