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Philippe Adell 1 , Hugh Barnaby 2 , Ron Schrimpf 3 and Bert Vermeire 2 1 Jet Propulsion Laboratory

Band-To-Band Tunneling (BBT) Induced Leakage Current Enhancement in Irradiated Fully Depleted SOI Devices. Philippe Adell 1 , Hugh Barnaby 2 , Ron Schrimpf 3 and Bert Vermeire 2 1 Jet Propulsion Laboratory 2 Arizona State University 3 Vanderbilt University. Motivation.

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Philippe Adell 1 , Hugh Barnaby 2 , Ron Schrimpf 3 and Bert Vermeire 2 1 Jet Propulsion Laboratory

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  1. Band-To-Band Tunneling (BBT) Induced Leakage Current Enhancement in Irradiated Fully Depleted SOI Devices Philippe Adell1, Hugh Barnaby2, Ron Schrimpf3 and Bert Vermeire2 1Jet Propulsion Laboratory 2Arizona State University 3Vanderbilt University Vanderbilt MURI meeting, June 14th&15th 2007

  2. Motivation Schwank et al. NSREC 2000 High current regime? Dependence of drain current degradation on gate voltage? What are the critical parameter responsible for the gate-to-source voltage dependence on the drain current total dose degradation in FD SOI devices? Vanderbilt MURI meeting, June 14th&15th 2007

  3. Device Characteristics High field region at the surface FD SOI n-channel transistor Gate length = 0.1, 0.2, 0.35 m Oxide thickness = 2 nm Si film thickness = 20 nm Low VD Floating body doping • NA = 1×1016 cm-3 Source/Drain doping • ND = 1×1018 cm-3 Device simulations were performed using SILVACO TCAD tools Vanderbilt MURI meeting, June 14th&15th 2007

  4. Band-To-Band Tunneling (BBT) • Large electric Es field resulting from positive drain bias and negative gate bias • Electrons tunnel from valence band on the body side of the junction into the conduction band where they readily transport to the drain terminal • Holes on the drain side will tunnel into the valence band and transport into the floating body region Steady-state band-to-band tunneling current B: tunneling constant W: width of the device A: constant depletion depth Es: large electric field Vanderbilt MURI meeting, June 14th&15th 2007

  5. FD SOI simulation results • Id vs Vgs characteristics for uniform Not layer (1×1012 cm-2) with/without BBT • BBT must be included to simulate the experimentally observed drain current increase at negative gate voltages Impact ionization model turned off, VD = 1V. Vanderbilt MURI meeting, June 14th&15th 2007

  6. Irradiation with BBT model • Id vs. Vgs for different Not (1011, 5×1011, 8×1011, 2×1012, 5×1012 and 1013 cm-2) • ID increases exponentially with Not up to 8×1011 cm-2 • For Not >5×1012 cm-2, ID entered a high current regime Impact ionization model turned off, VD = 1V. Vanderbilt MURI meeting, June 14th&15th 2007

  7. VG VD 0 1 2 Si hole flux n+ p n+ electron flux 3 tSi ++++++++++++++++++++++++++++++++ Buried Oxide (Box) x P-sub y L 0 Model Description • Proposed model describing the three current processes related to drain current degradation with total dose. Vanderbilt MURI meeting, June 14th&15th 2007

  8. Analytical Expression • Process 1: hole transport near the top-gate that arises as a result of BBT • Process 2: electrons back-injected from the source into the body • Process 3: electron flow along the back-gate that arises as a result of the charge buildup in the BOX BBT Not Vanderbilt MURI meeting, June 14th&15th 2007

  9. Silicon Thin Film BOX Gate oxide VG = -1.5V Not = 1012 cm-3 electron conc. (cm-3) VG = -1.2V Not = 1012 cm-3 VG = -1.2V Not = 0 cm-3 x (mm) Correlation between Not and BBT • An increase in Not from 0 to 1012 significantly increases the electron concentration at both interfaces • At fixed Not and reducing the gate voltage to -1.5 V clearly shows how a negatively biased gate couples with Not to further enhance back-channel current BBT Not Vanderbilt MURI meeting, June 14th&15th 2007

  10. Latch Issue • What is interpreted as a latch could be a steep sub-threshold slope due to the combined effects of BBT on the front side and Not on the backside. Vanderbilt MURI meeting, June 14th&15th 2007

  11. Scaling effect on ID vs. VG Simulation Flament et al. NSREC 2003 • Id vs Vgs characteristics for uniform Not layer (5×1012 cm-2) with BBT for two different channel length (0.35 m and 0.2 µm) • Results show that by reducing channel length, a high current regime can be triggered Vanderbilt MURI meeting, June 14th&15th 2007

  12. Conclusion • Band-to-band tunneling (BBT) is the critical mechanism responsible for the increase of leakage current in irradiated fully depleted SOI transistors • Drain current dependence on TID and negative gate bias results from the combination of BBT and charge buildup in the BOX, including the transition to the high current state • By reducing channel length, simulation indicates that a high current regime can be triggered in presence of Not. Vanderbilt MURI meeting, June 14th&15th 2007

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