Test2 summer 2007

Test2 summer 2007 - GEORGIA INSTITUTE OF TECHNOLOGY SCHOOL...

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ECE 3080: Test #2, Solutions July 20, 2007 Page 1 of 7 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 2007, Test #2 SOLUTION 1. Sketch the total minority carrier concentration in the quasi-neutral regimes of a pnp bipolar transistor biased in inverse 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. 2. Sketch the equivalent circuit of the hybrid-pi model for an npn bipolar transistor showing a finite Early voltage and explain when this equivalent circuit is used. Assume r μ !" . The hybrid-Pi model is used for low- frequency, small-signal analysis of the BJT in active mode bias. A finite output resistance r o is used to model the Early effect.
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ECE 3080: Test #2, Solutions July 20, 2007 Page 2 of 7 3. Both junctions of an npn bipolar transistor are reverse biased. What is the mode of operation of this transistor and what can you say about the minority carrier concentration in the base? The transistor is biased in cut-off mode and the minority carrier concentration in the base is essentially zero. 4. Explain the reason for doping the emitter of a homojunction BJT heavily compared with the base, and for doping the collector lightly. The emitter is doped higher than the base to improve the emitter efficiency; the collector is doped lower than the base to reduce the extension of the depletion region of the reverse- biased B-C junction into the base and, thus, reduce the base width modulation. 5. What is the origin of a graded base and why is it beneficial for the BJT operation? Due to the doping of the base using ion implantation from the surface of the wafer, the doping concentration is not constant across the base, but decreases from E-B junction to B-C junction. This establishes an electric field in the base region. The resulting carrier drift (on top of the carrier diffusion) aids minority carriers in crossing the base, thus increasing base transport factor and current gains. 6.
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Test2 summer 2007 - GEORGIA INSTITUTE OF TECHNOLOGY SCHOOL...

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