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Lecture 3

Lecture 3. Intrinsic Semiconductor. When a bond breaks, an electron and a hole are produced: n 0 = p 0 (electron & hole concentration) Also: n 0 p 0 = n i 2 Then: n 0 = p 0 = n i n i = intrinsic carrier concentration [cm -3 ]

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Lecture 3

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  1. Lecture 3

  2. Intrinsic Semiconductor • When a bond breaks, an electron and a hole are produced: n0= p0 (electron & hole concentration) Also: n0p0 = ni2 Then: n0= p0 = ni ni = intrinsic carrier concentration [cm -3 ] In Si at 300K (room temperature): ni = 1x 1010 cm -3

  3. Band Gap & Carrier Concentration Band gap and intrinsic carrier concentration for germanium, silicon and gallium arsenide at 300K

  4. Extrinsic Semiconductor: Donor • Nd = donor concentration [cm -3 ] • IfNd<< ni, doping irrelevant • Intrinsic semiconductor  n0= p0 = ni • IfNd>> ni, doping controls carrier concentrations • Extrinsic semiconductor  n0= Nd , p0 = ni2 /Nd Note: n0 >> p0 n-type semiconductor

  5. Extrinsic Semiconductor: Acceptor • Na = acceptor concentration [cm -3 ] • IfNa << ni, doping irrelevant • Intrinsic semiconductor  n0= p0 = ni • IfNa >> ni, doping controls carrier concentrations • Extrinsic semiconductor  p0= Na , n0 = ni2 /Na Note: p0 >> n0 p-type semiconductor

  6. Extrinsic Semiconductor: Donor & Acceptor • Carrier concentration can be engineered by addition of “dopants” (selected foreign atoms): • Pentavalent impurities (P, As, Sb)  n-type semiconductor: n0= Nd , p0 = ni2 /Nd • Trivalent impurities (B, Al, Ga)  p-type semiconductor: p0= Na , n0 = ni2 /Na

  7. Properties of Crystals • Two properties of crystals that are needed to calculate the current in a semiconductor: • First, we need to know how many fixed and mobile charges are present in the material. • Second, we need to understand the transport of the mobile carriers through the semiconductor.

  8. Carrier Transport • Two carrier transport mechanisms: • The drift of carriers in an electric field • The diffusion of carriers due to a carrier density gradient

  9. Carrier Drift • Electron and holes will move under the influence of an applied electric field since the field exert a force on charge carriers (electrons and holes). • These movements result a current of : drift current number of charge carriers per unit volume charge of the electron drift velocity of charge carrier area of the semiconductor

  10. Carrier Mobility , applied field mobility of charge carrier is a proportionality factor • So is a measure how easily charge carriers move under the influence of an applied field or determines how mobile the charge carriersare.

  11. V - + • n - type Si n – type Si e- Electric field Electron movement Current flow Currentcarriers are mostly electrons.

  12. V - + • p - type Si p – type Si hole Electric field Hole movement Current flow Current carriers are mostly holes.

  13. The PN Junction Diode • When a P-type semiconductor region and an N-type semiconductor region are in contact, a PN junction diode is formed. VD – + ID

  14. Diode Operating Regions • In order to understand the operation of a diode, it is necessary to study its behavior in three operation regions: equilibrium, reverse bias, and forward bias. VD = 0 VD < 0 VD > 0

  15. Carrier Diffusion across the Junction • Because of the difference in hole and electron concentrations on each side of the junction, carriers diffuse across the junction: Notation: nn electron concentration on N-type side (cm-3) pn hole concentration on N-type side (cm-3) pp hole concentration on P-type side (cm-3) np electron concentration on P-type side (cm-3)

  16. Depletion Region • A region in a semiconductor device, usually at the juncture of P-type and N-type materials, in which there is neither an excess of electrons nor of holes.

  17. Depletion Region • As conduction electrons and holes diffuse across the junction, they leave behind ionized dopants. Thus, a region that is depleted of mobile carriers is formed. • The charge density in the depletion region is not zero. • The carriers which diffuse across the junction recombine with majority carriers, i.e. they are annihilated. quasi-neutral region quasi-neutral region width=Wdep

  18. Carrier Drift across the Junction • Because charge density ≠ 0 in the depletion region, an electric field exists, hence there is drift current.

  19. Video Links • p-n-Juction-And-Diodes • http://www.youtube.com/watch?v=W6QUEq0nUH8 • http://www.youtube.com/watch?v=jWh06oaG6LA

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