1. io School of Microelectronic Engineering Lecture II Basic Semiconductor Devices

2. ` School of Microelectronic Engineering Objectives

3. ` School of Microelectronic Engineering Topics

4. ` School of Microelectronic Engineering Semiconductor Materials

5. ` School of Microelectronic Engineering What Is A Semiconductor?  Semiconductors are materials with electrical conductivity between conductors and insulators.  The most commonly used semiconductor materials are silicon and germanium.  Some compounds, such as GaAs, SiC and SiGe.  Most important property is its conductivity can be controlled by adding certain impurities in the process called doping.

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8. Periodic Table of The Elements

9. School of Microelectronic Engineering Periodic Table of The Elements

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11. ` Band Gap  Atom is basic building block of all materials  Classical mechanics – every atom has it own orbit structure.  Electron orbits are called shells.  The outermost shell is called valence shell.  When e leaves the valence shell, it becomes a free electron and can conduct electric current.

12. ` School of Microelectronic Engineering  When 2 or more identical atoms bond together to form solid materials, their orbit overlap and form so called energy bands. Can be represented by the energy band diagram  The bottom of conduction band is called Ec, and the top of the valence band is called Ev.  Eg = Ec – Ev  Eg is defined as the energy required to break a bond in semiconductor to free an e to cond band and leave the hole in the valence band.  Electrons in conduction band are free to move and can conduct electric current  Electrons in the valence band are bonded with nuclei and connot move freely, therefore cannot conduct electric current

13. ` School of Microelectronic Engineering Resistivity  Resistivity is the capability of a material resisting electric current.  A good conductor has a very low resistivity and a good insulator has a very high reistivity.  Unit: Ohm.cm

14. ` School of Microelectronic Engineering Resistivity and Band Gap  For most metals, conduction and valence bands almost overlap or very small band gap. Electron can easily jump from valence to conduction band. Therefore the conduction band has a lot of e.  For insulators, the band gap is so large that electrons cannot jump across it.

15. ` School of Microelectronic Engineering Semiconductor Materials and Its Applications

16. ` School of Microelectronic Engineering Crystal Properties of Semiconductors

17. ` School of Microelectronic Engineering Classification of Solids (Based on Atomic Arrangement)  Amorphous  Single Crystal  Poly Crystal

18. ` School of Microelectronic Engineering Crystal Structures

19. ` School of Microelectronic Engineering Amorphous Structure

20. ` School of Microelectronic Engineering Polycrystalline Structure

21. ` School of Microelectronic Engineering Single Crystal Structure

22. ` School of Microelectronic Engineering Silicon Crystal Structure  Silicon has four electrons in the outermost shell.  In a single crystal structure, every atom is bonded with four atoms shares a pair of electrons with each of them.

23. ` School of Microelectronic Engineering Crystal Lattice

24. ` School of Microelectronic Engineering Unit Cell

25. ` School of Microelectronic Engineering Unit Cell of Single Crystal Structure

26. School of Microelectronic Engineering Crystal Plane and Miller Indices

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28. Doping Semiconductors

29. School of Microelectronic Engineering Two types of Semiconductor Materials  Intrinsic Semiconductor  Extrinsic Semiconductor

30. School of Microelectronic Engineering Intrinsic Semiconductor  Pure semiconductor materials with no impurity atoms and no lattice defect.  At T=0 K, all energy states in valence band are filled with electrons, states in conduction band are empty.

31. School of Microelectronic Engineering Electrical Conduction in Intrinsic Semiconductor Si e-e- Silicon covalence bonding at T=0K

32. School of Microelectronic Engineering Si e-e- e Valance band Conduction band EgEg + e-e- As the temperature increase above 0K, a few valence bond electrons may gain enough thermal energy to break the bond and jump into the conduction band. As temperature increase further, more bonds broken, more electrons jump to the conduction band and more “empty states or holes” created in the valence band.

33. School of Microelectronic Engineering  In intrinsic material, electrons and holes are created in pairs by thermal energy. So the number of electrons in conduction band is equal to the number of holes in the valence band  Electron concentration = hole concentration  n i = p i and n i p i = n i 2 (MASS ACTION LAW) – the product of n p is always a constant for a given semiconductor material at given temperature

34. School of Microelectronic Engineering Extrinsic Semiconductor  Extrinsic s/c is defined as a semiconductor in which controlled amounts of specific dopant or impurity atoms have been added so that the thermal equilibrium electron and hole concentration are different from the intrinsic carrier concentration. Si P e-e- e-e- Intrinsic silicon lattice e Extrinsic silicon lattice

35. School of Microelectronic Engineering Doping of Semiconductors  The purpose of doping is to alter the conductivity of semiconductor materials.  Two types of dopant; p-type (B), n-type (P, As)  N-type dopants provide an electron in s/c materials, hence called donors.  P-type dopants provide a hole in s/c materials, hence called acceptor.

36. School of Microelectronic Engineering N-type Dopant  P and As have 5 electron valens  When doped into Si, 4 electrons used to form the covalence bond with Si  1 extra electron is left in the outermost shell and will occupy a new energy level called Donor Energy.

37. School of Microelectronic Engineering  Energy required to elevate donor electron is less than that for electron involved in covalence bonding.  With small thermal energy, donor electron is elevated to the conduction band  This process add electron to the conduction band without creating holes in the valence band.  The resulting material is referred as n-type semiconductor. valence band conduction band Ec Ev Ed Ec Ev Ed + + + - - -

38. School of Microelectronic Engineering P-type Dopant  B have 3 electron valens  When doped into Si, one empty state is created in the covalence bond  This empty state will occupy a new energy level called Acceptor Energy.

39. School of Microelectronic Engineering Si B e-e- Extrinsic silicon lattice doped with B Si B e-e- + Hopping of valence electron creating hole movement  Some valence electron gain a small amount of energy to move around the crystal lattice.  This electron would occupy the “empty” position associated with B atom.  The vacated electron position is considered as holes.

40. School of Microelectronic Engineering valence band conduction band Ec Ev Ea Ec Ev Ea + + + - - -  This process generate holes in the valence band without creating electrons in the conduction band.  The resulting material is referred as p-type semiconductor

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42. Dopant Concentration and Resistivity

43. School of Microelectronic Engineering Dopant Concentration and Resistivity WHY?

44. School of Microelectronic Engineering Basic Semiconductor Devices

45. School of Microelectronic Engineering Basic Semiconductor Devices

46. School of Microelectronic Engineering Resistor  The simplest electronic device.  In the IC fabrication, patterned doped silicon normally used to make resistors with resistance determined by the length, linewidth, junction depth and dopant concentration.  Poly silicon also used a resistor.

47. School of Microelectronic Engineering Example 1  Many people use polysilicon to form gates and local interconnect. Resistivity of polysilicon is determined by dopant concentration, about 10 22 cm -3, and ρ = 200  Ω.cm. Assume polysilicon gate and local interconnect line width, height, and legth are 1  m, 1  m and 100  m respectively. Calculate the resistance. R = ρ l / wh = 200  Ω.cm x (100 x 10 -4 ) cm / [(1x10 -4 cm) x 1X10 -4 cm) = 2 x 10 8  Ω = 200 Ω

48. School of Microelectronic Engineering Capacitor  One of the most important IC components  When two conducting materials are separated by a dielectric, a capacitor is formed. C = 00 h l d 00 - Absolute permittivity of vacuum (8.85 x 10 -12 F/m  - Dielectric constant

51.  Unwanted (parasitic) capacitor, as result of dielectric sandwiched between 2 metal layers. This will result in the RC delay of the IC circuit.  Major limitation for current IC device speed.  This application required low k dielectric and better conduction metal

52. Example 2  Calculate the capacitance for a capacitor shown with h = l = 10  m. Assume the dielectric between the 2 conducting plates is silicon dioxide, with k=3.9 and d=1000 Å. C = 00 h l d = 3.9 x 8.85x10 -12 x 10x10 -6 x 10x10 -6 1000x10 -10 = 3.45x10 -14 F

53. Example 3  Most IC chips use aluminum-copper alloy metal interconnection. The resistivity ρ = 3.2  Ω.cm, metal line geometry width w, height h, length l, and line spacing d are 1  m, 1  m, 1 m (1 million transistors connected by one metal line at 1  m between each transistor), and 1  m respectively. CVD silicon oxide lies between the metal line, with dielectric constant k = 4. Calculate the time delay. C = 00 h l d R = ρ l / wh Answer = 1.13 x 10 -8 sec

54. Diode  P-N junction  Allow electric conduction only in one way (positively biased)

55.  When p-type and n-type semiconductors join together, they form a p-n junction diode.  Holes in p-type region will diffuse to the n-type region, and electrons in n-type region will diffuse to the p-type region (at thermal equilibrium, without applied bias).  The area dominated by minority carriers is called the transistion region.  The voltage across the transistion region given by; For Si at room temperature, V 0 ~ 0.7 V I-V curve for diode

56. MOSFET

57. NMOS  Conducting gate (metal or polysilicon)  Heavily doped sour ce and drain  Ultra thin gate oxide  P-type substrate

58. NMOS  When no bias voltage is applied to the gate, no current flow.  When gate is positively biased, positive charge will appear at the gate.  Positive charge at the silicon surface will be expelled from the region.  At certain voltage (Threshold Voltage), electron will be accumulated at silicon surface to form channel, and allow the electron flow from source to drain.

59. PMOS  When no bias voltage is applied to the gate, no current flow.  When gate is negatively biased, negative charge will appear at the gate.  Negative charge at the silicon surface will be expelled from the region.  At certain voltage (Threshold Voltage), holes will be accumulated at silicon surface to form channel, and allow the holes flow from drain to source.

60. Basic Circuits  Bipolar  PMOS  NMOS  CMOS  BiCMOS

61. Bipolar

62. PMOS

63. NMOS

64. CMOS

67. BiCMOS

68. IC Device with different substrate

69. IC CHIPS  IC chips can be categorized into 3 main groups;  Memory  Microprocessor  ASIC

70. Memory

71. DRAM  Stands for Dynamic Random Access Memory. Random access means each memory cell in the chip can be accessed to read or write in any order.

72. Memory Cell of DRAM  Memory cell: location to store 1 bit of digital information (1 or 0) in a memory chip.  Memory cell of DRAM consists of 1 MOS transistor and 1 capacitor.  MOS serves as a switch. It allows e to flow into and store in the capacitor.  The capacitor needs to be recharged periodically by the power supply Vdd to compensate the e loss.  When power is removed from DRAM, the data are lost.

73. SRAM  Stands for Static Random Access Memory.

74. EPROM, EEPROM  Stands for electric-erasable programmable read-only memory.

75. EPROM, EEPROM Memory Cell TUNNEL OXIDE

76. EPROM, EEPROM Programming

77. SDA GNDA2A0A1 SCLWPVCC Charge Pump 16K-bits Memory Cell Timer Control Logic (Master) EEPROM Design Layout E/W circuit E/W circuit Decoder Xe Decoder Xr Dec Y Data ctrl Address block Decoder px

78. Microprocessor  Also called central processing unit (CPU) consists of 2 components;  a controller  arithmetic logic unit (ALU).  CPU is the brain of computers and other control system.  2 types of architecture;  complete instruction set computer (CISC) – IBM compatible  reduced intruction set computer (RISC) – Apple

79. ASICS  Application specific integrated circuits  Majority of chips belong to this cathegory; DSP, power devices, IC for TV, radio, internet, telecommunication, automobiles, etc

80. BASIC MOS IC PROCESS

82. MOS Technology

83. PMOS Process – 1960s

88. NMOS Process – Mid 1970s POLYSI GATE

91. CMOS Process – 1980s onwards