1. Semiconductors 8
2015년 10월 15일 중간고사 1: 9:30-11:00

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

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

5. 8.2 Intrinsic Semiconductors
At elevated temperature, Semiconductor become conducting
For intrinsic semiconductors,
Assuming, me*=mh* 5

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

7. Inserting that EF=-Eg/2, and effective mass ratio me*/m0,
Number of electrons in the conduction band per unit volume (cm3) 7

8. Fermi Level in Semiconductors:
Emax Ec EF EG Ev
When m*e=m, Nc =2.5x1019 cm-3
Effective density of states in the conduction band 8

9. Fermi Level in Semiconductors:
By the same token,
Emax Ec Eg Ev
The number should be constant for a given material for a given temperature. 9

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

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

12. Due to lattice vibration Increasing the number of carriers12

13. 8.3 Extrinsic Semiconductors
8.3.1 Donors and Acceptors
For intrinsic semiconductor, 109 electrons per cubic centimeter
P. Binding E = eV
Doping : adding small amounts of impurities (III or V) to intrinsic semiconductors
Dopant in substitutional manner 13

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

16. D0 -> D+ + e- A0 -> A- + h+ Ec Ed Ef Ea Ev
nA: density of electron occupied acceptors
ND-nD: density of electron un-occupied donors Ec Ed Ef Ea Ev 16

17. 8.3.3 Temperature Dependence of the Number of Carriers17

18. 8.3.4 Conductivity 18

19. 8.3.5 Fermi Energy
n-type semiconductor, Nd=1016 atoms per cubic centimeter 19

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). 20

21. + - 8.5 Hall Effect Lorentz Force: elecorn Bz For hole: For electron:
Electron velocity: -Vx Bz
Hole velocity: Vx
For hole:
For electron: 21

22. 8.5 Hall Effect Due to Ex Hall field Lorentz force Hall force
Hall constant
Negative value for electron
Positive value for hole 22

23. 8.6 Compound Semiconductors

25. 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:
electrons like to roll downwhill
Holes wan to drift upward
(Space charge region) 25

26. 8.7.2 Rectifying Contacts (Schottky Barrier Contacts)
Work function: energy difference between the Fermi energy and the ionization energy
(Vacuum level)
(electron affinity)
Contact potential
- Diffusion current: electrons from both sides cross the potential barrier at equilibrium state
- Drift current: the transport of thermally created electrons and holes 26

27. 27

28. 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

29. Reverse bias
Forward bias 29

30. A: Area of the contact, C: constant
Inet = ISM-IMS 30

31. 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
Consists of saturation current and a voltage-dependent term
A few advantages over p-n diode
No annihilation of electrons and holes, charge carrier , electron
Better heat removal 31

32. 8.7.3 Ohmic Contacts (Metallizations)
The formation of highly doped region to make an Ohmic contact. 32

33. Rectifier (Diode) 33

34. Diffusion of electrons in p-type region
The saturation current in the case of reverse bias is given by the Shockley equation, which is also called ideal diode law:
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
Einstein relation: 34

35. 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

36. 8.7.6 Solar Cell (Photodiode)
Reverse bias operation
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 mm depending on the quality of Si.
Quantum Efficiency:
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.
Should explain LED: forward bias operation 36

37. 8.7.8 Tunnel Diode (OK) Degenerated doping – high doping
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

38. 8.7.9 Transistors Smaller and higher resistivity - +
For signal amplification
Smaller and higher resistivity
n-p diode
p-n diode - +
Climb diffuse acceleration
The E-B diode is forward biased, whereas the B-C diode is strongly reverse biased.
Unbiased n-p-n bipolar junction transistor
Electron flow from E to C can be controlled
by bias voltage on the Base 38

39. Transistors : amplification of music or voice electronic switch (on & off) for logic and memory
(1) Bipolar : current flow through n-type as well as through p-type
Heavily doped # of holes kept to a minimum(light doping) or thin
doping level is not critical
The voltage applied between emitter and base modulates the transfer of the electrons from the emitter into the base region. 39

40. (2) Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)
Can be controlled
Electric field
Can be controlled
Unipolar : current flow only through n-type
Two types: depletion-type MOSFET
Normally on 40

41. * Enhancement-type MOSFET
Normally off
This type is dominating in IC circuit industry 41

42. CMOSFET (complementary MOSEFT)
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

43. (2) Junction Field-Effect Transistor (JFET)
Normally on, depletion type 43

44. Transistors
                   
Transistors
                    44

45. (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

46. 8.7.10 Quantum Semiconductor Devices
The energy level is separated due to the size quantization. 46

47. 47

48. 8.7.11 Semiconductor Device Fabrication48

49. 49

50. 50

51. 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

52. 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

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