4.11.4 fT and fmax for CMOS Transistors

Chapter 4.11.4 fT and fmax for CMOS Transistors

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 92 A Brief Review of Technologythe square law model especially at low overdrive voltages. However, ultimately, it is necessary to use simulators, such as Spectre, ADS, or SPICE, to verify the curves. Recently CMOS models have become much better at predicting transis-tor performance, although there can still be problems in the simulations of output conductance and phase shift at high frequencies. Other errors can occur due to the improper modeling of parasitics, process variations, coupling between neighboring circuits, or other effects. Thus, measurements are needed to verify any designs, or to refine the models. 4.11.4  fT and fmax for CMOS TransistorsAn expression for fT is the following ox2()()2 ()222GSTmmGSTTgsgdgsOXWCvvggvvLfCCCWLCLµµπππγπγæö-ç÷èø-=»==+ (4.35)where g » 2/3 in saturation for a long-channel device, and higher for short channel devices. An expression for fmax, similar to the expression for a bipolar transistor derived earlier, is [11]: max8Tg gdffr Cπ= (4.36)Given the expression for fmax, it is also possible to estimate for the maximum achievable power gain for an amplifier built with this transistor as follows [11] max28Tg gdfGr C fπ= (4.37)4.11.5  CMOS Small-Signal Model Including NoiseSimilar to the bipolar transistor, the thermal channel noise in a MOSFET can be modeled by placing a noise current source, ind in parallel with the output as shown in Figure 4.18. 24ndmikT gγ= (4.38)As before, the parameter g is about 2/3 for a long channel device in saturation, and larger (e.g., 1 to 2) for short channel devices.Gate resistance noise can be modeled with voltage source vng. 24nggvkTr= (4.39)In addition, CMOS transistors experience noise due to the distributed nature of the transistor. Such nonquasistatic effects not only cause a decrease in fmax but