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hw7_solutions - ECE 440 Homework VII Solutions Spring 2006...

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ECE 440 Homework VII Solutions Spring 2006 Due: Wednesday, February 22, 2006 1. The donor profile of a silicon sample is shown below (graph can be seen on the assignment sheet). Assume that the majority carrier mobility can be obtained from Fig. 3-23, and the sample is at thermal equilibrium at 300 K. (a) Determine the diffusion coefficients for majority carriers at points A, B and C, respectively. To find the diffusion coefficients, we have to first find the doping concentrations at those points. After we find those values, we can use the Einstein Relationship (4-29) to figure out the diffusion coefficients: (b) Find the majority carrier diffusion current densities along the cross sections at points A, B and C, respectively. Indicate not only the magnitude but also the direction. Before we can calculate the diffusion current densities, we need to find the equation for the majority carriers Nd(x) = n(x). Remember that the plot shown in the HW is a semilog plot. Therefore, we need to formally calculate the carrier concentration as a position of x, because the slope is different at every point! Luckily, with a mathematical trick and 2 points, we can easily

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