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Published byAmberly Matthews Modified over 6 years ago

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**Lecture 5 OUTLINE PN Junction Diodes I/V Capacitance Reverse Breakdown**

Large and Small signal models Reading: Chapter ,

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**Recap: Law of the Junction**

-b a

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**Recap: Minority Carrier Concentrations at the Edges of the Depletion Region**

x' -b ;x‘=0 x=a Notation: Ln electron diffusion length (cm)

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Hole Diffusion x’’ X’’

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**Distribution of Diffusion Current**

x -b a Assume: No Recombination in the depletion region Known: Total Current is the same everywhere

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**Diode Current under Forward Bias**

The current flowing across the junction is comprised of hole diffusion and electron diffusion components: J_total x -b a

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**I-V Characteristic of a PN Junction**

Current increases exponentially with applied forward bias voltage, and “saturates” at a relatively small negative current level for reverse bias voltages. “Ideal diode” equation:

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**Practical PN Junctions**

Typically, pn junctions in IC devices are formed by counter-doping. The equations provided in class (and in the textbook) can be readily applied to such diodes if NA net acceptor doping on p-side (NA-ND)p-side ND net donor doping on n-side (ND-NA)n-side ID (A) VD (V)

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How to make sure that current flow in a forward-biased p-n junction diode is mainly due to electrons?

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**Diode Saturation Current IS**

IS can vary by orders of magnitude, depending on the diode area, semiconductor material, and net dopant concentrations. typical range of values for Si PN diodes: to A/mm2 In an asymmetrically doped PN junction, the term associated with the more heavily doped side is negligible: If the P side is much more heavily doped, If the N side is much more heavily doped,

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**Depletion Width at Equilibrium**

(see slide 3 in lecture 4) r(x) qND on the P side: --(1) a -b x --(2) on the N side: -qNA --(3) V(x) Let us set the reference point at x=a Then V(a)=0 V(-b)=V0; Built in potential V0 -b a x

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**Depletion Width at Equilibrium**

V(x) --(1) V0 --(2) -b a x --(3) V(a)=0 V(-b)=V0; Built in potential

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**Depletion Width at Equilibrium**

V(x) V0 -b a x

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**Depletion Width at biased conditions**

V(x) V0 -b a x

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**PN Junction Depletion Capacitance**

A reverse-biased PN junction can be viewed as a capacitor, for incremental changes in applied voltage. esi F/cm is the permittivity of silicon

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**Voltage-Dependent Capacitance**

The depletion width (Wdep) and hence the junction capacitance (Cj) varies with VR. VD

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**Reverse-Biased Diode Application**

A very important application of a reverse-biased PN junction is in a voltage controlled oscillator (VCO), which uses an LC tank. By changing VR, we can change C, which changes the oscillation frequency.

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**Forward Bias Diffusion capacitance**

x’’ At small forward bias, putting a small ac signal changes the concentration of diffused minority carriers and therefore gives a capacitance. This is called a diffusion capacitance

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