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hw12_solutions - ECE 440 Homework XII Due Monday Fall 2008...

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ECE 440 Homework XII Fall 2008 Due: Monday, December 01, 2008 1. Field-effect transistors employ a mechanism different from that of BJT to control the output current by changing the charge density in the active region. The following problem is designed to illustrate the effect of changing charge density on the output current. Assume that a rectangular bar of silicon shown below consists of two layers, each of thickness t 1 and t 2 , respectively. The dimensions are D=100μm, W=4μm, t 2 =5μm and t 1 =100Å. The t 1 layer is uniformly doped with N d1 /cm 3 donors and N d2 =10 14 /cm 3 donors are also uniformly doped in the t 2 layer. A bias of 1 V is applied between x=0 and x=W. Determine the output current, I d , for different carrier densities in the t 1 layer. Specify the current for N d1 =10 17 /cm 3 , 1.1x10 17 /cm 3 , 10 18 /cm 3 , and 1.01x10 18 /cm 3 . Also, estimate the “Amplification Factor” of this device for N d1 =10 17 /cm 3 and N d1 =10 18 /cm 3 . The “Amplification Factor” is defined as AF= I d / N d1 .For simplicity, use an average electron mobility of 1000 cm
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