Semiconductor Devices

Transcription

1 Semiconductor Devices Lecture Course Part of SS Module PY4P03 Dr. P. Stamenov School of Physics and CRANN, Trinity College, Dublin 2, Ireland Hilary Term, TCD 17 th of Jan 14

2 Metal-Semiconductor Contacts How are they formed? Applications... What is the Physics of the barrier formation? Charge, potential, field and bands... Current models... IV-characteristics & temperature dependencies

3 Metal Deposition - Evaporation Bell Jar Quartz Monitor To Vacuum Pump Substrate Holder Shutter Evaporation Source Thermal, e-beam, laser, etc. versions Vacuum requirements < 10-4 mbar Difficult for refractory metals and for low melting point ones Can lead to astaehiometrisation Very directional coverage problems Poor deposition rate control apart from MBE Low energy process

4 Metal Deposition - Sputtering DC, RF and Mixed versions Vacuum requirements < 10-2 mbar Easy for refractory metals Can lead to astaehiometrisation Not directional no coverage problems Excellent rate control no need to monitor Higher energy process Difficult for magnetic materials

5 Schottky Diodes Contact Metal Oxide Insulation Semiconductor base Recipe Mask an aperture Remove oxide Deposit metal Contact Features Simple (...or not...) Important building block Fast Low forward drop Reverse bias breakdown not perfect Majority carrier device

6 Metal Fermi level Vacuum level Schottky Junction Before Formation Bottom of cond. band E E m s Fm Fs Work functions and affinities are both important! Top of valence band

7 The bands have to bend to accommodate! Before contact levels Schottky Junction In Equilibrium I Flat electrochemical potential! The alternative is a perpetuum mobile! Build-in potential EFm EFs m s bi m s qv 0 m s

8 Schottky Junction In Equilibrium II EFm EFs m s There are not too many examples of these...

9 Notes on Barrier Formation Mott-Schottky Model noninteracting interface 0 m s d d 0 m 1 Bardeen Model - 0 ( m s) (1 ) g (Gap States) Screening distance Heine Model 1 (Metal-induced States) 2 xg T 1 q Dg ( EFm ) s 0 Generalized Models Nat. oxide thickness s 1 q D (1 q D ) Extrinsic States 2 1 qds ( E 2 g s b m) 2 s s s 1 2 g g 1 q D x Interface Index Interface Capacitance g T m ox Thomas-Fermi screening length 0.1

10 Schottky Junction Under Bias Positive bias Zero bias Negative bias All within a rigid band approximation for the shifts in depth on S side.

11 Hole Electron potential Charge density Charge, Field, Potential and Bands Electric field Band energy Full Depletion Approximation (bends are for illustration only)

12 Full Depletion Approximation Only at the very edge of the depleted region is there an appreciatable difference between the FDA and the exact solution. The Full Depletion Approximation is quite valid for a typical SC at RT.

13 Main Current Components a) Thermionic emission b) Tunnelling c) Recomb. in dep. region d) Deep recombination After: Rhoderick and Williams (1988) Metal-Semiconductor Contacts

14 The Schottky Model Diffusion I Einstein s Relation Continuity Drift-Diffusion Equation 1D Poisson Equation Depleted Region Width

15 The Schottky Model Diffusion II Barrier height (V) Donor activation corrected barrier Deposited charge Barriers for the electrons and the holes Total current Junction capacitance

16 The Bethe Model - Emission Thermionic Emission Richardson s Constant Total current Projected Effective Mass

17 The Sze Model Diffusion + Emission Recomb. Velocity Drift- Diffusion Velocity Correct Recomb. Velocity Total current

18 Tunnelling Current Schrodinger s Equation Need only factor in the part of the DOS with significant tunnelling probability Recombination Richardson Velocity Total tunnelling current (Note the weak temperature dependence)

19 Thanks and Acknowledgements Thank You Very Much for Your Attention!