Enhance Performance of GaN based LEDs Under High Currents

1
Enhance Performance of GaN based LEDs Under High
Currents

• Luke Seale Barry Zhong
• Shuji Nakamura Steven DenBaars

2
Diodes

• An electrical device that, with bias voltage,
  allows varying degrees of current to flow
• A typical diode consists an electron rich N-type
  material and a hole rich P-type material

N
?
N
OFF
ON
?
P
P
3
Gallium Nitride-based Materials

• Doped with Magnesium for P-type, Silicon for
  N-type
• Grown epitaxially with MOCVD (Metallic Organic
  Chemical Vapor Deposition)
• Trimethol Gallium and ammonia gas are flown over
  a substrate at 1000º C, so they get deposited
• InGaN is used for the active region, from which
  light is emitted

4
How an LED Works

• An efficient LED uses a direct bandgap material,
  such as Nitrides
• A bandgap is the energy difference between the
  conduction and valence energy bands
• When an electron recombines with a hole, it emits
  a photon with the equivalent energy of the band
  gap.
• Diffusion area

Gallium Nitride
N
Conduction Band (electrons)
3.2 eV released
Diffusion length
Valence Band (holes)
P
5
Double Heterostrucure and Quantum Well LEDs

• InGaN layer with smaller bandgap
• Higher carrier concentration
• R Bnp

N
electrons
N
3.2 eV
holes
P
P
Band diagram of double heterostructure (invented
by Herbert Kroemer)
Band diagram of quantum well structure
6
Consequences of Using Quantum Well Design

• Current overflow
• Possible solutions
• Increasing quantum well numbers
• Larger surface area to reduce current densities

7
How LEDs are Made

• MOCVD for epitaxial layer growth
• Rapid Thermal Annealing for P-type GaN activation
• ITO for transparent P contacts
• Photolithography
• Plasma etching to form mesas
• E-Beam evaporation for metal contacts

8
Finished Device
A cross-section of a device (not proportional)
Top view of the device
1 mm
9
Test Subjects

• Small .25mm x .5 mm device
• Large 1mm x 1mm device (Powerchip)

10
Testing the Devices

• Use a function generator to bias the device and
  measure current
• Use a spectrometer for wavelength
• Use a photometer for output power

11
I-V Curve

• The dark line is the powerchip, the dashed is
  the small device.
• The small devices curve ends earlier because we
  were afraid of burning it
• The powerchip has lower voltages for the same
  drive current

12
L-I Curve

• The thick line is the powerchip, the dashed is
  the small device
• While the small device performs better at lower
  currents, the powerchip is more intense at higher
  currents, which we care about.

13
Conclusion

• The Powerchip performed better in both criteria
• Since it is larger, the injecting area for
  carriers is larger and the device can be operated
  at higher currents

? An awesome powerchip LED being tested under
high current
14
What I learned

• That semiconductor physics is fascinating,
  complex, and very intriguing
• That college textbooks use dense language to
  describe relatively simple things
• That clean rooms are full of expensive machines
• That LEDs are the future
• That Barry is helpful and hilarous

15
Acknowledgements

• Barry Zhong for being my mentor
• Shuji, Steve, and Jim for advising Barry
• The other AR interns for being good company
• Derrick, Ben, and Jordan for being in the office

16
The End