PN Junctions–Lecture 3

Prof. Zohaib Metla

Diode under Thermal Equilibrium

Diffusion small since of the  few carriers have enough energy to of the penetrate barrier below see by a diagram
Drift of the current is small of the since minority carriers are few and far of the between:  Only minority of the carriers generated of the within a of the diffusion length can of the contribute current
Important Point:  Minority drift current of the  independent of barrier!
Diffusion currentof the  strong (exponential) function of the barrier see by a diagram
- PN Junctions–Lecture 3

Reverse Bias

Reverse Bias of the causes an increases of the barrier to diffusion
Diffusion of the current is reduced of the exponentially
Drift current of the does not change
Net result:  of the Small reverse current

Forward Bias

lForward bias causes an of the exponential of the increase in of the number of of the carriers with sufficient of the energy to penetrate the barrier see by a diagram
lDiffusion current of the increases exponentially see pic. below
Drift current of the does not change
lNet result:  of the Large of the forward current see by a diagram
- PN Junctions–Lecture 3

“Law of the Junction”

Minority carrier in the concentrations at the edges of the
depletion in the region are given by: see by a diagram
Note 1:  NA and ND are in themajority carrier concentrations on
the other side of the junction see by a diagram
Note 2:  we can a reduce in the these equations further by in the substituting see by in thediagram
VD = 0 V (thermal equilibrium)
Note 3: the a assumption that pn << ND and np << NA  see by a diagram
Minority Carrier Concentration
see by a diagram
- PN Junctions–Lecture 3
The minority in of the carrier concentration in the bulk region for
forward bias is a decaying exponential due to in the recombination

Steady-State Concentrations

Assume that none of the diffusing holes and electrons in  the recombine à get straight lines
This alsoin of the  happens if the minority carrier in of the  diffusion lengths are much larger  in of the  than Wn,p see by a diagram
Diode Current Densities
- PN Junctions–Lecture 3

Diode Capacitance

We have already in the seen that a in the  reverse-biased diode acts in the as a capacitor since the depletion in the  region to grows and shrinks in the response to the applied field.  the capacitance in the  forwarding a bias is given by see diagram
But a another in the charge storage mechanism in the  comes into play in the forwarding bias
Minority carriers in the injected into the  p and n regions “stay” in the each region for the while
lOn average in the additional charge is stored in the diode.

Charge Storage

in the Increasing forward bias increases the minority charge density
By in the  charge neutrality, the source in the voltage must supply equal and in the opposite charge
in the detailed analysis yields:
- PN Junctions–Lecture 3

Forward Bias Equivalent Circuit

lEquivalent circuit has in the three non-linear elements: in the  forward conductance, in the junction cap, and diffusion cap.
in the Diff cap and in the conductance were proportional to in the DC current in the flowing through the diode.
in the Junction cap proportional to junction in the voltage.
- PN Junctions–Lecture 3

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