Narrow Base PN junction Diode

1
Narrow Base P-N junction Diode
What happens if the diode is smaller than the
minority carrier diffusion length(s)? Diffusion
lengths can be 20-30 microns
2
Narrow base diodes

• Surface of diode will cause recombination of all
  minority carriers
• New boundary conditions
• Minority carrier diffusion equations

3
Solutions
This time, B,D?0
4
Solutions

• Using general form and boundary conditions, can
  show

If WltltL and xltL then Sinh arguments are small.
5
Find current density from continuity equation
Similarly for Jn
6
Total diode current
Compare to result from wide base ideal diode
?Replace minority carrier diffusion length with
diode width
7
Charge control methodology

• Analyze by examining injected minority carrier
  charge
• e.g. electrons injected into p side of FB diode
• Total negative charge on p-side

?np
x
-xp
8
Charge control method

• Approximate total charge by diffusion length
  times charge at boundary of QN-depletion regions
• Non-equilibrium injected electrons with average
  lifetime of ?n
• Recombination Ratecharge/timecurrent

9
Charge control
but
Similarly for holes on the n-side
10
Total current

• Same result as before but we didnt have to solve
  the minority carrier diffusion equations
• Stored charge and recombinationcurrent needed to
  resupply