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Chapter 5 Lecture - Part 4

# Transient for p n junctions make the n region

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Unformatted text preview: cations • How to deal with this stored charge (and How transient voltage)? transient – For p+-n junctions, make the n-region shallower -n than Lp so that very little charge can be stored than – Add recombination center impurities (Au) into the Add n-region n-region Transient and AC Conditions Transient • Reserve Recovery Transient – In switching applications, the junction is switched In from forward bias to reverse bias modes frequently. – This affects the time-dependent charge – A reverse current that is much larger than the reverse normal reverse saturation current flows normal – Analysis is best performed using a p+-n junction -n with a square wave input with Storage delay time Storage in a p+-n diode in (a) circuit and input square wave; (b) hole distribution in the n-region as a function of time during the transient; (c) variation of current and voltage with time; (d) sketch of transient current and voltage on the device I–V characteristic. Transient and AC Conditions Transient • As the stored charge becomes depleted from the As junction, the following occurs: junction, – For Δpn positive, v(t) is positive and small, i ~ E/R until For Δpn goes to zero – When the charge is depleted and Δpn becomes negative, When v(t) becomes negative, • The source voltage, e(t), becomes divided between R and the The junction junction • Ir decreases as more of the source voltage appears across the reverse biased junction reverse – The time required for the stored charge (junction voltage) The to become zero is called the storage delay time (tsd) to Transients and AC Conditions Transients • Storage delay time: – – Its an error function Characteristic: It should be small relative to the Characteristic: switching time switching – How to decrease tsd • Shorten the lightly doped region • Introduce recombination centers (decrease carrier Introduce lifetimes) lifetimes) Effects of storage delay time on switching signal: (a) switching voltage; (b) diode current....
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