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The Devices: Diode EE314 Basic EE II 1.Semiconductors 2.Doping concept 3.n & p-type semiconductors 4.Si diode 5.Forward & reversed bias 6.Examples 7.Diode Characteristic Chapter 10: Diodes Engineer-In-Training Reference Manual http://www.amazon.com/Engineer-Training-Reference-Michael-Lindeburg/dp/0912045566 EE314 Basic EE II Outline Motivation and Goals Semiconductor Basics Diode Structure Operation » Static model EE314 Basic EE II 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. EE314 Basic EE II What do we know about an atomic structure? EE314 Basic EE II 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 EE314 Basic EE II 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 EE314 Basic EE II 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 EE314 Basic EE II 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. EE314 Basic EE II 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 EE314 Basic EE II 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 EE314 Basic EE II The Diode B A Al SiO 2 p n Cross section of pn-junction in an IC process EE314 Basic EE II N-type region P-type region doped with donor impurities (phosphorus, arsenic) doped with acceptor impurities (boron) The Diode Simplified structure A p Al A n The pn region is assumed to be thin (step or abrupt junction) EE314 Basic EE II B One-dimensional representation B diode symbol Different concentrations of electrons (and holes) of the p and ntype regions cause a concentration gradient at the boundary 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 EE314 Basic EE II Depletion Region •Zero bias conditions hole diffusion electron diffusion p •p more heavily doped than n (NA > NB) •Electric field gives rise to potential difference in the junction, known as the built-in potential (a) Current flow. n hole drift electron drift Charge Density + x Distance - Electrical Field (b) Charge density. x (c) Electric field. V Potential -W 1 EE314 Basic EE II W2 x (d) Electrostatic 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 EE314 Basic EE II Reverse Bias 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 EE314 Basic EE II Diode Current Ideal diode equation: EE314 Basic EE II