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EECS 130, Fall 98
Final Exam
Professor C. Hu
Problem #
(25 pts.) A silicon sample is doped with boron atoms to N (^) A = 10 15 cm.
a) (3 pts.) What are the electron n and hole p concentrations at room temperature? b) (4 pts.) What are n and p at T = 600 K to within an order of magnitude? c) (4 pts.) Sketch log 10 ( p ) versus temperature between 300K and 1200K on the axes below.
d) (4 pts.) Consider a GaAs sample (Eg = 1.4 eV) also doped with NA = 10^15 cm-3. Using a dotted line, add a 2nd curve to the axes in part (c) showing log 10 ( p ) vs. T for the GaAs sample.
e) (3 pts.) GaAs is preferred over Si for high-frequency transitors. What semiconductor property accounts for this?
f) (4 pts.) Is the hole mobility μ (^) p at 600 K larger than, smaller than, or the same as μ (^) p at 300 K?
What scattering mechanism is dominant in this temperature range? g) (3 pts.) A hot-probe experiment is performed on this sample as illustrated below. With an arrow, indicate the current flow direction
Problem #
(20 pts.) IC Processing Questions a) (4 pts.) Give one significant advantage of shallow trench isolation over LOCOS isolation. b) (5 pts.) What are the two reasons for doing a high temperature anneal after source/drain implantation? c) (5 pts.) What is the meaning of wet oxidation? Does wet oxidation proceed at a faster, slower, or same rate compared to dry oxidation? d) (2 pts.) What does the acronym CVD stand for? e) (4 pts.) What is epitaxial growth used for in the fabrication of bipolar transitors?
Problem #
(20 pts.) Comparison of BJT and MOSFET transconductances a) (3 pts.) For a long-channel MOSFET biased at Vg > VT, write down the expression for the transconductance g (^) msat.
b) (3 pts.) Write the expression of gm for a bipolar junction transistor biased at collector current IC.
c) (6 pts.) Under the condition that ICBJT^ = IDsatMOS, what is the ration gmBJT/g (^) mMOS?
Which transistor has the larger gm?
d) (8 pts.) Repeat (c) for the MOSFET biased in the subthreshold regime (Vg < VT).
Problem #
(10 pts.) The CV measurements for MOS capacitors A (solid line) and B (dashed line), each having the same area, are shown below.
a) (5 pts.) Are the substrates p-type of n-type? How do you know this? b) (5 pts.) Circle A or B to indicate the capacitor having larger
Xox: A B VFB: A B Xdmax: A B N (^) sub: A B VT: A B
Problem #
(30 pts.) A MOS capacitor has the following doping profile in the silicon substrate.
a) (6 pts.) Draw the energy band diagram at the flat-band condition, i.e. when there is no band bending at the Substrate-SiO 2 interface (EOX = 0).
b) (6 pts.) Calculate the flat-band voltage for part (a). c) (6 pts.) What is the depletion region width in the substrate when Vg > VT?
d) (6 pts.) Qualitatively sketch the high-frequency and low-frequenct CV curves for this MOS capacitor. Label the curves HF and LF respectively. e) (6 pts.) What is the high-frequency capacitcance value (units of F/cm 2 ) at Vg = VT
