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

Recap: Law of the Junction -b a

Recap: Minority Carrier Concentrations at the Edges of the Depletion Region x' -b ;x‘=0 x=a Notation: Ln  electron diffusion length (cm)

Hole Diffusion x’’ X’’

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

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

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:

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)

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

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/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,

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

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

Depletion Width at Equilibrium V(x) V0 -b a x

Depletion Width at biased conditions V(x) V0 -b a x

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

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

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.

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