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Microlectronics 2

Electrical Engineering 2. Lecture 12. Microlectronics 2. Dr. Peter Ewen. (Room G08, SMC; email - pjse). electron hole. I llumination h  > E g. E. p. n. - +. Fig. 67: Schematic structure of the photodiode. electron drift flow. hole drift flow. W. Depletion Region.

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Microlectronics 2

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  1. Electrical Engineering 2 Lecture 12 Microlectronics 2 Dr. Peter Ewen (Room G08, SMC; email - pjse)

  2. electron hole Illumination h > Eg E p n - + Fig. 67: Schematic structure of the photodiode electron drift flow hole drift flow W Depletion Region Ip is the optically generated current R V Ip Light Intensity, P; Ip~ mA for P ~ mWcm-2

  3. 0 mWcm-2 0.075 mWcm-2 0.150 mWcm-2 0.225 mWcm-2 0.300 mWcm-2 IF / A 50 40 30 20 10 • Good linearity (Ip P) • Fast response time (~10-9s) 0 0.1 0.7 Light Intensity, P mWcm-2 0.2 0.6 0.3 0.5 0.4 0 mWcm-2 0.1 0.2 0.3 0.4 0.5 -3 -2 -1 0 V / volts -30 -60 -90 -120 -150 Fig. 68: Typical photodiode characteristics IR / A

  4. IL  Rload VL Fig. 70: Photodiode used in photovoltaic mode to convert optical to electrical power. A V  Fig. 69: Photodiode used in photoconductive mode to detect light. Alternative symbol

  5. IF / A 0 mWcm-2 0.075 mWcm-2 0.150 mWcm-2 0.225 mWcm-2 0.300 mWcm-2 50 40 30 20 10 Fig. 68: Typical photodiode characteristics IL = 0 = Isc  Rload VL Voc = =0 0.1 0.2 0.3 0.4 0.5 -3 -2 -1 0 V / volts Voc -30 -60 -90 -120 -150 load line Slope of load line is -1/Rload Isc Photovoltaic region Photoconductive region IR / A

  6. C.B. defect levels phonons V.B. Fig. 72: Non-radiative recombination.  = hc/Eg For visible emission 1.4 < Eg < 3 eV GaAs – red light GaP – green light GaAsP – yellow light InGaN – blue light

  7. THE SEMICONDUCTOR LASER p-type n-type Heavily doped pn junction EC Eg electrons EC EV holes forward bias electrons EV Fig.75: Semiconductor laser EC Eg EV holes EF EF electrons EC electrons zero bias EV electrons EC EC Eg strong forward bias EV holes EV

  8. LECTURE 12 • The Abrupt-Junction model Electric field in the depletion region Potential in the depletion region  Field-effect transistors • Structure of the MOS transistor

  9. A diode has the I-V characteristic shown below. Its dynamic resistance, rd , at 300 K is 10  at the operating point shown. What are the values of Is and  for this device? I / mA 5 4 3 2 1 Is A 1 mA 1 B -1 mA 2 C 1 nA 1 D 1 nA 2 E -1 nA 1 F -1 nA 2 operating point 0.8 1.6 -1.6 -0.8 V / volts -0.5 -1 -1.5 -2 -2.5 I / nA

  10. Which one of the following combination of processes is the basis for the operation of a semiconductor laser? • Stimulated emission and spontaneous emission • Stimulated emission and non-radiative recombination • Stimulated emission and population inversion • Spontaneous emission and non-radiative recombination • Spontaneous emission and population inversion • Non-radiative recombination and population inversion

  11. The Abrupt Junction Diffusion Implantation Linearly-graded junction model Abrupt junction model NA Dopant concentration NA or ND Dopant concentration NA or ND p n p n xj xj ND ND Depth into wafer, x Depth into wafer, x ABRUPT JUNCTION LINEARLY-GRADED JUNCTION

  12. Electric field and potential in the depletion region of an abrupt pn junction Poisson’s equation is a fundamental equation of electromagnetics which relates the potential or electric field to the charges in a region: Poisson’s Equation • E – electric field • x – distance •  – permittivity of the material in the region • c– charge density, i.e. the net amount of charge per unit volume in the region

  13. junction - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + - - + + - + - + - + - + - + - + - + - + n-type p-type + - + - + - + - + - + - + - + - + - + - -ve +ve The charges in the depletion region are those on the carriers and on the charged impurity ions fixed in the lattice. E depletion region Taking the sign of the charges into account: c = e(p – n + ND – NA) dE e dx   = (p – n + ND – NA) For simplicity take n = 0 & p = 0 – the depletion approximation

  14. Depletion region - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + - - + + - + - + - + - + - + - + - + - + + - + - + - + - + - + - + - + - + - + - n-type p-type -lp x=0 ln junction Charge density variation through a pn junction Charge density, c eND 0 Distance, x -eNA Fig. 76.2(a)

  15. To find the Electric Field For the p-type side we have: Poisson’s equation on p-type side Since NA is constant (abrupt junction) Since E = 0 outside the depletion region, i.e. at x  -Ip Similarly, for the n-type side:

  16. - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + - - + + - + - + - + - + - + - + - + - + + - + - + - + - + - + - + - + - + - + - n-type p-type -lp x=0 ln Electric field, E 0 Distance, x Ep En Emax E Fig. 76.2(b) Electric field variation through a pn junction

  17.  At x = 0, Ep must equal En, since the electric field at any one point can only have a single value: Ep = En NAlp = NDln i.e. the depletion region penetrates a shorter distance into the more heavily doped side. If the doping on one side is much larger than on the other, the junction or diode is said to be one-sided.  The maximum electric field in the depletion region occurs at x = 0 and is:

  18. To find the Potential The electric field, E, is defined by: For the p-type side we have: If we take the zero of potential to be at x = 0 then C' = 0.

  19. - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + - - + + - + - + - + - + - + - + - + - + Hence: Similarly for the n-type side: + - + - + - + - + - + - + - + - + - + - n-type p-type -lp x=0 ln Potential variation Vn Potential, V Fig. 76.2(c) VB 0 Distance, x Vp

  20. 8. Maximum electric field at a junction For a one-sided abrupt Si junction with NA = 1025 m-3 and ND = 1022 m-3, calculate the maximum field at zero bias if VB = 0.874 V. [εr = 11.9 for Si; ε0 = 8.85 x 10-12 Fm-1]

  21. 8. Maximum field at a junction n-type p-type -lp x=0 ln Vn Potential, V VB 0 Distance, x Vp NA = 1025 m-3 ND= 1022 m-3 VB = 0.874 V NA >> ND lp << ln and is small.

  22. n-type p-type -lp x=0 ln Vn Potential, V VB 0 Distance, x Vp For a one-sided junction with NA >> ND , lp ≈ 0. Hence the potential variation through the depletion region will just be that on the n-type side: Hence Vn(ln), the potential at the boundary of the depletion region on the n-type side, must equal VB, i.e.

  23. NA = 1025 m-3 ND= 1022 m-3 VB = 0.874 V

  24. 9. Electric field in a semiconductor A 1m3 cubic region of an n-type semiconductor has ND = 1022 m-3. All the donors are ionised and there are no other charges in the region. If the electric field, E, on one face of the cube is zero, use Poisson’s Equation below to find the electric field at the opposite face. (r = 11.9 for Si) 1m 1m E=0 E=? 1m x = 1μm x = 0μm

  25. 9. Electric field in a semiconductor 1m 1m E=0 E=? 1m x = 1μm x = 0μm Taking the E = 0 face to be at x = 0 implies A (the constant of integration) is also 0

  26. - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + - - + + - + - + - + - + - + - + - + - + + - + - + - + - + - + - + - + - + - + - n-type p-type -lp x=0 ln eND Charge density, c 0 0 0 Distance, x -eNA Electric field, E E Distance, x Ep En Emax VB Vn Potential, V Vp Distance, x

  27. Fig. 77 Transistors Bipolar (BJT) Field-effect npn pnp MOS Junction (JFET) n-channel p-channel Enhancement mode Depletion mode n-channel p-channel n-channel p-channel

  28. Fig. 78: Changes in market share for different transistor technologies GaAs 1% 100 90 80 70 60 50 40 30 20 10 0 ECL 9% TTL Percentage of market Analogue bipolar % market share PMOS 85% NMOS CMOS BiCMOS 5% 1985 1990 1995 2005 Year

  29. Fig. 79 Gate p-type semiconductor + electrode Gate voltage VG holes – + Source Drain − electrode insulation

  30. Fig. 79 Gate p-type semiconductor + electrode Gate voltage VG holes – + Source Drain − electrode insulation

  31. MOS Transistor – Basic Structure Gate Source Drain n-channel device +Vg Metal Oxide Semiconductor SiO2 n+ n+ Channel p-type substrate Fig. 80

  32. Summary • THE ABRUPT JUNCTION • In the abrupt junction model it is assumed that there is a step-like changeover from p- to n-type material at the junction. • This is a good approximation for junctions produced by shallow diffusion or implantation.

  33. POISSON’S EQUATION • Poisson’s Equation is a fundamental equation of electromagnetics relating the electric field to the charge density in a region: • THE DEPLETION APPROXIMATION The carrier concentrations in the depletion region are small – in the depletion approximation it is assumed that n = 0 and p = 0.

  34. POTENTIAL VARIATION ELECTRIC FIELD VARIATION E

  35. MOS STRUCTURE • Two heavily doped regions at wafer surface – the SOURCE and DRAIN. • Conducting electrode formed between source and drain, insulated from the Si by a layer of SiO2 – the GATE. • Gate electrode controls the current flowing between source and drain.

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