Download UC Berkeley EE 105 Midterm: Microelectronic Devices & Circuits and more Exams Microelectronic Circuits in PDF only on Docsity! UNIVERSITY OF CALIFORNIA, BERKELEY College of Engineering Department of Electrical Engineering and Computer Sciences EE 105: Microelectronic Devices and Circuits Spring 2008 MIDTERM EXAMINATION #1 Time allotted: 80 minutes NAME: _____SOLUTIONS_______ _________________________ _________________________ (print) Last First Signature STUDENT ID#: ____________________ INSTRUCTIONS: 1. Use the values of physical constants provided below. 2. SHOW YOUR WORK. (Make your methods clear to the grader!) 3. Clearly mark (underline or box) your answers. 4. Specify the units on answers whenever appropriate. PHYSICAL CONSTANTS Description Symbol Value Page 1 Electronic charge q 1.6×10-19 C Boltzmann’s constant k 8.62×10-5 eV/K Thermal voltage at 300K VT = kT/q 0.026 V Note that VT ln(10) = 0.060 V at T=300K PROPERTIES OF SILICON AT 300K Description Symbol Value Band gap energy EG 1.12 eV Intrinsic carrier concentration ni 1010 cm-3 Dielectric permittivity εSi 1.0×10-12 F/cm Electron and Hole Mobilities in Silicon at 300K SCORE: 1 __________ / 25 2 __________ / 25 3 __________ / 30 Total: __________ / 80 Page 2 Problem 1 [25 points]: Semiconductor Basics a) A Si resistor is doped with 1017 cm-3 of phosphorus and 2x1017 cm-3 of boron impurities. i) What are the electron and hole concentrations, n and p, in this sample at room temperature? [4 pts] 10 cm 2 10 cm 10 cm 10 cm ii) Estimate the resistivity of this sample. [5 p ] ts 1 1 We can find in the mobility chart 3 10 cm . Doing so, we find using 250 cm /V s 0.25 Ω cm iii) Qualitatively (no calculations required), how would the resistivity change when the temperature goes up to 100ºC? Explain briefly. [4 pts] Increasing the temperature will result in more mobile carriers (i.e., more electrons and holes will have the thermal energy necessary to be conduction electrons/holes), meaning the resistivity will decrease . c) Answer this question qualitatively. For the two BJTs in Part a), which BJT will have larger Early voltage? Why? [4 pts] Device B will have a larger Early voltage (i.e., it will suffer less from the Early effect than device A). This is because the base width in B is larger than it is in A. This means that any change in the base width due to a change in the reverse bias on the base-collector junction will result in a smaller relative change in the base width in B than in A. d) Solve the bias point of the following PNP transistor (IC, VEB, VEC). Assume [5 pts] 1710 A, =100, and VS AI β= = ∞ Assume 10 A (correction made during exam). 1 KER = Ω 3VCCV = 1 1 ln 2.12 mA Page 5 e) Draw the small-signal model of the circuit in Part d). Specify all the small signal parameters used (e.g., , mg rπ , etc). [4 pts] 81.5 mS 1.226 kΩ Page 6 Problem 3 [30 points]: BJT Amplifiers a) Consider the BJT amplifier shown below with 1 mABIASI = . Assume 1710 A, =100, and V 10S AI Vβ= = . 1 KER = Ω 3VCCV = 10 KLR = Ω 1 K SR = Ω inv outv CC BIASI i) Find the v . [4 pts] alue of BEV Assume 2 V (given during exam) and 10 A (correction made during exam). 2 V 1 mA fixed by the current source 1 1.01 V 0.99 V 1 ln 1 836 mV ii) Is the BJT in the active mode? Why? [4 pts] Yes. The base-emitter junction is forward biased and the base-collector junction is reverse biased (i.e., ). iii) Find the small signal parameters of the BJT under this bias condition. [4 pts] Page 7 1 26 38.5 mS 2.6 kΩ 10 kΩ
