ECSE-6290 Semiconductor Devices and Models II Lecture 18

1. ECSE-6290Semiconductor Devices and Models IILecture 18 Prof. Shayla M. Sawyer Bldg. CII, Room 8225 Rensselaer Polytechnic Institute Troy, NY 12180-3590 Tel. (518)276-2164 FAX (518)276-2990 e-mail: ssawyer@ecse.rpi.edu

2. Lecture Outline • Monday April 25th • Laser Diodes and Photodetectors • Assign Homework 4 • Monday May 2nd • 15-20 minute presentations, class may end at 4:30 p.m. • Monday, May 9th • Homework 4 due • Final paper due (electronic preferred) • Thursday, May 12th • Awards announced by email

3. Lecture Outline • Introduction to Thin Film Transistors • History • Technology • Amorphous Silicon (a-Si) TFTs • Polysilicon (poly-Si) TFTs • Summary

4. TFT Introduction What is the niche market for thin film transistors? “Thin film transistors have in the past 10 years become the main stable of the electronic flat panel industry, just as silicon chips were earlier called staple of the electronic computer revolution.” Kagan http://www.unbeatable.co.uk/ articles/lg_100_inch_tft_lcd.jpg

5. TFT Introduction What is the niche market for thin film transistors? • Crystalline wafers • Fragile • Relatively expensive • Limited in size • Amorphous or polycrystalline materials • Relatively inexpensive • Large area • Drawbacks: very small mobility of electrons and holes

6. TFT Introduction: Polycrystalline History • P.K. Weimer at RCA Laboratories (1962) • Top gated staggered CdS TFT • Used thin films of polycrystalline cadmium sulfide (typically photodetector material) • Used insulating films such as silicon monoxide • Glass substrate

7. Introduction: Polycrystalline History • P.K. Weimer at RCA Laboratories (1962) • Achieved transconductances of 25,000 μA/V for gate capacitance of 50 pF • Maximum oscillation frequency would be 80 MHz but actual values were 20MHz • Using analysis in linear regime, • High transconductance corresponded to mobilities of 140 cm2/Vs

8. Introduction: Polycrystalline History • In 1960s competition to provide basis for lower performance logic a very low cost, relative to the cost of crystalline silicon • In 1962 IGFET (or MOSFET) demonstrated high economic potential • Decline of TFT development by end of 1960s

9. Introduction: Polycrystalline History • In 1970s TFTs found niche in need for large arrays of low cost electronics from LCD invention in 1960s • Simple x-y electronics caused too much cross talk • “Passive” pixel matrix • 1971 Lechner et al. proposed TFT plus capacitor • 1973 demonstrated LCD with 120 x 120 picture elements in a 6”x6” format • Each intersection had a CdSe TFT • “Active” pixel matrix

10. Introduction: History • Liquid Crystal Operation • LC sandwiched between glass plates which are rubbed at right angles to align the LC molecules • Perpendicular polarizers • No voltage: LC molecules twist to align with rubbing of glass plates (pixel on) • Voltage: LC molecules untwist to align with electric field (pixel is off) http://nina.ecse.rpi.edu/tft/research_summary/sld010.htm

11. Introduction: History • First demonstration of amorphous silicon (a-Si) Spear and LeComber (1972) • Used glow discharge decomposition of (SiH4) • Amorphous Si obtained high concentration of H which tied up dangling bonds present • Dangling bonds become localized states • At high concentration they pin the Fermi level • Amorphous Si:H reduces the number of localized states by many orders of magnitude • Localized states determine transport properties

12. Introduction: History • Advantages (a-Si) • Very large area (2x4 feet and larger) • Inexpensively produced in a continuous process • Disadvantage (a-Si) • Low carrier mobility • Effective electron mobility 1 cm2/Vs • Applications (a-Si) • Large area driver for Liquid Crystal Displays • Basic Integrated Circuits • Addressable image sensing arrays http://regmedia.co.uk/2006/02/03/dell_lcd_tv.jpg

13. Introduction: History • 1990s Organic TFTs • Garnier reported TFTs evaporated hexathiophene as an active material • Claiming mobilities comparable to those of a-Si • Future rugged, lightweight displays that can be rolled up like a map http://students.washington.edu/jetpeach/EE341_Organic_Transistors_Presentation.ppt

14. Introduction: History • Comparison • Amorphous silicon has taken over due to its economics, cheap set up, and reproducibility • Low temperature processing, reduced device performance • Polycrystalline silicon is not as reproducible because high mobility is associated with large grains and short channels • Causes variation from transistor to transistor • High temperature processing; better device performance • Try new materials????

15. Introduction J. Yeon et al, Electronic Materials Letters, (2011) K. Nomura et al., Letters to Nature, (2004)

16. Introduction: Display Technology • AMLCD (Active Matrix LCD) • Each pixel has a TFT • Gate is attached to a horizontal row electron • Drain attached to a column electrode • Data applied through vertical lines, changing the polarization and optical transparency of the liquid crystal cell • Source is attached to the LC electrode • Display is activated one row at a time by activating gate lines • Column electrons carry data voltages • When TFT is turned on, the date line charges up the LC capacitor to the appropriate voltage • Then the TFT is turned off so that the charge is held on the capacitor until the next refresh time • Data voltage is isolated from the other rows of the display by the TFT (crosstalk is very low even with large number of rows)

17. Introduction: Display Technology AMLCD • Advantage: • Thin, light-weight, and low power consumption • Applications: • Portable, compact systems and high-definition TV • Market: • 1993: 2 billion • 1998: 10-15 billion • 2005: 74 billion • 2010: 100 billion • Disadvantage: • Cost, large upfront investment and manufacturing difficulties CRT • Advantage: • Low cost per pixel and high quality of display • Applications: • TV, computer, oscilloscope and measuring device • Market: • 1993: 11 billion • 1998: 18 billion • Disadvantage: • Environmental and safety issue

18. Introduction: Display Technology • AMOLED (Active Matrix OLED) • Backplane TFT • Android, IPod etc….. http://www.isuppli.com/display-materials-and-systems/marketwatch/pages/oled-shortages-cause-concerns-in-smart-phone-market.aspx

19. Material Properties • Major difference between crystalline Si and amorphous or polycrystalline Si is the presence of localized states in the energy band gap • These localized states are the center of operation of the TFT

20. Amorphous Si: Material Properties • a-Si is a direct band gap material with an energy gap around 1.7 eV • Much larger absorption coefficient than Si • Presence of a large number of localized states in the energy gap • Behave as • Acceptor-like states (tail and deep) • Donor-like states (tail and deep)

21. Amorphous Si: Material Properties • Electron and hole band mobilities are on the order of 10 cm2/Vs • Distribution of donor-like states and acceptor-like states is not symmetrical (more donor-like states) • Position of Fermi level in an undoped uniform a-Si sample, EF0, is shifted closer to the conduction band

22. Amorphous Si: Principe of operation • Upper half of the energy gap of a-Si:H at the a-Si gate insulator interface • Gate bias induces electron into the a-SiH channel • Electrons fill the localized states and shift the Fermi level toward the conduction band • In deep localized states, modest electron concentration = large shift of Fermi level • Below-threshold regime

23. Amorphous Si: Principe of operation • Below-threshold regime • Nearly all induced charge goes in to deep acceptor-like states and surface states at a-Si insulator interface (reason current value is small) • How is this different from MOSFET? • Large shift in Fermi level corresponds to large relative increase of electron concentration in conduction band since

24. Amorphous Si: Principe of operation • Point at which Fermi level enters tail states corresponds to threshold voltage • Fermi level enters tail states, position changes less with further gate bias • Concentration of tail states increase rapidly with energy

25. Amorphous Si: Principe of operation • Above-threshold regime • Free carrier concentration is a sizable fraction of the total induced charge • Large fraction still in localized states (do not contribute to channel conductance) • Field effect mobility • A function of surface electron charge induced in the channel

26. Amorphous Si: Principe of operation • Transitional regime • Induced charge is increased further • Tail states at the a-Si insulator interface are almost completely filled • Fermi level touches the bottom of the conduction band • Divided induced charge between conduction band and tail states farther from the a-Si interface

27. Amorphous Si: Principe of operation • Crystalline regime • Fermi level at the a-Si insulator interface moves high into conduction band so most induced charge goes into conduction band • Gate-source voltage necessary is currently too large for practical applications (50-100V)

28. Amorphous Si: Principe of operation • Numerically calculated densities of free and trapped carriers (ns and nt) • Regimes • EFo-Ev is less than approx. 1.4 eV • EFo-Ev is less than approx. 1.7 eV and larger than 1.4 eV • EFo-Ev greater than 1.7 eV Below threshold Above threshold Crystalline

29. Amorphous Si: Characteristics • Use gradual channel approximation model to derive TFT characteristics • In an a-Si TFT, ns is not a simple linear function of the gate source voltage • Field effect mobility as a function of nind has different regions which in the relevant range of the induced charge, can be approximated by m and μoare constants no =1012 cm-2 (convenient scaling factor)

30. Amorphous Si: Characteristics • Equation relating drain current ID to the longitudinal electric field in the channel μ is the band mobility W is the gate width V is the channel voltage ns can be related to the sheet density of induced electrons nind and the field effect mobility μFET by

31. Amorphous Si: Characteristics • According to the gradual channel approximation we can write Where ninds is the sheet charge concentration in the channel per unit area by the source side of the channel or Where nindd is the sheet density of induced electrons at the drain

32. Amorphous Si: Characteristics • NOTE: In a conventional MOSFET • If this is substituted into • It yields the standard equation for a crystalline long channel MOSFET in the linear regime

33. Amorphous Si: Characteristics http://nina.ecse.rpi.edu/tft/research_summary/sld042.htm

34. Amorphous Si: Characteristics http://nina.ecse.rpi.edu/tft/research_summary/sld042.htm

35. Amorphous Si: Characteristics http://nina.ecse.rpi.edu/tft/research_summary/sld042.htm

36. Amorphous Si: Characteristics http://nina.ecse.rpi.edu/tft/research_summary/sld042.htm

37. Poly-Si: Material Properties • Consists of small crystallites (grains) separated by grain boundaries • Electron and hole mobilities increase with grain size • Carrier transport across grains plays dominate role • Mobilities are much greater than a-Si but much less than crystalline

38. Poly-Si: Material Properties • There are charge traps within the grain boundary • Double Schottky barrier is formed at electrostatic and thermal equilibrium • Thermionic emission process and the drift- diffusion process are used for carrier transport http://arxiv.org/ftp/cond-mat/papers/0308/0308537.pdf

39. Poly-Si: Material Properties • Two regions for operation • Subthreshold • Based on diffusion model • Above threshold • Based on MOSFET model • μFET accounts for trap states

40. Poly-Si: Material Properties • Leakage current is high during off state

41. Comparison a-Si and Poly-Si

42. Comparison a-Si and Poly-Si

43. Summary: Comparison a-Si and Poly-Si

44. ECSE-6290Semiconductor Devices and Models IILecture 19: LEDs Shayla M. Sawyer Bldg. CII, Room 8225 Rensselaer Polytechnic Institute Troy, NY 12180-3590 Tel. (518)276-2164 FAX (518)276-2990 e-mail: ssawyer@ecse.rpi.edu

45. Lecture Outline • Introduction • Applications • Basic Physics (Overall Processes) • LED Physics • Laser Physics

46. Introduction • First Electroluminescence Phenomenon 1907 • H.J. Round (experiment with SiC) • “On applying a potential of 10 volts between two points on a crystal of carborundum, the crystal gave out yellowish light…There seems to be some connection between the above effect and the e.m.f produced by a junction of carborundum and another conductor….” • Electrical World, February 9, 1907

47. Introduction • 1920s and 1930s Lossev • 1949 Development of the pn junction • Other materials studied Ge and Si (low efficiency) • 1962 Direct bandgap GaAs • Semiconductor laser • Four papers almost simultaneously published Hall et al., Nathan et al., Quist et Al. • On GaAsP Holonyak and Bevacqua • 1964-1965 Indirect band gap material advancement through isoelectronic impurities • Impact on GaAsP and GaP • Most recent development GaN-based for Green, Blue, and UV

48. Introduction

49. Applications • 100 Years after Electroluminescencediscovery Communications Displays http://crave.cnet.com/i/bto/20070522 /LG_displays_04.GIF http://assets.zarlink.com/PI /zl60003pr.jpg Shaw, G. A., et al., Unattended Ground Sensor Technologies and Applications VII, Proc. Of SPIE 5796, 214, (2005). Optical Sensors Solid State Lighting Yang, Y, “UV Reflectance Spectroscopy Probes DNA and Protein Changes in Human Breast Tissue”, Journal of Clinical Laser Medicine and Surgery, Vol. 19, No. 1, p. 35-39, 2001. OIDA Roadmap (2002)

50. Applications • “Like inorganic semiconductor transistors, which displaced vacuum tubes for computation, SSL-LED is a disruptive technology that has the potential to displace vacuum or gas tubes (like those used in traditional incandescent or fluorescent lamps) for lighting.” Optoelectronic Industry Development Association (OIDA) Technology Roadmap Update 2002 http://www.netl.doe.gov/ssl/workshop/Report%20led%20November%202002a_1.pdf