1 / 21

Electronic structure vs. Transfer characteristics of Amorphous oxide TFT

Electronic structure vs. Transfer characteristics of Amorphous oxide TFT. Density of states. a-IGZO TFT Device parameters. Field-effect mobility , Current on–off ratio Subthreshold swing. Highly dependent. a-IGZO material parameters. Density of states. Density of states.

olympe
Download Presentation

Electronic structure vs. Transfer characteristics of Amorphous oxide TFT

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Electronic structure vs. Transfer characteristics of Amorphous oxide TFT

  2. Density of states a-IGZO TFT Device parameters Field-effect mobility, Current on–off ratio Subthresholdswing Highly dependent a-IGZO material parameters Density of states

  3. Density of states Density of states Permitted energy state in given k-vector (Or given energy value) Crystalline semiconductor’s density of states

  4. Density of states • Amorphous semiconductor’s density of states Anderson model of localization By random potential distribution, density of states have localized states (Bandtail) By qualitative modeling, bandtail has exponential distribution of density of states

  5. Multiple trapping & release MTR conduction Electrons are trapped in localized states and, Are released and move forward by thermal excitation Percolation condiction In high temperature: Shorter but higher-barrier path (more localized state are occupied) (higher thermal energy) In low temperature: Longer but lower-barrier path

  6. Amorphous IGZO Temperature-dependent field-effect measurement RF-sputtered a-IGZO TFT As temperature increases, (20ºC ~ 80ºC) Field effect mobility ↑ Threshold voltage ↓ Subthreshold swing ↑ (These parameters are related to density of states) S and IOFF are insensitive to temperature, unlike Si:H TFT.

  7. Amorphous IGZO Temperature-dependent field-effect measurement The temperature dependency (μ and VT) → Similar to a-Si:H TFTs → Explained by the multiple trapping model μ follows Arrhenius equation ( 9 to 11 cm2/Vs, activation energy of 26meV ) Vt linearly dependent to temperature ( -17mV/ºC ) S slightly increases with temperature ( 0.4 to 0.6 V/dec )

  8. Amorphous IGZO Temperature-dependent field-effect measurement Arrhenius plot of ID Drain current is thermally activated Ea and ID0 is determined

  9. Meyer-Neldel rule Temperature-dependent field-effect measurement Plotting Ea vs. ID0 Slope: ID00 Intercept : A A is material’s intrinsic property A is independent to Fermi energy position A become ~0 above threshold regime (>1V) A become <0 (VGS>5V)

  10. Amorphous IGZO Analysis of band bending and charge density Ea is not only a function of VGS but Also a function of x!

  11. Amorphous IGZO Analysis of band bending and charge density Solving Poisson’s equation Charge density Band bending By applying material constants and Ea, ID00, A values → Determines band bending and charge density

  12. Amorphous IGZO Analysis of band bending and charge density Density of states N(E) is calculated from Differential value of this plot

  13. Flatband voltage Real MOS structures is further affected by the presence of charge in the oxide or at the oxide-semiconductor interface → Band bending at 0 external electric field Flatband voltage Gate voltage that compensates band bending in semiconductor VFB = χm – χS – (EC – EF)

  14. Amorphous IGZO Determining flat band voltage A proper VFB value can be obtained by Eacurve fitting In this a-IGZO case, VFB was -1.5V

  15. Solution-processed ZTO Thermally activated transport vs. Band transport Thermally activated transport: Mobility increases with temperature → More thermal excitation to overcome activation energy Band transport: Mobility decreases with temperature → Increased phonon scattering

  16. Mobility edge Mobility edge Highest energy at which states are localized (Sharp energy separating) EC: mobility edge EA: injected electrons in equilibrium Above mobility edge, only extended states are remaining → Coexistence of localized states and extended states is not possible

  17. Solution-processed ZTO Thermally activated transport vs. Band transport By increasing VGS, → Fermi energy level shift toward the mobility edge → Electrons fill localized states (bandtail) → Band transport is dominant

  18. Solution-processed ZTO Transition point Solution-processed Zinc tin oxide TFT Combination of thermally activated & extended state transport Mobility edge is well defined Band transport dominates at high carrier concentration Data fit well with MTR transport model

  19. Summary • MTR conduction in amorphous oxide semicondictor like IGZO or ZTO • Conduction is thermally activated process and mobility increases with temperature • To increase field effect mobility, activation energy should be reduced • → When localized states are occupied activation energy become close to zero • - Fermi energy (Donor concentration) • - Bandgap energy • To increase field effect mobility, DOS in localized states should be minimized • - Prediction of amorphous semiconductor’s electronic structure: very difficult • - To apply this inspiration into material design, much more works are required

  20. Future works • Percolation conduction model (MTR transport) • Relation with many phenomenon with solution-processed TFT • Ex) Vt & mobility relationship

  21. Future works • Semiconductor/dielectric interface & dielectric thickness

More Related