Lecture 5 OUTLINE PN Junction Diodes IV Capacitance
- Slides: 18
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 Fall 2011 Lecture 5, Slide 2 Prof. Salahuddin, UC Berkeley
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. Salahuddin, UC Berkeley
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 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 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. 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 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 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 -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) 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 x Lecture 5, Slide 13 Prof. Salahuddin, UC Berkeley
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 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 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 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 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
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