09SECE108Ch9scalingpresentation

09SECE108Ch9scalingpresentation - Esener ece108 9 SCALING...

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Unformatted text preview: Esener ece108 9. SCALING OF MOS LOGIC Scaling is used to improve circuit speed and packing density. When MOS transistors are scaled in 5 dimensions (3 space voltage and doping conc.) the field patterns remain same in scaled devices Second order effects limit scalability •Velocity saturation •Parasitic source drain resistance •Built-in junction potentials •Silicon bandgap 9. 1 Esener ece108 9.1. First order Effects of MOS Scaling p-type L n W x V S (SOURCE) V D (DRAIN) V G (GATE) z V BG (SUBSTRATE) n p-type L n W x V S (SOURCE) V D (DRAIN) V G (GATE) z V BG (SUBSTRATE) n 9. 2 9.2. Technology Scaling Models • Full Scaling (Constant Electrical Field) • Fixed Voltage Scaling • General Scaling ideal model — dimensions and voltage scale together by the same factor S most common model until recently — only dimensions scale, voltages remain constant most realistic for todays situation — voltages and dimensions scale with different factors 3 4 Scaling Relationships for Long Channel Devices Esener ece108 SCALING PARAMETERS: Feature Size and Voltage Supply 9. 5 Propagation Delay Scaling 6 Technology Evolution 7 Esener ece108 V DS SAT V DS SAT 9.3. SHORT CHANNEL MOSFET BEHAVIOR 9. 8 Short-channel effects: • Lack of pinch-off – Early SAT • Threshold-voltage shift • Increased leakage current • Increase of output conductance Esener ece108 • A MOSFET is called a short channel device if its channel length is approximately equal to the source and drain junction depth x j The behavior of transistors with very short channel lengths (called short-channel devices ) deviates considerably from the resistive and saturated models. • The first main culprit is the limitations imposed on carrier drift characteristics in the channel through the velocity saturation and mobility degradation effects. So far we have assumed that the carrier mobility is a constant. However, when (horizontal) field strength, along the channel reaches a critical value E c the velocity of the carriers tends to saturate due to scattering effects (collisions suffered by the carriers). • The saturation velocity for electrons and holes is approximately the same: 10 7 m/s in the bulk of silicon. • The critical field for bulk silicon depends upon the doping levels and is about 10 5 V/cm for electrons. In short channel devices the carrier velocity is also a function of the normal (vertical) electric field component. Since the vertical field influences the scattering of carriers in the surface region, the surface mobility is reduced with respect to the bulk mobility. • We will assume that the saturation velocity for electrons under both horizontal and lateral fields in a short channel MOSFET is v sat =10 5 m/s and the Critical Field E c varies between 1 and 5 V/ μ m ....
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09SECE108Ch9scalingpresentation - Esener ece108 9 SCALING...

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