chapter R2 - CHAPTER H2 fl is] “A wide-base diode is...

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Unformatted text preview: CHAPTER H2 fl is] “A wide-base diode is assumed. {h} ...Tbe R-G current is the dominant cement component. to] ...Avalanching is the dominant process causing breakdown if V33 :3- - 4.5 V. to}@.-.c1 varies as 1N Vbi—Vg it one has a step junction. £2. (a) {i} forward biased ...pn(x=xn) :- pnn = ntszD. One has a carrier excess at the n-edge of the depletion region. {b} ...Per the Eq. {6.18) Ittoundarg.I condition sperm") = (eitheitett’siti— n The ningD factor must be the same for both diodes since the rrtinorit},r carrier concentrations in the two diodes approach the same value as x —+ an. Also. npntmxn} is the same for both diodes. It therefore follows that VA must he the same for both diodes. (c) (i) significantly larger than “As emphasized in Subsection 8.1.2 [see Eq. 8.2}, the current flowing in an ideal diode is directly proportional to dopnfritipxn. Inspectng the figure associated with the problem, we find the magnitude of dfiuwidxixzx" = dpnfdtifixn is greater for Diode E. (d) (ii) roughly the same as]...Ii‘anND is the same in twop+'n Si diodes maintained at room temperature, then the No doping meat be the same in the two diodes. Now, the FER of a p+—rt junction varies in an inverse manner with the n—side ND doping. Thus, even though the p+—side NA doping may be different. having the same ND doping, the diodes should exhibit rougth the same breakdown voltage. ' Rl—l £3. {a} Forward biased . There is an excess of rninoritg,r carriers adjacent to the edges of the depletion region. {b} — low level injection does prevail, As required for low—level injection conditions to prevail, the majorityr carrier concentrations in the quasineutral regions are essentiall},r unperturbed, and the minm‘lty can‘ier concentrations in these regions are much less than the majority.F carrier concenn‘ations. (c) The diffusion capacitance {CD} results dinnctlyr front the oscillation of minority carrier charge piled-up near the depletion region edges in response to an applied a.c. signal. {d} In going from the forwardvhlas “on"rstate to the reverse—bias “off”—state, the store of minority carriers adjacent to the edges of the depletion region must be removed. Since this cannot be accomplished instantaneously, there is a lag time known as the storage delayr time (:5) that is observed during the turn—off transient. RZ—Z EA {a—f) The required dashed~line or “no effect" answers are given in the figures on the next page. An explanation of the answers {which is not technically required) is provided below. [:11 The increase in ND leads to two major modifications in the .LV characteristic. First, since FER is approximately proportional to UNI), increasing ND by a factor of 2 decreases VBR by about a factor of 2. Second, from the shape of the f—rV characteristic, it is clear that the R—G current demimttes in the given device. {This is also to be expected. since the characteristics were said to be derived from a Si diode maintained at room temperature.) For reverse biases greater than a few kaq volts, Aw I- =—"’ 1 .s.4s no in} E41 ( 3 where In w = “smtvwm Eq. (5.33) fora p+—n chip At biases where 43,; 3c:- i’bi, W a: 1N ND, and the factor of 2 increase in ND reduces W and therfore It“; by a factor of «ii. At small —VA. Veil Whiflh 15 slightly iJ’ICTEaSfid by increasing ND, causes the decrease in W and IRJG to be slightly less than {1. {£11 Noting C] = K'sfty‘ii'ii”, we conclude based on the discussion in part {a} that C] a: NW will increase by a factor of xi? if —Vg no» Vht. At small —Vg, the slight increase in Vb; with the increase in ND will lead to an increase in C; that is slightly less than Vii. (£1 The storage delay time relationships developed and cited in Chapter 3 (Eqs. 3.3 and 3.9) are not a function of the semiconductor doping. [d1 As noted in the part (a) explanation, fRfi a: List} is clearly the dominant current component. Thus, increasing it] by a factor of 2 1will decrease the observed current by a factor of 2. fig The junction capacitance is not a function of the TI} and in carrier lifetimes. {fl :5 as given by either Eq. (8.8] or (8.9) is directly proportional to rp. Thus, increasing II, by a factor of 2 causes .I.’S to increase by a factor of 2. 122-3 {a} NU EFFECT [ii-4 ...
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chapter R2 - CHAPTER H2 fl is] “A wide-base diode is...

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