Materials Beyond Silicon

1. Materials Beyond Silicon By Uma Aghoram

2. Moore’s Law • MSI LSI VLSI • Moore’s Law states that the number of transistors on a chip doubles about every two years. • At each stage of scaling fundamental limits are being reached

3. SCALING LIMITS • Lithographic Limits • XRay and E-beam in future • Short channel effects • Vt roll off • DIBL • Oxide thickness scaling reaching a few atomic layers of silicon • Large leakage currents • Higher power dissipation

4. Exponential Forever ? 130nm􀂼􀂼90nm􀂼􀂼60nm􀂼􀂼45nm􀂼􀂼30nm􀂼􀂼?

5. KEEPING MOORE’S LAW ALIVE • Strained Silicon • Novel Device structures • High K dielectrics • Carbon Nanotubes, SET • Alternate channel materials

6. REPLACING SILICON • Cost effective • Easy incorporation into existing technology • Reliable oxide • Performance enhancement

7. Germanium ADVANTAGES • Has high electron and hole mobility as compared to silicon – High speed • high-Kdielectrics • Good compatibility with III-V materials DISADVANTAGES • No reliable native oxide • Small Bandgap may lead to larger tunneling currents

8. Gallium Arsenide and other III-V materials ADVANTAGES • Has 6X larger electron mobility as compared to silicon – High speed • Have excellent Optoelectronic properties • High resistance to radiation damage • High flexibility to forming alloys • Heat resistant

9. Gallium Arsenide and other III-V materials DISADVANTAGES • No reliable native oxide • Composite nature leads to high defect density-Unstable Vt • More expensive to manufacture pure GaAs to meet industry standard • Lower thermal conductivity than Silicon • More fragile • More quantum effects • Small recombination time results in poor performance of GaAs BJTs. • High leakage current in narrow gap III-V materials

10. Where do we stand? Germanium • Strained Si Mosfets with SiGe layers have been manufactured • Dual channel heterostructure MOSFETS are being researched. Gallium Arsenide: • Bell Labs and Freescale recently reported that they have grown high quality Ga2O3 on GaAs with low interface states and achieved enhancement and depletion mode MOSFETs. • HBT’s, HEMT, Power transistor

11. Toy Problem • Position of charge centroid in a MOSCAP for three different materials namely: • Silicon • Germanium • Gallium Arsenide • The position where the charge peaks is of relevance as it determines additional thickness of the dielectric and effectiveness of gate control of device

12. Assumptions • One dimensional problem • One band effective mass Hamiltonian • Voltage on gate directly applies to the channel • 20A of oxide and varying well widths • Wave functions are strongly excluded form the oxide • Lattice spacing and dimension varies with material • (100) surface • Inversion charge density was a constant at 1e13/cm2 Vg X insulator drain channel source insulator Z 0

13. PROCEDURE • 1-D Schrodinger Poisson Solver [Hz+U]fm= em fm n(z) U(r)

14. Procedure • From Schrödinger part of the solution we get the eigen values and eigen functions. • We can then calculate the electron density n(z) • Using the Poisson's equation we then solve for the self consistent potential, and use this new potential in the Schrödinger's equation and thus solve self consistently. • For the Hamiltonian the mass to be used is the out of plane effective mass and for the density of states expression use the DOS mass.

15. Results Si Ninv=1e13/cm2 Ge GaAs InSb

16. CONCLUSIONS • As the mass reduces the quantum confinement effects are more significant and peaking of the charge concentration takes place deeper in the substrate. • This means that the effective gate capacitance is smaller in Ge and GaAs as compared to silicon • Also there is better gate control of channel in silicon devices as compared to GaAs and Ge • Higher voltages are required by III-V materials to reach the same inversion charge density when compared to silicon • Lower transconductance in materials with lower mass.

17. Existing Results

18. Current Research Germanium • The main supporters of Germanium are Sematech, IBM, Umicore and Soitech. A lot of research is also being carried out in this field by IMEC in partnership with Umicore and Soitech. GaAs • Intel is one of the main supporters of GaAs • Quintec researches in this area

19. References and Acknowledgement • Quantum Transport – Atom to transistor Supriyo Dutta Cambridge university press • Physics of Strain Effects in Semiconductors and MOSFETs Y. Sun, S. E. Thompson, and T. Nishida, awaiting publication • Gallium Arsenide –GaAs as a semiconductor, its turbulent past, shaky present and promising but distant future Aseem Srivastava IEEE 1989 • http://www.edn.com/article/CA6314526.html?ref=nbra • http://www.investorrelations.umicore.com/en/pressReleases/2003/germanium_E.pdf • Fundamentals of modern Vlsi devices Taur and Ning • http://mems.caltech.edu/courses/EE40%20Web%20Files/effective%20mass%20explanation.pdf • Indium Phosphide and related materials, 2005 International conference on • www.intel.com • ftp://download.intel.com/research/silicon/Gordon_Moore_ISSCC_021003.pdf • www.compoundsemiconductor.net/articles/news/10/1/25/1

20. Thank you!! Questions???