No Slide TitleProtons, Electrons & Neutrons.
An Atom requires balance, an equal No. of Protons & Electrons.
When an atom has one more particle (protons or electrons) it acquires a charge:
+ Ion has more Protons than Electrons,
- Ion has more Electrons than Protons.
Atom
*
covalently bonded to atoms
http://www.masstech.org/cleanenergy/solar_info/images/crystal.gif
In an intrinsic (no doping) material, ni=pi
At 300K, ni=pi is low (1010cm-3)
Use doping to improve conductivity
*
Substantially higher densities (1015cm-3)
Majority and minority carriers
Random recombination and thermal generation
*
Has loosely bound electrons in its outer or Valence ring,
they are easily displaced.
Has tightly bound electrons in its outer or Valence ring,
they cannot be easily displaced.
Semiconductor;
Has at least 4 electrons in the outer or Valence ring, it is neither a conductor nor an insulator.
In its pure state it makes a better insulator than conductor. 4 electrons allows easy bonding w/ other materials.
Conduction
covalently bonded to atoms
*
In an intrinsic (no doping) material, ni=pi
At 300K, ni=pi is low (1010cm-3)
Use doping to improve conductivity
Extrinsic carriers
Substantially higher densities (1015cm-3)
Majority and minority carriers
Random recombination and thermal generation
*
P-type region
N-type region
*
PN Junction made of two homogenous regions of p and n-type material, separated by a region of transition from one type of doping to another.
Region is assumed THIN; abrupt (step) junction.
P-type: hole rich, doped with ACCEPTOR impurities (Boron).
N-type: electron rich, doped with DONOR impurities (Phosphorus, Arsenic)
Bringing n-type and p-type regions together causes a large CONCENTRATION GRADIENT at the boundary.
Gradient causes electrons to DIFFUSE from n to p and holes to diffuse from p to n.
When charges leave, they leave behind IMMOBILE IONS.
The DEPLETION or SPACE CHARGE REGION is the region at the junction where the majority carriers have been removed.
The charges create an ELECTRIC FIELD which prevents further diffusion of majority carriers.
EE314 Basic EE II
diode symbol
*
PN Junction made of two homogenous regions of p and n-type material, separated by a region of transition from one type of doping to another.
Region is assumed THIN; abrupt (step) junction.
P-type: hole rich, doped with ACCEPTOR impurities (Boron).
N-type: electron rich, doped with DONOR impurities (Phosphorus, Arsenic)
Bringing n-type and p-type regions together causes a large CONCENTRATION GRADIENT at the boundary.
Gradient causes electrons to DIFFUSE from n to p and holes to diffuse from p to n.
When charges leave, they leave behind IMMOBILE IONS.
The DEPLETION or SPACE CHARGE REGION is the region at the junction where the majority carriers have been removed.
The charges create an ELECTRIC FIELD which prevents further diffusion of majority carriers.
EE314 Basic EE II
Concentration Gradient causes electrons to diffuse from n to p, and holes to diffuse from p to n
This produces immobile ions in the vicinity of the boundary
Region at the junction with the charged ions is called the depletion region or space-charge region
Charges create electric field that attracts the carriers, causing them to drift
Drift counteracts diffusion causing equilibrium ( Idrift = -Idiffusion )
Depletion Region
hole diffusion
electron diffusion
p more heavily doped than n (NA > NB)
*
Mobile carriers drift through the dep. region into neutral regions
become excess minority carriers and diffuse towards terminals
hole diffusion
electron diffusion
Diffusion current is reduced
Diode works in the reverse bias with a very small drift current
hole diffusion
electron diffusion