Lecture 5 OUTLINE PN Junction Diodes IV Capacitance

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

Lecture 5 OUTLINE • PN Junction Diodes – I/V – Capacitance – Reverse Breakdown – Large and Small signal models Reading: Chapter 2. 2 -2. 3, 3. 2 -3. 4 EE 105 Fall 2011 Lecture 5, Slide 1 Prof. Salahuddin, UC Berkeley

Recap: Law of the Junction Law of the junction: -b 0 a EE 105

Recap: Law of the Junction Law of the junction: -b 0 a EE 105 Fall 2011 Lecture 5, Slide 2 Prof. Salahuddin, UC Berkeley

Recap: Minority Carrier Concentrations at the Edges of the Depletion Region -b 0 x'

Recap: Minority Carrier Concentrations at the Edges of the Depletion Region -b 0 x' ; x‘=0 x=a Notation: Ln electron diffusion length (cm) EE 105 Fall 2011 Lecture 5, Slide 3 Prof. Salahuddin, UC Berkeley

Hole Diffusion X’’ EE 105 Fall 2011 0 x’’ Lecture 5, Slide 4 Prof.

Hole Diffusion X’’ EE 105 Fall 2011 0 x’’ Lecture 5, Slide 4 Prof. Salahuddin, UC Berkeley

Distribution of Diffusion Current x -b 0 a • Assume: No Recombination in the

Distribution of Diffusion Current x -b 0 a • Assume: No Recombination in the depletion region • Known: Total Current is the same everywhere EE 105 Fall 2011 Lecture 5, Slide 5 Prof. Salahuddin, UC Berkeley

Diode Current under Forward Bias • The current flowing across the junction is comprised

Diode Current under Forward Bias • The current flowing across the junction is comprised of hole diffusion and electron diffusion components: J_total x -b EE 105 Fall 2011 0 a Lecture 5, Slide 6 Prof. Salahuddin, UC Berkeley

I-V Characteristic of a PN Junction • Current increases exponentially with applied forward bias

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: EE 105 Fall 2011 Lecture 5, Slide 7 Prof. Salahuddin, UC Berkeley

Practical PN Junctions • Typically, pn junctions in IC devices are formed by counter-doping.

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 ID (A) – NA net acceptor doping on p-side (NA-ND)p-side – ND net donor doping on n-side (ND-NA)n-side VD (V) EE 105 Fall 2011 Lecture 5, Slide 8 Prof. Salahuddin, UC Berkeley

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

How to make sure that current flow in a forward-biased p-n junction diode is mainly due to electrons? EE 105 Fall 2011 Lecture 5, Slide 9 Prof. Salahuddin, UC Berkeley

Diode Saturation Current IS • IS can vary by orders of magnitude, depending on

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: 10 -14 to 10 -17 A/mm 2 • 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, EE 105 Fall 2011 Lecture 5, Slide 10 Prof. Salahuddin, UC Berkeley

Depletion Width at Equilibrium (see slide 3 in lecture 4) r(x) q. ND a

Depletion Width at Equilibrium (see slide 3 in lecture 4) r(x) q. ND a -b -q. NA x EE 105 Fall 2011 on the N side: --(2) Let us set the reference point at x=a Then V(a)=0 V(-b)=V 0; Built in potential V 0 0 --(1) --(3) V(x) -b on the P side: a x Lecture 5, Slide 11 Prof. Salahuddin, UC Berkeley

Depletion Width at Equilibrium V(x) --(1) V 0 -b --(2) 0 a x --(3)

Depletion Width at Equilibrium V(x) --(1) V 0 -b --(2) 0 a x --(3) V(a)=0 V(-b)=V 0; Built in potential EE 105 Fall 2011 Lecture 5, Slide 12 Prof. Salahuddin, UC Berkeley

Depletion Width at Equilibrium V(x) V 0 -b EE 105 Fall 2011 0 a

Depletion Width at Equilibrium V(x) V 0 -b EE 105 Fall 2011 0 a x Lecture 5, Slide 13 Prof. Salahuddin, UC Berkeley

Depletion Width at biased conditions V(x) V 0 -b EE 105 Fall 2011 0

Depletion Width at biased conditions V(x) V 0 -b EE 105 Fall 2011 0 a x Lecture 5, Slide 14 Prof. Salahuddin, UC Berkeley

PN Junction Depletion Capacitance • A reverse-biased PN junction can be viewed as a

PN Junction Depletion Capacitance • A reverse-biased PN junction can be viewed as a capacitor, for incremental changes in applied voltage. esi 10 -12 F/cm is the permittivity of silicon EE 105 Fall 2011 Lecture 5, Slide 15 Prof. Salahuddin, UC Berkeley

Voltage-Dependent Capacitance • The depletion width (Wdep) and hence the junction capacitance (Cj) varies

Voltage-Dependent Capacitance • The depletion width (Wdep) and hence the junction capacitance (Cj) varies with VR. VD EE 105 Fall 2011 Lecture 5, Slide 16 Prof. Salahuddin, UC Berkeley

Reverse-Biased Diode Application • A very important application of a reverse-biased PN junction is

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. EE 105 Fall 2011 Lecture 5, Slide 17 Prof. Salahuddin, UC Berkeley

Forward Bias Diffusion capacitance x’’ At small forward bias, putting a small ac signal

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 EE 105 Fall 2011 Lecture 5, Slide 18 Prof. Salahuddin, UC Berkeley