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- Current spreading has one of the
major issues in Deep UV LEDS that is Self Heating Effect
and to realize reverse polarized structures.
- Higher voltage drop can increase
the heating effect of the device.
- The reverse-biased tunnel contact
junction provides lateral current spreading (without any
semitransparent electrode) and the uniform luminescence exhibiting
an improved radiative efficiency.
- The tunnel junction is an effective
method to make possible hole injection through a lateral electron
current, with a very small voltage drop which leads to reduce in
the heating effect.
EXPLAINATION
Self Heating
Effect
- If the band gap is high,then a
large band gap results in low tunneling probability and high
tunneling resistance in interband tunnel junctions, and give
additional voltage drop that will be unacceptably high.
- However, polarization engineering
can help to get over the intrinsic barriers to tunneling in these
materials. This can be achieved by aligning conduction &
valence bands on either side of the junction by utilizing the high
electric fields from the interfacial polarization-induced dipoles
at junctions.
- This will result in reduction in
depletion width (or tunneling distance) as well as the reduction in
energy barrier.
- The band gap will become lower
andbarrier leads to a high reverse current density in GaN tunnel
junctions.The forward characteristics will result in negative
resistance.
- tunnel junctions act as a carrier
conversion center enabling tunnel injection of holes into the
active device
layer. In a reverse biased tunnel junction, an electron in the
valence band of p-type material tunnels into
the empty states in the conduction band of the n-type material,
leaving behind a hole in the p-type
material, leading to hole injection into the p-type material.
- tunnel junctions can eliminate the
need for thick p- layer which otherwise is required for
hole injection into another p-type layer. Since the low band gap
layer required is thinner and the associated absorption loss will
also be reduced.
Realizing reverse polarized
structures
The p-down structure is essential to
realize the reverse polarization structures along with the +c
orientation, which is a challenge due to current crowding and poor
ohmic p-contact on etched
surfaces. A tunnel junction based structure would overcome the both
p-region spreading and enable inverted polarity LEDs for
potentially higher efficiency.
For example-for A Ga-face p-down or
N-face p-up structure, the depletion field of the heavily doped p-n
junction and the polarization field due to the sheet charges at
GaN/InGaN interface are in the same direction, and this
configuration is considered favorable for tunneling and helps
polarizing in C structure.
Hope that helps :)