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1.1 Medupi power station uses coal of calorific value of 5880 kcal/kg to generate electricity. They use 14.3 tonnes of coal every 25 minutes to generate the maximum demand of the power station. The thermal and electrical efficiency of the power station are 24.2% and 81.1%, respectively. The cost of purchasing the coal is R800 per tonne.
1. Determine the overall efficiency of the power station (%).
Find the compound amount at the end of 3 years if P2000 is invested at 8% compounded semi - annually.
A 20 kVA closet transformer located inside a building is used to step down the voltage for the building. It is connected to a primary 4800 V AC power source. The ratio of the number of primary windings to the number of secondary windings on the transformer is 20 to 1 and 40 to 1.
a. Approximate the secondary voltage provided.
b. Approximate the current available, in amps
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(5) Compared to a PN junction with NA = ND = 1014 cm-3, which one of the following
statements is TRUE for a PN junction with NA = ND = 1020 cm-3? ( ).
A. Reverse breakdown voltage is lower and depletion capacitance is lower
B. Reverse breakdown voltage is higher and depletion capacitance is lower
C. Reverse breakdown voltage is lower and depletion capacitance is higher
D. Reverse breakdown voltage is higher and depletion capacitance is higher
(6) Gold is often diffused into silicon PN junction devices to ( ).
A. increase recombination rate B. reduce recombination rate
C. make silicon a direct gap semiconductor D. make silicon semimetal
(3) Which of the following statements is (are) true about reverse bias breakdown due
to impact ionization and Zener tunneling? ( ).
A. Tunneling breakdown generally occurs at lower voltages than impact ionization.
B. Tunneling generally has a weaker temperature dependence that impact ionization.
C. Significant tunneling only occurs when the junction is heavily doped.
D. All of the above.
(4) Consider an abrupt PN junction. Let Vbi be the built-in potential of this junction and
VR be the applied reverse bias. If the junction capacitance (Cj) is 1 pF for Vbi + VR =
1 V, then for Vbi + VR = 4 V, Cj will be ( ).
A. 4 pF B. 2 pF C. 0.25 pF D. 0.5 pF
1. Multiple Choice (Choose the ONE answer that best fits the question/statement.)
(1) When the “nfactor” of a diode ID = I0(eqV/nkT −1) is 2 at low forward bias voltages,
what do we suspect? ( ).
A. That the current is dominated by high level injection in the lightly doped, quasi-
neutral region.
B. That the current is dominated Zener tunneling
C. That the current is dominated by recombination in the space charge region.
D. That the current is dominated by generation in the space charge region.
(2) How does series resistance affect on diode characteristic? ( ).
A. It leads to the sharp increase of the current
B. The direct current, depending on the voltage, increases slower than exponential
law
C. V-I characteristic, starting with the smallest value of voltage, becomes ohmic
D. Negative differential conductance region appears on V-I characteristic of the diode
The excess hole concentration within the quasi-neutral n-type region of a silicon p+n
step junction of area A = 1mm × 1mm maintained at T = 300K is plotted on a linear
scale below. The hole lifetime τp = 10-6 s. ni = 1010 cm-3.(1) Calculate the applied voltage, VA.
(2) Calculate the minority-carrier charge stored in this diode, QP.
(3) Calculate the diode current, I.
If the slope of the (1/Cdep)2 vs. VA characteristic of a Si PN step junction is 2 × 1023
F-2V-1, the intercept is 0.84 V, and A is 104 μm2, find the lighter and heavier doping
concentrations Nl and Nh.
Compare Ge diodes to similarly structured and doped Si diodes for the following
(give reasons):
(1) Reverse saturation current
(2) Avalanche breakdown voltage
