Vanderbilt MURI meeting, June 14 th &15 th 2007 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

Vanderbilt MURI meeting, June 14 th &15 th 2007 Motivation 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? Schwank et al. NSREC 2000

Vanderbilt MURI meeting, June 14 th &15 th 2007 Device Characteristics FD SOI n-channel transistor Gate length = 0.1, 0.2, 0.35  m Oxide thickness = 2 nm Si film thickness = 20 nm Low V D Floating body doping N A = 1×10 16 cm -3 Source/Drain doping N D = 1×10 18 cm -3 High field region at the surface Device simulations were performed using SILVACO TCAD tools

Vanderbilt MURI meeting, June 14 th &15 th 2007 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 14 th &15 th 2007 FD SOI simulation results I d vs V gs characteristics for uniform N ot layer (1×10 12 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, V D = 1V.

Vanderbilt MURI meeting, June 14 th &15 th 2007 Irradiation with BBT model I d vs. V gs for different N ot (10 11, 5×10 11, 8×10 11, 2×10 12, 5×10 12 and cm -2 ) I D increases exponentially with N ot up to 8×10 11 cm -2 For N ot > 5×10 12 cm -2, I D entered a high current regime Impact ionization model turned off, V D = 1V.

Vanderbilt MURI meeting, June 14 th &15 th 2007 Model Description Proposed model describing the three current processes related to drain current degradation with total dose. electron flux n+n+ n+n+ Buried Oxide (B ox ) P-sub t Si p x 0 Si VDVD VGVG hole flux y 0 L

Vanderbilt MURI meeting, June 14 th &15 th 2007 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 N ot

Vanderbilt MURI meeting, June 14 th &15 th 2007 Correlation between N ot and BBT An increase in N ot from 0 to significantly increases the electron concentration at both interfaces At fixed N ot and reducing the gate voltage to -1.5 V clearly shows how a negatively biased gate couples with N ot to further enhance back-channel current V G = -1.2V N ot = 0 cm -3 V G = -1.2V N ot = cm -3 V G = -1.5V N ot = cm -3 x (  m) electron conc. (cm -3 ) Gate oxide BOXSilicon Thin Film BBTN ot

Vanderbilt MURI meeting, June 14 th &15 th 2007 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 N ot on the backside.

Vanderbilt MURI meeting, June 14 th &15 th 2007 Scaling effect on I D vs. V G I d vs V gs characteristics for uniform N ot layer (5×10 12 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 Flament et al. NSREC 2003 Simulation

Vanderbilt MURI meeting, June 14 th &15 th 2007 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 N ot.