Floating-Gate Devices / Circuits

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Presentation transcript:

Floating-Gate Devices / Circuits Prof. Paul Hasler Georgia Institute of Technology

Channel Current Dependence on Gate Voltage 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 10 -11 -10 -9 -8 -7 -6 Gate voltage (V) Drain current (A) k = 0.58680 Io = 1.2104fA In linear scale, we have a quadratic dependence In log-scale, we have an exponential dependence return

Basic Scaling Rules for MOSFETs Early effect increases (Early voltage) decreases with length --- to keep the same margin from punchthrough as the previous generation, the doping must increase doping  1 / L2 (keeps constant field) To keep sufficiently high k (subthreshold slope),we must decrease the oxide thickness. This slope determines the ratio of on-current to off-current. (as supplies go down, we need a higher slope) oxide thickness  1 /L

Scaling of MOS Parameters Relationship between Substrate Doping and Oxide Thickness - square relationship keeps a constant coupling into the surface potential by keeping the gate and depletion capacitances the same Relationship between Oxide Thickness and Gate Length At this 4nm oxide thickness boundary: - the drawn gate length is 160nm - the substrate doping is 1018 cm-3 - a 1V barrier produces a one-sided depletion width of 40nm

Effect of Velocity Saturation VT A 76 nm MOSFET

Maximum Operating Frequencies fT = frequency where gain is unity at a given current

DIBL in PFETs in 2mm Process 1 2 3 4 5 6 7 10 -1 -9 -8 -7 -6 Drain Current (A) Drain Voltage (V) Gate Voltage = 0V L = 1.25 m m L = 1.50 m m L = 1.75 m m V d g Gate voltage = 0

DIBL in PFETs in 2mm Process 1 1.5 2 2.5 3 3.5 4 4.5 10 -11 -10 -9 -8 -7 -6 -5 V d g Drain current (A) I = I0 e e Vd / V0 kVg / UT V0 = 450mV; k = .422 Drain voltage

Impact Ionization The mean rate of an impact-ionization collision is highly energy dependant Impact Current is proportional to source current

Overview of Floating-Gate Devices Information Storage Floating-Gate Transistor Modifying Floating-Gate Charge UV photo-injection Electron tunneling Hot-electron injection

Floating-Gate Charge Modification Decrease Floating-Gate charge by hot-electron injection Increase Floating-Gate charge by electron-tunneling

Electron Tunneling Increasing the applied voltage decreases the effective barrier width The range of tunneling currents span many orders of magnitude.

How do we measure these currents? V1 Measurement scheme used to characterize floating-gate devices Cf C2 V2 C3 Vout V3 Vref V4 Used in memory cells --Epots [Harrison, et. al.] C4

Scaling Theory for Electron Tunneling Tunneling Curve (Theory) for 5nm oxides 3 3.5 4 4.5 10 -18 -17 -16 -15 -14 -13 -12 Oxide Voltage Oxide Current (A) Itun = Itun0 e -Eo/Eox = Itun0 e - toxEo/Vox 108mV Itun = 0.2 e -(125V)/Vox

Tunneling in normal FET operation 1 2 3 4 5 6 7 8 9 10 -70 -60 -50 -40 -30 -20 -10 Oxide thickness (nm) Oxide current at equilibrium 10 20 10 15 10 10 1 year Hold time on 1fF capacitor 10 5 1 hour 10 10 -5 10 -10 1 1.5 2 2.5 3 3.5 4 4.5 5 Oxide thickness (nm) To store a value (10 years), minimum oxide = 35nm Tunneling occurs normally in MOSFET operation

Electron Transport in a Hot-Electron Injecting nFET

Electron Transport in a Hot-Electron Injecting nFET

Electron Transport in a Hot-Electron Injecting nFET

Measurements and Modeling of Hot-Electron Injection

pFET Hot-Electron Injection The injected electrons are generated by hole impact ionizations. Vinj = 430mV Injection current is proportional to source current, and is an exponential function of Fdc.

Circuit Model of Injection

A 0.25mm Floating-Gate MOSFET UT/k = 35mV, Ith = 10uA (minimum size) tox = 5nm, NA = 5 x 10 17 Injection pFET: 10-6 efficiency at 3.3V Vinj = 90mV at 3.3V Iinj = 10-6 (Is/ Iso) e -DVd/Vinj 0.6 Tunneling: Vx = 108 mV 0.32 -DVfg/Vx Iinj = 2.1 x 10-15 (Is/ Iso) e

Summary of Scaling FG Devices Near future: Very good floating-gate devices, and “classical” theory holds reasonably well (down to L = 100nm, ~2006) Long term: We have many possibilities (at 50nm and below) - Storage by thicker oxide FETs - Basic FETs will tunnel / inject in normal operation