UNIVERSITY OF CALIFORNIA Department of Electrical Engineering and Computer Sciences EE130 Fall 2004 Prof. Subramanian Test 4 1) You are given two NMOSFETs with identical doping, gate oxide thickness, and junction depth. • MOSFET “A” has a punchthrough voltage of 10V • MOSFET “B” has a punchthrough voltage of 20V a) Which device has a shorter L? Give reasons for your answer. b) Suppose MOSFET “A” has a VT of 0.5V. Sketch a band diagram for MOSFET “A” along slice Q-Q’ (shown below) for VDS = 1.0V and VGS = 1.5V. Also sketch a band diagram for the same device along the W-W’ slice. Make sure you clearly identify any differences between the two slices. X Y Q Q’ 2 pts W W’ 5 ptsc) Which device (“A” or “B”) would you expect to have a higher small-signal output resistance? Why? d) Suppose I took MOSFET “A”, and increase the channel doping. Assume I change the gate workfunction so that the threshold voltage stays unchanged. i) What would be the impact on the output resistance? Give reasons. i) When I changed the gate workfunction – should I have increased it or decreased it to keep the VT constant. e) Suppose I were to operate MOSFET “A” at VDS = 0.5V and VGS = 0.4V (As in part (b), assume the VT is 0.5V). What phenomenon is responsible for current to flow between the source and the drain? Explain the mechanism behind this. f) Assume MOSFET “A” has a series resistance that is approximately 25% of the large-signal channel resistance (IDsat/VDS). Draw a band diagram for MOSFET along slice X-Y for MOSFET A, assuming VT = 0.5V, VG = 1V, and VD = 1V. Make sure you include the source/drain regions in your band diagram. 2 pts 2 pts 1 pt 2 pts 5 pts2) In recent years, for various reasons, IDsat in MOSFETs is no longer increasing with decreasing channel length; rather, it has become almost constant from generation to generation for devices with the same channel width, despite the fact that L is decreasing steadily. Given this fact, why do we continue to scale L? Justify your answer in terms of both speed and power consumption metrics. (Hint: consider the impact of constant field scaling on capacitance) 3) One of the important parameters that must be scaled is tox. Discuss any problems that might exist with the continued scaling of tox in terms of the following: i) The use of a polysilicon gate ii) The effect of the quantum mechanical inversion charge centroid iii) The effect of tunneling current through the gate oxide as the gate thickness is reduced. 3 pts 1 pt 1 pt 1
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