1 / 21

Intrinsic semiconductor

Intrinsic semiconductor. Doped semiconductor N-type. Doped semiconductor P-type. Electric current in semiconductors. Drift of charged carriers. Electric current in semiconductors. J p,drift = q μ p pE J n,drift = q μ n nE. v d = μ p E. I p,drift = qpv d A.

tuwa
Download Presentation

Intrinsic semiconductor

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Intrinsic semiconductor ESO / L1 / J.Boušek

  2. Doped semiconductor N-type ESO / L1 / J.Boušek

  3. Doped semiconductor P-type ESO / L1 / J.Boušek

  4. Electric current in semiconductors Drift of charged carriers ESO / L2 / J.Boušek

  5. Electric current in semiconductors Jp,drift = qμppE Jn,drift = qμnnE vd = μpE Ip,drift = qpvd A Jp,drift = qpvd ESO / L2 / J.Boušek

  6. ESO / L2 / J.Boušek

  7. Dependence on dopant concentration ESO / L2 / J.Boušek

  8. Diffusion ESO / L2 / J.Boušek

  9. Diffusion 1. Fick-Law: ESO / L2 / J.Boušek

  10. Diffusion + Drift Einstein equation ESO / L2 / J.Boušek

  11. Generation and recombination • Generation = need energy = generation in pairs: (electron + hole) • - photo-generation • - thermal excitation of the crystal lattice • - high energy electron • Recombination = loss of energy = recombination in pairs: (el. + hole) : • - large number of complicated processes - direct (interband) - undirect (recombination centres, traps) - surface • generation lifetime recombination • electrones… n holes…. p ESO / L2 / J.Boušek

  12. Lifetime of the carriers Doped semiconductor: Type N n >> p ; Type P p >> n Usually : n , p ≈ 1 s High quality silicon : n , p ≥ 1 ms High density of traps / of recombination centres : n , p ≈ 1 s ÷1 ns - High speed devices: Intentionally ... Au (Al) - Low quality production: Crystal distortions, Impurities ESO / L2 / J.Boušek

  13. Thermal equilibrium p0 n0 = ni2equilibrium state (index "0") Distortion of thermal equilibrium: n = n0 +n ; p = p0 +p (n a pconcentration of non-equilibrium carriers) Injection : np > ni2 low(n << n0) - medium (n  n0) - high (n >> n0) Extraction : np < ni2. ESO / L2 / J.Boušek

  14. PN-Junction in equilibrium state Depletion region ESO / L2 / J.Boušek

  15. PN-Junction in equilibrium state Concentration of dopants: ND = 1019 m-3 NA = 1020 m-3 Electrons in N: nn = ND = 1019 m-3 Electrons in P: np = ni2 / NA = 1032 m-6 / 1020 m-3 =1012 m-3 Difference in concentration 107 electron diffusion to P !!!!! In N only ionized donors (ND +) standing firmly in the lattice Holes in P: pp = NA = 1020 m-3 Holes in N: pp = ni2 / ND = 1032 m-6 / 1019 m-3 =1013 m-3 Difference in concentration 107 diffusion of holes to N !!!!! In N only ionized donors (ND +) standing firmly in the lattice Ionized dopants create space charge !!!!!!!!! ESO / L2 / J.Boušek

  16. Space charge in depletion area Electrical field Potential ESO / L2 / J.Boušek

  17. PN-Junction in equilibrium state Density of the space charge given by dopants concentration Junction area with lower dopants concentration ís wider Consequence : Electrical field in depletion area Emax- in metalurgical junction !!!! Potential difference between P and N : Diffusion voltage. Actual potential value given by the shape of electrical field ESO / L2 / J.Boušek

  18. Band-diagram of PN-Junction ESO / L2 / J.Boušek

  19. Band-diagram of PN-Junction 1) The position of EF in both areas P and N must correspond to the type of semiconductor / type of conductivity. (shift EF to EV in case of “P-type“ or to EC in case of “N-type“) 2) In Thermal equilibrium the value of Fermi level EF is constant. To fulfuill both 1) + 2) : a) mutual shift of Conductive and Valence bands (band-bending) b) The shift corresponds to qUD . qUD : energetic treshold - prevents diffusion of majority carriers. ESO / L2 / J.Boušek

  20. PN junction in FORWARD polarisation Diffusionvoltage - barrieragainstdiffusionof majority carriers Equilibriumstate : Onlysmalldiffusioncurrentwhichiscompensatedwiththe drift casused by potentialdifference in spacecharge area. majority carriers - diffusion minoritní carriers - drift In forward polarisation : external voltage acts against the potential in depletion area - the barrier / treshold is lower !!! Forward current is made by DIFFUSSION of majority carriers !! ESO / L2 / J.Boušek

  21. PN junction in REVERSE polarisation • Polarity of external voltage is the same as the polarity of electrical field in the space charge region: • !! Electrical field in the space charge region grows !! • Electrical field in space charge region enhance the drift of minority carriers from quasineutral parts of the junction: • The concentration of minority carriers in quasineutral parts of the junction drops. • When increasing the reverse voltage the reverse current does not increase !!!! Reverse current : DRIFT current of minority carriers !!!! ESO / L2 / J.Boušek

More Related