1. 1 8 10 월 19 일 중간고사 1: 6:00 – 8:00 pm

2. 2

3. 3 Hybridization of s- and p-states Two s+p bands, lower filled higher empty for Ge, Si Group IV 8.1 Band Structure Sp3 state

4. 4 Calculated band structure for Si. Eg, at T=0 K C : 5.48 Si : 1.17 Ge : 0.74 Sn(gray) : 0.08  D : Debye Temperature Dependence of band-gap on temperature

5. 5 At elevated temperature, Semiconductor become conducting For intrinsic semiconductors, 8.2 Intrinsic Semiconductors Assuming, m e *=m h *

6. 6 Z(E): density of states N(E): number of electrons per unit energy N*: number of electrons

7. 7 Number of electrons in the conduction band per unit volume (cm 3 ) Inserting that E F =-E g /2, and effective mass ratio m e */m 0,

8. 8 Fermi Level in Semiconductors: E max EcEc EvEv EFEF When m * e =m, N c =2.5x10 19 cm -3 EGEG

9. 9 Fermi Level in Semiconductors: E max EcEc EvEv EgEg By the same token, The number should be constant for a given material for a given temperature.

10. 10 Intrinsic Semiconductor: The electron density of free electrons equals the density of free holes. Therefore, if n i is the intrinsic carrier density,

11. 11 Mobility Ohm’s law For intrinsic semiconductor,

12. 12 Due to lattice vibrationIncreasing the number of carriers

13. 13 For intrinsic semiconductor, 10 9 electrons per cubic centimeter Doping : adding small amounts of impurities (III or V) to intrinsic semiconductors Dopant in substitutional manner P. Binding E = 0.045 eV 8.3 Extrinsic Semiconductors 8.3.1 Donors and Acceptors

14. 14

15. 15 CB VB Donor electrons & thermally excited electrons n-type, major carrier: electrons P, As, Sb p-type, major carrier: holes B, Al, Ga, In Impurity states; donor or acceptor levels 8.3.2 Band Structure

16. 16 Ec Ev Ef Ed Ea

17. 17 8.3.3 Temperature Dependence of the Number of Carriers

18. 18 8.3.4 Conductivity

19. 19 n-type semiconductor, N d =10 16 atoms per cubic centimeter 8.3.5 Fermi Energy

20. 20 8.4 Effective Mass In the presence of electric field, electrons at the bottom of conduction band and holes at the top of the valence band move in opposite directions in real space (same sign mass but different sign charge), whereas electrons and holes both at the top of the valence band move in the same direction (different sign mass cancels different sign charge).

21. 21 8.5 Hall Effect Hall field Hall constant Negative value for electron Positive value for hole Due to E x

22. 22 8.6 Compound Semiconductors

23. 23

24. 24 (Space charge region) Holes wan to drift upward electrons like to roll downwhill 8.7 Semiconductor Devices 8.7.1 Metal-Semiconductor Contacts (a) rectifying contact: convert AC to DC (b) ohmic contact: electrons can easily flow in both directions draw the I-V curve:

25. 25 - Diffusion current: electrons from both sides cross the potential barrier at equilibrium state - Drift current: the transport of thermally created electrons and holes (Vacuum level) 8.7.2 Rectifying Contacts (Schottky Barrier Contacts) Work function: energy difference between the Fermi energy and the ionization energy (electron affinity) Contact potential

26. 26

27. 27 The width of the depletion region in the n-type semiconductor: Assuming full depletion model (there are no free electrons in the depletion region and that the only charge there is the charge on spatially uniform ionized donors.

28. 28 Reverse bias Forward bias

29. 29 A: Area of the contact, C: constant

30. 30 Consists of saturation current and a voltage-dependent term For low enough temperatures, Fermi level lies close to the conduction band, See Fig. 8.10 A few advantages over p-n diode No annihilation of electrons and holes, charge carrier, electron Better heat removal

31. 31 8.7.3 Ohmic Contacts (Metallizations) The formation of highly doped region to make an Ohmic contact.

32. 32 8.7.4 Rectifier (Diode)

33. 33 Ideal diode law : Einstein relation: The saturation current in the case of reverse bias is given by the Shockley equation, which is also called ideal diode law: The electrons in the p-type region and the holes in the n-type region can diffuse to the junction area and be swept away when the reverse bias voltage is applied. Diffusion of holes in n-type region Diffusion of electrons in p-type region

34. 34 8.7.5 Breakdown Voltage and Zener Diode When the reverse voltage of a p-n diode is increased above a critical value, the high electric field strength caused some electrons to become accelerated to a velocity at which impact ionization occurs. The breakdown voltage, which is the result of this avalanching process, depends on the degree of doping: the higher the doping the lower the breakdown voltage. Tunneling or Zener breakdown occurs when the doping is heavy and thus the barrier width becomes very thin. -takes place at low reverse voltages

35. 35 8.7.6 Solar Cell (Photodiode) Quantum Efficiency: A photodiode consists of a p-n junction. A Si PV device yields an inherent voltage of 0.6 V. Diffusion length of carriers: 10 – 200  m depending on the quality of Si. Si: 20 – 28% efficiency The goal is to produce for terrestrial applications inexpensive solar cells having 20% efficiency or better and a lifetime of about 20 years.

36. 36 8.7.8 Tunnel Diode (OK) Show negative current-voltage characteristics. Degenerated doping – high doping Fermi level lies in the conduction and valence band Depletion width is very narrow (~10 nm)

37. 37 n-p diode p-n diode Unbiased n-p-n bipolar junction transistor For signal amplification Climb diffuse acceleration Smaller and higher resistivity + - Electron flow from E to C can be controlled by bias voltage on the Base 8.7.9 Transistors The E-B diode is forward biased, whereas the B-C diode is strongly reverse biased.

38. 38 Transistors : amplification of music or voice electronic switch (on & off) for logic and memory Heavily doped # of holes kept to a minimum(light doping) or thin doping level is not critical (1) Bipolar : current flow through n-type as well as through p-type The voltage applied between emitter and base modulates the transfer of the electrons from the emitter into the base region.

39. 39 Unipolar : current flow only through n-type Two types: depletion-type MOSFET Electric field Can be controlled Normally on (2) Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

40. 40 Normally off This type is dominating in IC circuit industry * Enhancement-type MOSFET

41. 41 N-MOSFET P-MOSFET If both are integrated on one chip and wired in series, this technology is labeled CMOSFET (complementary MOSEFT) For information processing, low operating voltage, low power short channel for High speed. MOSFET = MOST (metal-oxide-semiconductor transistor) = MISFET (metal-insulator-semiconductor field-effect transistor)

42. 42 Normally on, depletion type (2) Junction Field-Effect Transistor (JFET)

43. 43 Transistors Transistors

44. 44 (2) GaAs MESFET Use of computer still demands higher switching speed device- GaAs seems to be the answer with its higher electron mobility Source, Drain – Ohmic contact Gate – Schottky contact

45. 45 8.7.10 Quantum Semiconductor Devices The energy level is separated due to the size quantization.

46. 46

47. 47 8.7.11 Semiconductor Device Fabrication

48. 48

49. 49

50. 50 8.7.12 Digital Circuits and Memory Devices 8-39: AND device (A(G) and B(S) on makes On)8-40: inverter circuit (Gate on – OFF, Gate off – On) 8-41: NAND (NOT-AND) device with one load MOSFET and two input MOSFET transistors, A and B on – OFF Either A or B off - On 8-42: OR device: Either A or B on - On

51. 51 8-43: NOR device: Either A or B on - OFF 8-44: SRAM memory device called R-S flip-flop with latch 8-46: DRAM memory device

52. 52