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diode_spice_old - EE151 Class Notes Diode Spice Models S.K...

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EE151 Class Notes: Diode - Spice Models S.K. Tewksbury Sept. 29, 1997 Contents 1 Introduction 1 2 IS: Saturation Current 1 3 RS: Parasitic Series Resistance of Neutral Regions 2 4 N: Emission Coefficient (or Ideality Factor 3 5 TT: Transit Time 4 6 CJ0: The Zero Bias p-n Capacitance 7 7 VJ: Junction Potential 8 8 M: Junction-Gradient Coefficient 8 9 XTF: IS Temperature Coefficient 9 1 Introduction This handout summarizes the Spice modeling of a silicon PN diode. Table 1 lists the Spice parameters, which are described individually in the sections below. 2 IS: Saturation Current The total DC diode current density J d (Amps/cm 2 ) and current I d = AJ d (Amps) is related to the voltage V across the diode and the saturation current J s under reverse bias by J d = q D p p n 0 L p + D n n p 0 L n · e q ( V bi - V ) /kT (1) = J s · e q ( V bi - V ) /kT (2) where J s = q D p p n 0 L p + D n n p 0 L n (3) 1
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Table 1: Spice parameters for silicon PN diode. Parameters marked with a ”*” in column 2 are proportional to the area (end-face) of the diode. Name Area Model parameter Units Default Typical IS * Reverse saturation current Amps 1E-14 1-14 RS * Parasitic resistance of neutral regions Ohms 0 10 N Emission Coefficient 1 0.5 to 1 (also called ideality factor) TT Transit time seconds 0 0.1NS CJO * Zero bias p-n capacitance Farads 0 2pF VJ Junction potential Volts 1 0.6 M Junction-gradient coefficient 0.5 0.5 EG Activation energy Electron Volts 1.11 1.11 (same as energy gap) XTF IS temperature coefficient 3 3 KF Flicker noise coefficient 0 AF Flicker noise exponent 1 FC Forward bias depletion 0.5 capacitance coefficient BV Reverse breakdown voltage Volts 50 IBV * Reverse breakdown current Amps 1E-10 J d and J s are current densities, with the total currents I d and I s determined by the cross sectional area A through which the current enters and leaves the diode, i.e., I s = J s · A (4) I d = I s · e q ( V bi - V ) /kT (5) The Spice parameter IS is I s . Three cases arise in obtaining I s . 1. An estimate of J s can be obtained if you know the physical parameters used in (3). This “physics model” also allows you to explore the effect of various variations in fabrication (e.g., doping densities in the N- and P-type regions, minority carrier lifetimes, etc.). I s is then obtained by multiplying by the area of the device. 2. The saturation current density J s (or the saturation current I s ) may be specified directly, either on a data sheet for a commercial diode or as a specification for diodes in a given VLSI circuit. 3. The saturation current may be estimated from current voltage characteristics (either data sheets or experimental measurement). 3 RS: Parasitic Series Resistance of Neutral Regions The neutral regions of the diode are the regions between the depletion layer edges and the contacts to the diode. There is a non-zero resistance in these regions, which lead to a voltage drop between the external 2
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voltage applied V appl and the voltage V across the diode in (2). If the distance between the junction (point where N-type region changes to P-type region) and the N-side contact is l n , the distance d
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