lec4-nonideal.ppt - Nonideal Transistor Theory 1 Outline Nonideal Transistor Behavior \u2013 High Field Effects \u2022 Mobility Degradation \u2022 Velocity

# lec4-nonideal.ppt - Nonideal Transistor Theory 1 Outline...

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Nonideal Transistor Theory 1
2 Outline Nonideal Transistor Behavior High Field Effects Mobility Degradation Velocity Saturation Channel Length Modulation Threshold Voltage Effects Body Effect Drain-Induced Barrier Lowering Short Channel Effect Leakage Subthreshold Leakage Gate Leakage Junction Leakage Process and Environmental Variations
3 Ideal Transistor I-V Shockley long-channel transistor models 2 cutoff linear saturatio 0 2 2 n gs t ds ds gs t ds ds dsat gs t ds dsat V V V I V V V V V V V V V
4 Ideal vs. Simulated nMOS I-V Plot 65 nm IBM process, V DD = 1.0 V
5 ON and OFF Current I on = I ds @ V gs = V ds = V DD Saturation I off = I ds @ V gs = 0, V ds = V DD Cutoff
6 Electric Fields Effects Vertical electric field: E vert = V gs / t ox Attracts carriers into channel Long channel: Q channel E vert Lateral electric field: E lat = V ds / L Accelerates carriers from drain to source Long channel: v = E lat
7 Coffee Cart Analogy Tired student runs from VLSI lab to coffee cart Freshmen are pouring out of the physics lecture hall V ds is how long you have been up Your velocity = fatigue × mobility V gs is a wind blowing you against the glass (SiO 2 ) wall At high V gs , you are buffeted against the wall Mobility degradation At high V ds , you scatter off freshmen, fall down, get up Velocity saturation Don’t confuse this with the saturation region
8 Mobility Degradation High E vert effectively reduces mobility Collisions with oxide interface
9 Velocity Saturation At high E lat , carrier velocity rolls off Carriers scatter off atoms in silicon lattice Velocity reaches v sat Electrons: 10 7 cm/s Holes: 8 x 10 6 cm/s Better model
10 Vel Sat I-V Effects Ideal transistor ON current increases with V DD 2 Velocity-saturated ON current increases with V DD