1. The Devices: Diode

2. n & p-type semiconductors Si diode Forward & reversed bias Examples
Doping concept
n & p-type semiconductors
Si diode
Forward & reversed bias
Examples
Diode Characteristic
Engineer-In-Training Reference Manual
Chapter 10: Diodes

3. Outline Motivation and Goals Semiconductor Basics Diode Structure
Operation
Static model

4. Atom Composed of 3 Basic particles: Protons, 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.

5. What do we know about an atomic structure?

6. Semiconductor Basics  I
Electrons in intrinsic (pure) Silicon
covalently bonded to atoms
“juggled” between neighbors
thermally activated: density  eT
move around the lattice, if free
leave a positively charged `hole’ behind

7. Semiconductor Basics  II
Two types of intrinsic carriers
Electrons (ni) and holes (pi)
In an intrinsic (no doping) material, ni=pi
At 300K, ni=pi is low (1010cm-3)
Use doping to improve conductivity

8. Semiconductor Basics  III
Extrinsic carriers
Also two types of dopants (donors or acceptors)
Donors bring electron (n-type) and become ive ions
Acceptors bring holes (p-type) and become ive ions
Substantially higher densities (1015cm-3)
Majority and minority carriers
if n>>p (n-type) electrons majority and holes minority
Random recombination and thermal generation

9. Conduction Conductor;
Has loosely bound electrons in its outer or Valence ring,
they are easily displaced.
Insulator;
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.

10. Semiconductor Basics  I
Electrons in intrinsic (pure) Silicon
covalently bonded to atoms
“juggled” between neighbors
thermally activated: density  eT
move around the lattice, if free
leave a positively charged `hole’ behind

11. Semiconductor Basics  II
Two types of intrinsic carriers
Electrons (ni) and holes (pi)
In an intrinsic (no doping) material, ni=pi
At 300K, ni=pi is low (1010cm-3)
Use doping to improve conductivity
Extrinsic carriers
Also two types of dopants (donors or acceptors)
Donors bring electron (n-type) and become ive ions
Acceptors bring holes (p-type) and become ive ions
Substantially higher densities (1015cm-3)
Majority and minority carriers
if n>>p (n-type) electrons majority and holes minority
Random recombination and thermal generation

12. The Diode N-type regionB A
SiO 2 Al
Cross section of pn-junction in an IC process
N-type region
doped with donor impurities (phosphorus, arsenic)
P-type region
doped with acceptor impurities (boron)
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.

13. The Diode Simplified structureA B n p Al
One-dimensional
representation
diode symbol
The pn region is assumed to be thin (step or abrupt junction)
Different concentrations of electrons (and holes) of the p and n-type regions cause a concentration gradient at the boundary
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.

14. Depletion Region
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 )
hole diffusion
electron diffusion p n
hole drift
electron drift

15. Depletion Region Zero bias conditions
p more heavily doped than n (NA > NB)
Electric field gives rise to potential difference in the junction, known as the built-in potential

16. Forward Bias
hole diffusion
electron diffusion p n
hole drift
electron drift + -
Applied potential lowers the potential barrier, Idiffusion > I drift
Mobile carriers drift through the dep. region into neutral regions
become excess minority carriers and diffuse towards terminals

17. Reverse Bias - + Applied potential increases the potential barrier
hole diffusion
electron diffusion p n
hole drift
electron drift - +
Applied potential increases the potential barrier
Diffusion current is reduced
Diode works in the reverse bias with a very small drift current

18. Diode Current
Ideal diode equation: