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Unformatted text preview: CHAPTER H2 ﬂ
is] “A widebase diode is assumed.
{h} ...Tbe RG 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 nedge 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 piledup 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. 1223 {a} NU EFFECT [ii4 ...
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 Fall '07
 HAMBLEN

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