TEST2 summer 2005

# TEST2 summer 2005 - GEORGIA INSTITUTE OF TECHNOLOGY SCHOOL...

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ECE 3080: Test #2, Solutions July 31, 2005 Page 1 of 6 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 2005, Test #2 SOLUTIONS 1. Sketch the bandstructure of a pn-junction under forward bias , showing E C , E V , E i and E F (sketch E F only in the quasi-neutral regions). See e.g. handouts, Chapter 3.1, page 15, Figure (b). 2. Does the saturation current density of a pn junction depend on temperature? Justify your answer. The saturation current density of a pn junction strongly increases with temperature. I S is proportional to the square of the intrinsic carrier concentration n i , which exponentially increases with temperature. 3. The switching speed of a pn-junction can be improved by doping both sides of the junction with gold atoms (in addition to N A and N D ). Why? Doping of silicon with gold results in R-G centers located in the center of the bandgap. With increasing density of R-G centers, the R-G center recombination is improved, resulting in a reduced minority carrier lifetime and thus an increased switching speed. 4. You measure the I-V characteristic of a silicon pn-junction at room temperature and observe that the current flowing under reverse bias is larger than what you would expect from the ideal diode equation. What might be the origin of this non-ideality? Justify your answer. The reason for the increased current could be the additional generation current originating from the depletion region in case of a reverse biased junction. Under reverse bias, the carrier concentrations in the depletion region are reduced below equilibrium, resulting in carrier generation and an additional current component. Alternatively, junction breakdown (Zener or avalanche) could be responsible for the increased current.

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ECE 3080: Test #2, Solutions July 31, 2005 Page 2 of 6 5. What is known as the Zener breakdown in a pn-junction? Zener breakdown is associated with the sudden increase of the current flowing in a reverse
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TEST2 summer 2005 - GEORGIA INSTITUTE OF TECHNOLOGY SCHOOL...

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