OE_50200: Compound Semiconductors

1
OE_50200 Compound Semiconductors Lecture
9 LED Current flow ??????????? National
Dong-hwa University, Hualian, Taiwan 2008.
05.09
2
Outlines

• Design of current flow

• Current spreading
• Current crowding
• Current blocking
• Lateral injection
• Flip-chip packaging

3
P-contact
p
Current spreading layer
n
n-contact
Vertical
Horizontal
4
Current Spreading Structures for AlGaInP
AlGaAs window layer is lattice matched to GaAs
and it is somewhat absorbing, because of the
indirect bandgap. Al is difficult to grow by
MOCVD. So AlGaAs window is usually inferior.
GaP window layer non-absorbing but is lattice
mismatched to GaAs
5
Effect of Window Layer Thickness (1)
6
Effect of Window Layer Thickness (2)

• Thin layer has problem with the light blocking by
  the top contact.
• Thick layer will push the current to the edge of
  the die, but high surface recombination hinders
  the eff., increased absorption, increased
  resistance, long growth time causes dopant
  diffusion in the active layer to lower the eff..

7
Transparent Conductive Contact
8
Theory of Current Spreading - 1
For linear strip geometry,
, Ls current spreading length
At x rcLs, j 1/e, then applying ohms law
9
Theory of Current Spreading - 2
For circular geometry,
10
Current Crowding in LEDs on Insulating Substrates
Vj-Vj-1 (Jj-1dxW) (r/tn//wdx) Vj1-Vj
(JidxW) (r/tn/w dx) Jj - J-1
joexp(eVj/kT)-1
11
Voltage across the p-n junction and the
p-resistors
Perform the 2nd derivative, inserting into the
eq. above
12
For
and
The solution is
J(x) Joexp(eVj/kT) Joexp(eVj(0)/kT)
exp(-x/Ls)
The current spreading length is
For small rcrptp, the crowding effect
increases!
13
Experimental Data on Current Crowding
14
Interdigitated Design
Guo and Schubert, APL 78, 33337 (2001) D. A.
Steigerwald US Patent 6,307, 218 (2001)

• Larger resistance can alleviate the current
  crowding, but may generate more heat,
• Current crowding will be important, as the
  resistances improve and the chips become large,
• Interdigitated structure with WpltLs shown above
  may solve the problem!

15
Lateral Injection Schemes
If the n-type sheet resistance is much larger
than the p-type ones, the current prefer to flow
in the laterally in the n-region and current
crowds near the p-type contact.
16
Current Dependence of the Lateral Injection
J(x) J(0) exp(-x/Ls) Ls
Large Ls favors uniform light generation. This
means high doping and thick layer.
2Va
J(0)(rp/tp) (rn/tn)
17
Current Blocking Layer

• N-type current blocking layer can force the
  current flow outside the contact to minimize the
  contact effect.
• The use of epitaxial regrowth will add up the
  cost and decrease the yield.

18
Flip Chip Packaging

• Large p-type, Non-blocking contacts
• Better heat conduction through Solder

19
Exercises for Lecture 9
Current crowding occurring at very high
current levels in devices with current-spreading
layer. In device structures with vertical current
flow (current flowing from the top to bottom of
chip), the current-spreading layer ensures that
the current spreads out over the entire p-n
junction area. However, as the current increases
to very high levels, the current tends to crowd
under the top contact as illustrated below. Why?