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ECE 3080: Test #2, Solutions July 9, 2009 Page 1 of 8 G EORGIA I NSTITUTE OF T ECHNOLOGY S CHOOL OF E LECTRICAL AND C OMPUTER E NGINEERING ECE 3080: Semiconductor Devices for Computer Engineering and Telecommunication Systems Summer Semester 2009, Test #2 SOLUTIONS Short Questions (20 questions, 3 points each): 1. Sketch the total minority carrier concentration in the quasi-neutral regimes of a pnp bipolar transistor biased in active mode. Mark the emitter, base and collector regime as well as the depletion regions and the equilibrium minority carrier concentrations in the three regimes. E B C 2. Sketch the equivalent circuit of a simplified hybrid-pi model for an npn bipolar transistor and explain when this equivalent circuit is used. See Chapter 4.4, page 11 of my handouts (schematic on the top of the page). The simplified hybrid-Pi model is used for low-frequency, small-signal analysis of the BJT in active mode bias. 3. You want to improve the Early voltage of a BJT, i.e. you want to reduce the influence of the associated non-ideality. Do you try to increase or decrease the Early voltage? Justify your answer. You try to increase the magnitude of the Early voltage (note: the Early voltage is defined as a positive voltage, even though it is the voltage at which the extrapolated I C -V CE intersects with the negative V CE axis). With increasing (magnitude of the) Early voltage, the slope of the I C -
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ECE 3080: Test #2, Solutions July 9, 2009 Page 2 of 8 V CE characteristic is reduced (reduced base-width modulation). An ideal BJT has an Early voltage V A = . 4. What is the most efficient way to improve the emitter efficiency of a homojunction BJT? Justify your answer. Increase the emitter doping with respect to the base doping: N E >> N B . As a result, under active mode bias, more carriers are injected from the emitter into the base than vice versa, thus increasing the emitter efficiency. 5. A good BJT should have a “small” base. When do we consider a base to be “small”? Small compared to which characteristic length? Which transistor parameter is mainly affected by making the base “small”? The base length should be small compared to the minority carrier diffusion length in the base: W << L B . A small base improves the base transport factor, i.e. ensures that carriers injected from the emitter into the base can diffuse across the base without recombination. 6.
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This note was uploaded on 07/30/2009 for the course ECE 3080 taught by Professor Staff during the Spring '08 term at Georgia Tech.

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