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Unformatted text preview: Subject_14: Outline pn Junction Transient & SmallSignal ACResponse * pnjunction admittance * Reversebias capacitance CV curves & dopant profiling * Forwardbias admittance Frequency dependence * Transient response of pn junctions Switching diodes pnJunction Admittance To discuss the response of pn junctions to TIMEVARYING voltages we must define the pnjunction ADMITTANCE (Y) with RESISTIVE and CAPACITIVE components * Eq. 14.1 follows from the fact that pn junctions can be well represented by an EQUIVALENT CIRCUIT with THREE main components A PARALLEL combination of a CAPACITOR (C) and CONDUCTOR (G) whose values VARY as a function of the applied bias and a SERIES resistance (R s ) due to the parts of the diode AWAY from the junction this is SMALL compared to 1/G EXCEPT for large forwardbias voltages ) 1 . 14 ( C j G Y + = p n G C R s pn JUNCTION CONNECTED IN AN ELECTRICAL CIRCUIT AND ITS CORRESPONDING EQUIVALENT CIRCUIT THE DEPLETION REGION IS MODELED AS A CAPACITOR IN PARALLEL WITH A CONDUCTOR AND THE VALUES OF BOTH OF THESE ELEMENTS ARE DEPENDENT UPON THE APPLIED BIAS THE SERIES RESISTANCE REPRESENTS THE DOPED QUASINEUTRAL REGIONS AND THEIR ASSOCIATED CONTACTS THE SERIES RESISTANCE IS NORMALLY MUCH SMALLER THAN THE JUNCTION RESISTANCE EXCEPT FOR STRONG FORWARD BIAS ReverseBias Capacitance An important application of REVERSEBIASED pn junctions is as CAPACITORS in integrated circuits * The depletion region of such junctions may be viewed as analogous to a CAPACITOR * In contrast to a conventional capacitor however BOTH the stored charge AND the width of the depletionregion capacitor VARY as a function of the reversebias voltage hence these capacitors are referred to as VARACTORS nTYPE DEPLETION REGION V a > 0 V a < 0 V a = 0 pTYPE SMALL CHANGES IN THE EXTERNAL BIAS PRODUCES CORRESPONDINGLY SMALL CHANGES IN THE DEPLETION REGION WIDTH x V a > 0 V a < 0 V a > 0 V a < 0 THE NET EFFECT THEREFORE APPEARS TO BE A MODULATION OF THE CHARGE STORED AT THE ENDS OF THE DEPLETION REGION For SMALL ac modulations of the external bias the reversebiased junction therefore looks like a PARALLEL PLATE capacitor and its capacitance may be computed as * In this expression A is the crosssectional area of the junction and W is the BIAS DEPENDENT width of the depletion region (see Equations 10.9, 10.10 & 10.11) * Since the capacitor charges are provided by the MAJORITY carriers they can respond VERY QUICKLY...
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 Winter '07
 Vis
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