The Devices: Diode

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 http://www.amazon.com/Engineer-Training-Reference-Michael-Lindeburg/dp/0912045566

Outline Motivation and Goals Semiconductor Basics Diode Structure Operation Static model

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.

What do we know about an atomic structure?

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

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

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

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.

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

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

The Diode N-type region B 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.

The Diode Simplified structure A 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.

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

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

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

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

Diode Current Ideal diode equation: