EE 130 MOS Caps Week 11 NotesProblem 1 (from Prof. Ali Javey)What are the advantages/disadvantages in using metal drain and source? Draw the band diagram for the case of on and off states.Topics to discuss:Formation of channel (band bending)Effective Mobility (mobility degredation at high Vgs)subthreshold leakage current/subthreshold swinghow to lower swing?1. increase Cox2. decrease Cdep3. Decrease TempQuestion: Can the subthreshold swing ever be better than 60 mV/dec?Velocity saturation (scattering in channel)V_T roll-offseries resistanceProblem 2: Dynamic Threshold MOSFET(a) How can the threshold voltage of a MOSFET be adjusted? (Describe 3 ways.)(b) Describe the VT design trade-off for short-channel MOSFETs. (Why is it desirable to have low VT? Why is it desirable to have high VT?)(c) One way to improve the VT design trade-off for a short-channel MOSFET is to change the VT dynamically (that is, to adjust its value during circuit operation) by changing the body bias. Consider a very short channel NMOSFET, designed to operate at relatively low power-supply voltage (VDD < 0.6 V). Suppose the body bias (VB) can be varied between 0V and VDD: i. When the NMOSFET is in the ON state (VGS = VDD), what should VB be, in order to maximize the transistor current? ii. When the NMOSFET is in the OFF state (VGS = 0V), what should VB be, in order to minimize the sub-threshold leakage current? iii. Considering your answers to parts (i) and (ii) above, what is the advantage of connecting the body to the gate? iv. What is the disadvantage of connecting the body to the gate?Problem 2: Dynamic Threshold MOSFET a) Three ways to adjust VT: - adjust Tox - adjust channel doping - adjust the gate material work function b) It is desirable to have a low VT in order to maximize the transistor current when it is ON, to maximize Idsat It is desirable to have a high VT in order to minimize the transistor leakage current when it is OFF, to reduce the static power consumption. c) i) VB should be connected to VDD to reduce VT and hence increase Idsat ii) VB should be connected to GND in order to maximize VT and hence minimize sub- threshold leakage current ii) The main advantage of connecting VB to VG is to dynamically optimize VT for high speed (in the ON state) and low static power consumption (in the OFF state). iii) The disadvantage of connecting VB to VG is an increase power consumption when the transistor is turned ON, due to the forward-bias pn-junction diode current associated with the source and drain regions. If the area of the source and drain junctions is small (e.g. if SOI technology is used), this power consumption penalty will be
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