# Category: chapter 1 microelectronics by sedra and smith

# Category: chapter 1 microelectronics by sedra and smith

#### *3.29 A short-base diode is one where the widths of the p and n regions are much smaller than Ln and Lp, respectively. As a result, the excess minority-carrier distribution in each region is a straight line rather than the exponentials shown in Fig. 3.12. (a) For the short-base diode, sketch a figure corresponding to Fig. 3.12 and assume as in Fig. 3.12 that NA ND. (b) Following a derivation similar to that given in Section 3.5.2, show that if the widths of the p and n regions are denoted W p and Wn then I = Aqni2Wn −Dxp nND + Wp −DnxpNA eV/VT −1 and Qp = 1 2 Wn −xn2 D p Ip 1 2 W 2 n D p Ip , for Wn xn (c) Also, assuming Q Qp, I Ip, show that Cd = τ T VT I where τ T = 1 2 W 2 n D p (d) If a designer wishes to limit Cd to 8 pF at I = 1 mA, what should W n be? Assume Dp = 10 cm2/s.

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#### 3.23 A pn junction for which the breakdown voltage is 12 V has a rated (i.e., maximum allowable) power dissipation of 0.25 W. What continuous current in the breakdown region will raise the dissipation to half the rated value? If breakdown occursfor only 10ms in every 20ms, what average breakdown current is allowed?

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#### 3.22 A p+n junction is one in which the doping concentration in the p region is much greater than that in the n region. In such a junction, the forward current is mostly due to hole injection across the junction. Show that I I p = Aqni2 Dp Lp ND eV/VT −1 For the specific case in which ND = 1017/cm3, Dp = 10 cm2/s, Lp = 10 μm, and A = 104 μm2, find IS and the voltage V obtained when I = 1 mA. Assume operation at 300 K where n i = 1.5×1010/cm3.

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#### 3.21 Assuming that the temperature dependence of IS arises mostly because IS is proportional to ni2, use the expression for n i in Eq. (3.2) to determine the factor by which ni2 changes as T changes from 300 K to 305 K. This will be approximately the same factor by which IS changes for a 5°C rise in temperature. What is the factor?

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