Professor Levi`s recent research program includes the study of limits to semiconductor device size, functionality and integrability. An example of his research into the limits of scaling semiconductor devices is the development of a new very small laser diode based on a microdisk resonator. Devices have been fabricated which exhibit single mode 1.5 µ m wavelength, room temperature operation with a sub-millamp threshold current. Lasers with submicron dimension lase with a mode spacing which is approximately one tenth of the energy of the semiconductor band gap. In this situation a significant fraction of spontaneous emission feeds directly into the lasing mode giving rise to an ill-defined lasing/nonlasing transition in the device power input/output characteristics. The results of this work contribute to our understanding of concepts such as "threshold lasing". Future work of interest will include the study of nonequilibrium carrier effects in microlaser devices.
The scaling of ultra-fast heterojunction bipolar transistors has been investigated both experimentally and theoretically. The results of this work indicate that nonequilibrium electron transport plays a critical role in determining both the steady-state and the transient behavior of such devices. The practical importance of this work is due to the fact that optimization of high performance device and integrated circuit design must take such effects into account.
Another area of study is the fundamental properties of semiconductors and their relation to the behavior of semiconductor devices. An example of this work are the results of an experimental study of net gain at laser threshold which show that large photo n fluctuations exist in the cavity modes of a Fabry-Perot laser diode. This fluctuation phenomena causes a large extra current to be extracted from the laser that is not present in the corresponding superluminescent LED. Significantly, these fluctuations act as a feedback mechanism which causes laser threshold current to increase with increasing temperature and, as such, this knowledge is of practical importance for laser diode applications.
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