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High Frequency MOSFET Model. Models for Computer Simulation Simple dc Model Small Signal Model Frequency-Dependent Small Signal Model Better Analytical.

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Presentation on theme: "High Frequency MOSFET Model. Models for Computer Simulation Simple dc Model Small Signal Model Frequency-Dependent Small Signal Model Better Analytical."— Presentation transcript:

1 High Frequency MOSFET Model

2 Models for Computer Simulation Simple dc Model Small Signal Model Frequency-Dependent Small Signal Model Better Analytical dc Models Sophisticated Model for Computer Simulations Better Models for Predicting Device Operation

3 Strategy Identify and include relevant parasitic capacitors that inherently exist in a MOS transistor

4 Question Are the parasitic capacitors relevant?

5 Observation Parasitic Capacitors are Small Consider a minimum-sized transistor 2 3 4

6 Process Parameters from AMI 0.5u Process =.35 microns 2473 424

7 Size of Capacitances Gate-Channel Capacitance = 6 2 x 2.47fF/  2 = 1.82fF Source Diffusion-Substrate Capacitance = 12 2 x.424fF/  2 + 14 x.315fF/  =.624fF + 1.54fF =2.16fF Are these negligible?

8 Process Parameters from AMI 0.5u Process

9 Size of Capacitances Gate-Channel Capacitance = 6 2 x 2.47fF/  2 = 1.82fF Source Diffusion-Substrate Capacitance = 12 2 x.424fF/  2 + 14 x.315fF/  =.624fF + 1.54fF =2.16fF Are these negligible? Impedance of Triode Region Device R FET = L/(  C OX W[V GS -V T ]) = 2/(118E-6 x 3 x 1)=5.6K Time Constant R FET C GC = 1.82E-15 x 5.6K = 10.2psec

10 Are these negligible? These small capacitors play the dominant role in the speed limitations of most digital circuits These small capacitors play a major role in the performance of many linear circuits It is essential that these capacitors (parasitic capacitors) be considered and managed when designing most integrated circuits today!

11 Types of Capacitors 1.Fixed Capacitors a. Fixed Geometry b. Junction 2. Operating Region Dependent a. Fixed Geometry b. Junction

12 Parasitic Capacitors in MOSFET

13 Parasitic Capacitors in MOSFET Fixed Capacitors Overlap Capacitors: C GDO, C GSO C GDO C GSO

14 Parasitic Capacitance Summary D S G B C GS C GD

15 Parasitic Capacitors in MOSFET Fixed Capacitors C BS1 C BD1 Junction Capacitors: C BS1, C BD1

16 Parasitic Capacitors in MOSFET Fixed Capacitors Overlap Capacitors: C GDO, C GSO C GDO C GSO C BS1 C BD1 Junction Capacitors: C BS1, C BD1

17 Parasitic Capacitance Summary D S G B C GS C GD C BS C BD

18 Parasitic Capacitors in MOSFET Operation Region Dependent -- Cutoff C GBCO Cutoff Capacitor: C GBCO

19 Parasitic Capacitors in MOSFET Operation Region Dependent -- Cutoff C GBCO Cutoff Capacitor: C GBCO Note: A depletion region will form under the gate if a positive Gate voltage is applied thus decreasing the capacitance density

20 Parasitic Capacitors in MOSFET Operation Region Dependent -- Cutoff Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1 C GDO C GSO C BS1 C BD1 C GBCO Cutoff Capacitor: C GBCO

21 Parasitic Capacitance Summary D S GB C GS C GD C BS C BD C BG

22 Parasitic Capacitors in MOSFET Operation Region Dependent -- Ohmic Ohmic Capacitor: C GCH, C BCH C GCH C BCH Note: The Channel is not a node in the lumped device model so can not directly include this distributed capacitance in existing models Note: The distributed channel capacitance is usually lumped and split evenly between the source and drain nodes

23 Parasitic Capacitors in MOSFET Operation Region Dependent -- Ohmic Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1 C GDO C GSO C BS1 C BD1 Ohmic Capacitor: C GCH, C BCH C GCH C BCH

24 Parasitic Capacitance Summary D S GB C GS C GD C BS C BD C BG

25 Parasitic Capacitance Summary D S GB C GS C GD C BS C BD C BG

26 Parasitic Capacitors in MOSFET Operation Region Dependent -- Saturation Saturation Capacitors: C GCH, C BCH C GCH C BCH Note: Since the channel is an extension of the source when in saturation, the distributed capacitors to the channel are generally lumped to the source node

27 Parasitic Capacitors in MOSFET Operation Region Dependent -- Saturation Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1 C GDO C GSO C BS1 C BD1 Saturation Capacitors: C GCH, C BCH C GCH C BCH

28 Parasitic Capacitance Summary D S GB C GS C GD C BS C BD C BG

29 Parasitic Capacitance Summary D S GB C GS C GD C BS C BD C BG

30 Process Parameters from AMI 0.5u Process =.35 microns

31 Small Signal Model Summary

32 Equivalent circuit showing dependent sources In many applications simpler small signal model is adequate

33 Small Signal Model Summary Simplified Equivalent Circuit Adequate for Many Applications When Source is Grounded (C BS need not be shown when source connected to ground)

34 That’s all folks!

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