EECE 310 Homework 6 with Solution, Fall 2007

American University of Beirut Department of Electrical and Computer Engineering EECE 310 – Electronics Fall 2007 – 2008 Homework 6 Problem 1. a) The mobility of electrons in the channel of an NMOS device is 1100 cm2/V.sec. Find the transconductance parameter k’n for this MOSFET if its oxide thickness is 40 nm. The doping level in the channel is NA = 1016 cm-3. The relative permittivity of the oxide (SiO2) is 3.9, and that of Silicon is 11.7. b) The threshold voltage Vt0 for this NMOS transistor is 0.8 V. Find the required dimensions of the gate of the NMOS transistor (W and L) to get a drain current of 0.1 mA when the MOSFET is biased at (VGS = 2.5 V, VDS = 2.5 V, VBS = 2 V). The minimum dimension is 0.25 μm. c) The drain current of the NMOS transistor is ID1 at (VGS = 2.5 V, VDS = 2.5 V). When VDS becomes 2 V, the current decreases by 10% to 0.9 ID1. Find the channel length modulation parameter λ of this transistor. Problem 2. The circuit shown below is a voltage-controlled attenuator. Assume VGG = 2.5 V, R = 3.3 KΩ, and for the MOSFET: Vt = 0.8 V, k’(W/L) = 0.12 mA/V2. a) For what range of output voltages does the MOSFET behave as a (voltage-controlled) resistor? Assume that the square term is negligible when it is less than 5% of the linear term in the MOSFET current equation. b) Find the value of VOV for the MOSFET and the value of its resistance rDS when vO satisfies the condition of part (a). c) Plot the attenuation factor (vo/vs) as a function of VGG. Problem 3. Two identical enhancement MOSFETs with k’n(W/L) = 200 μA/V2, Vt = 0.75 V, and λ = 0.12 V-1 are connected as shown below. The MOSFET drain current is 0.25 mA. Assume that VDD = 5 V. a) Find the value of VGS for the lower MOSFET. In what region is this MOSFET operating? b) Find the voltage at the gate of the upper MOSFET. In what region is this MOSFET operating? c) Find the resistance R1 when R2 = 820 KΩ.