New Twists in LEDs and HFETs based on Nitride Semiconductors
|Hadis Morkoc , Umit Ozgur , A. Matulionis|
Virginia Commonwealth University (VCU), Department of Electrical Engineering, Richmond, VA, United States
Despite notable progress in light emitting and transport based devices in GaN heterostructures, there is still controversy as to the light emission characteristics at high injection levels in LEDs and hot carrier scattering in FETs. For LEDs to be inserted into conventional lighting systems, reasonably high efficiencies would have to be maintained at high injection levels to meet intensity requirements which is not borne out by experiments. The efficiency degradation at high injection levels, beyond that which is expected due to heating, has been attributed to mainly Auger recombination and carrier spill over. In FETs, the conventional wisdom of increased carrier concentrations leading to better devices does not seem to hold beyond a certain point. This is due to strong electron LO phonon coupling in this highly ionic material which leads to hot phonon population. Hot phonon lifetime decreases with increasing carrier concentration up to a point owing to Plasmon phonon interaction. But beyond the frequency at which the plasma frequency and phonon frequency resonate, the phonon lifetime begins to increase again. Increased phonon lifetimes lead to reduced carrier velocity and thus performance degradation ensues. Another intriguing feature is that the aforementioned phenomenon is electric field dependent at least because increased field in FETs means widening of the channel and thus for the same volume density the resonance occurs at higher sheet densities.
The details of carrier recombination in the context of LEDs at high injection levels, and hot carrier scattering related physics in the context of HFETs based on the GaN material family will be elucidated.
Presentation: Keynote lecture at E-MRS Fall Meeting 2009, Symposium A, by Hadis Morkoc
See On-line Journal of E-MRS Fall Meeting 2009
Submitted: 2009-05-26 17:56 Revised: 2009-06-07 00:48
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