1 / 31

EE 5340 Semiconductor Device Theory Lecture 17 – Spring 2011

EE 5340 Semiconductor Device Theory Lecture 17 – Spring 2011. Professor Ronald L. Carter ronc@uta.edu http://www.uta.edu/ronc. Summary of V a > 0 current density eqns. Ideal diode, J s expd ( V a /( h V t )) ideality factor, h Recombination, J s,rec exp ( V a /(2 h V t ))

infinity
Download Presentation

EE 5340 Semiconductor Device Theory Lecture 17 – Spring 2011

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. EE 5340Semiconductor Device TheoryLecture 17 – Spring 2011 Professor Ronald L. Carter ronc@uta.edu http://www.uta.edu/ronc

  2. Summary of Va > 0 current density eqns. • Ideal diode, Jsexpd(Va/(hVt)) • ideality factor, h • Recombination, Js,recexp(Va/(2hVt)) • appears in parallel with ideal term • High-level injection, (Js*JKF)1/2exp(Va/(2hVt)) • SPICE model by modulating ideal Js term • Va = Vext - J*A*Rs = Vext - Idiode*Rs

  3. ln(J) Plot of typical Va > 0 current density equations data Effect of Rs Vext VKF

  4. For Va < 0 carrierrecombination in DR • The S-R-H rate (tno = tpo = to) is

  5. Reverse bias (Va<0)=> carrier gen in DR • Consequently U = -ni/2t0 • t0 = mean min. carr. g/r lifetime

  6. Reverse bias (Va< 0),carr gen in DR (cont.)

  7. Ecrit for reverse breakdown (M&K**) Taken from p. 198, M&K**

  8. Reverse biasjunction breakdown • Avalanche breakdown • Electric field accelerates electrons to sufficient energy to initiate multiplication of impact ionization of valence bonding electrons • field dependence shown on next slide • Heavily doped narrow junction will allow tunneling - see Neamen*, p. 274 • Zener breakdown

  9. Reverse biasjunction breakdown • Assume-Va= VR >> Vbi, so Vbi-Va-->VR • Since Emax~ 2VR/W = (2qN-VR/(e))1/2, and VR = BV when Emax = Ecrit(N- is doping of lightly doped side ~ Neff) • BV = e (Ecrit)2/(2qN-) • Remember, this is a 1-dim calculation

  10. Junction curvatureeffect on breakdown • The field due to a sphere, R, with charge, Q is Er = Q/(4per2) for (r > R) • V(R) = Q/(4peR), (V at the surface) • So, for constant potential, V, the field, Er(R) = V/R (E field at surface increases for smaller spheres) Note: corners of a jctn of depth xj are like 1/8 spheres of radius ~ xj

  11. BV for reverse breakdown (M&K**) Taken from Figure 4.13, p. 198, M&K** Breakdown voltage of a one-sided, plan, silicon step junction showing the effect of junction curvature.4,5

  12. Diode equivalentcircuit (small sig) ID h is the practical “ideality factor” IQ VD VQ

  13. Small-signal eqcircuit Cdiff and Cdepl are both charged by Va= VQ Va rdiff Cdepl Cdiff

  14. Diode Switching • Consider the charging and discharging of a Pn diode • (Na > Nd) • Wn << Lp • For t < 0, apply the Thevenin pair VF and RF, so that in steady state • IF = (VF - Va)/RF, VF >> Va, so current source • For t > 0, apply VR and RR • IR = (VR + Va)/RR, VR >> Va, so current source

  15. Diode switching(cont.) VF,VR >> Va F: t < 0 Sw RF R: t > 0 VF + RR D VR +

  16. Diode chargefor t < 0 pn pno x xn xnc

  17. Diode charge fort >>> 0 (long times) pn pno x xn xnc

  18. Equationsummary

  19. Snapshot for tbarely > 0 pn Total charge removed, Qdis=IRt pno x xn xnc

  20. I(t) for diodeswitching ID IF ts ts+trr t - 0.1 IR -IR

  21. Ideal diode equation for EgN = EgN Js = Js,p + Js,n = hole curr + elecurr Js,p = qni2Dpcoth(Wn/Lp)/(NdLp), [cath.] = qni2Dp/(NdWn), Wn<< Lp, “short” = qni2Dp/(NdLp), Wn>> Lp, “long” Js,n = qni2Dncoth(Wp/Ln)/(NaLn), [anode] = qni2Dn/(NaWp), Wp<< Ln, “short” = qni2Dn/(NaLn), Wp>> Ln, “long” Js,n<<Js,p when Na>>Nd, Wn & Wpcnrwdth

  22. Ideal diode equationfor heterojunction • Js = Js,p + Js,n = hole curr + elecurr Js,p = qniN2Dp/[NdLptanh(WN/Lp)], [cath.] = qniN2Dp/[NdWN], WN << Lp, “short” = qniN2Dp/(NdLp), WN >> Lp, “long” Js,n = qniP2Dn/[NaLntanh(WP/Ln)], [anode] = qniP2Dn/(NaWp), Wp<< Ln, “short” = qniP2Dn/(NaLn), Wp>> Ln, “long” Js,p/Js,n ~ niN2/niP2 ~ exp[[EgP-EgN]/kT]

  23. The BJT is a “Si sandwich” Pnp (P=p+,p=p-) or Npn(N=n+, n=n-) BJT action: npn Forward Active when VBE> 0 and VBC< 0 E B C p n P VEB VCB Depletion Region Charge neutral Region Bipolar junctiontransistor (BJT)

  24. IE IC IB x’ x x” xB x”c x’E 0 0 0 z 0 -WE WB+WC WB N-Emitter p-Base n-Collector Depletion Region Charge Neutral Region npn BJT topology

  25. BJT boundary andinjection cond (npn)

  26. BJT boundary andinjection cond (npn)

  27. IC npn BJT(*Fig 9.2a)

  28. References * Semiconductor Physics and Devices, 2nd ed., by Neamen, Irwin, Boston, 1997. **Device Electronics for Integrated Circuits, 2nd ed., by Muller and Kamins, John Wiley, New York, 1986.

  29. References * Semiconductor Physics and Devices, 2nd ed., by Neamen, Irwin, Boston, 1997. **Device Electronics for Integrated Circuits, 2nd ed., by Muller and Kamins, John Wiley, New York, 1986.

More Related