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Lec15 - Subject_14: Outline pn Junction Transient-...

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Unformatted text preview: Subject_14: Outline pn Junction Transient- & Small-Signal AC-Response * pn-junction admittance * Reverse-bias capacitance C-V curves & dopant profiling * Forward-bias admittance Frequency dependence * Transient response of pn junctions Switching diodes pn-Junction Admittance To discuss the response of pn junctions to TIME-VARYING voltages we must define the pn-junction 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 forward-bias 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 QUASI-NEUTRAL REGIONS AND THEIR ASSOCIATED CONTACTS THE SERIES RESISTANCE IS NORMALLY MUCH SMALLER THAN THE JUNCTION RESISTANCE EXCEPT FOR STRONG FORWARD BIAS Reverse-Bias Capacitance An important application of REVERSE-BIASED 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 depletion-region capacitor VARY as a function of the reverse-bias voltage hence these capacitors are referred to as VARACTORS n-TYPE DEPLETION REGION V a > 0 V a < 0 V a = 0 p-TYPE 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 reverse-biased junction therefore looks like a PARALLEL- PLATE capacitor and its capacitance may be computed as * In this expression A is the cross-sectional 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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Lec15 - Subject_14: Outline pn Junction Transient-...

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