The Devices: Diode

1. The Devices:Diode

2. Semiconductors • Doping concept • n & p-type semiconductors • Si diode • Forward & reversed bias • Examples • Diode Characteristic Engineer-In-Training Reference Manual Chapter 10: Diodes http://www.amazon.com/Engineer-Training-Reference-Michael-Lindeburg/dp/0912045566

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 http://www.masstech.org/cleanenergy/solar_info/images/crystal.gif

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

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

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

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. hole diffusion electron diffusion p n hole drift electron drift + - Forward Bias • 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. hole diffusion electron diffusion p n hole drift electron drift - + Reverse Bias • 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: