Master of Engineering Balram Sahu - Embedded Sensing ...

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2.2. Challenges in Sub-threshold operation 8From equation 2.2, The MOS sub-threshold ON current can be given as:I ON ≈ β.e V DDn.v t (2.9)where V DD ≫ v t is assumed. This states that in sub-threshold regime, the reduction inV DD determine an exponential degradation in the delay τ D as shown in its classical CV/Iexpression in equation 2.10.τ D =C . V DDI ON 2= C 2β . V DD(2.10)e V DDn.v tFigure 2.3 depicts the same for an FO4 Inverter delay. The FO4 trend in sub-thresholdis approximately exponential, as expected from equation 2.10.Figure 2.3: Normalized FO4 delay vs. V DD [7]Another problem in sub-threshold operation is the leakage current. From equation 2.2,We can write the off current of a MOS device as follows.I OF F = βe λ DS V DDn.vt (1 − e −V DDv t ) ≈ β (2.11)Hence From equation 2.9 and 2.11, we can write:I ONI OF F= e V DD/n.v t(2.12)Which is exponentially depending on the supply and reduces as we reduce supply voltage.It means in sub-threshold OFF current of transistor becomes significantly comparable tothe ON current. Hence it has a stronger impact on power compared to super-thresholdcircuits.
9 2.2. Challenges in Sub-threshold operationFigure 2.4: I ON to I OF F ratio of an InverterIn Figure 2.4, I ON to I OF F ratio of an FO4 inverter is plotted against the supply voltage.It rolls off exponentially in sub-threshold regime as expected from equation 2.12.Another factor affecting the sub-threshold operation is stacked devices. Stacking helpsin super-threshold operation in reducing power consumption. When transistors are inseries, there overall strength is lower than that of a single transistor by a well knownstacking factor. The stacking factor for 0.13µm technology for 2 and 3 stacked devicesare plotted in the figure 2.5.Figure 2.5: Stacking Factor for I ON and I OF F for 2 and 3 stacked NMOS transistors
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Page 4 and 5: CONTENTSiv4 Flip-Flop Characterizat
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Page 12 and 13: 1.4. Thesis Organization 4harvestin
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Page 28 and 29: 4.3. Generation of LEF 20each metal
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Page 36 and 37: 4.4. Behavioral Model 28Figure 4.7:
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Page 42 and 43: 5.2. Process Flow 34changes and thu
Page 44 and 45: 5.3. Results 365.3 ResultsCORDIC al
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Page 56 and 57: A.2. SoC Encounter Flow 48Do the se
Page 58 and 59: A.2. SoC Encounter Flow 50Figure A.
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BIBLIOGRAPHY 58[11] FARADAY cell li
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