Metal piezoelectric semiconductor field effect transistors for piezoelectric strain sensors

In this letter, we examine the potential of a functional device that can have good transistor and stress sensor properties. The device examined is based on the use of a thin oxide with high piezoelectric coefficients under the gate region. Channel charge and current are controlled by gate voltage or by stress. We examine the performance of two classes of heterostructures that are important semiconductor technologies: (i) Si∕SiO2∕BaTiO3Si∕SiO2∕BaTiO3 heterostructure junctions that would be an important breakthrough for silicon sensor technology and (ii) GaN∕AlN∕BaTiO3GaN∕AlN∕BaTiO3 heterostructure field effect transistors. The calculations show that with a very thin piezoelectric layer we can have a highly sensitive stress sensor and transistor. For optimum performance, the piezoelectric layer thickness should be ∼30–60 ∼30–60 Å.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71320/2/APPLAB-85-7-1223-1.pd

Gate leakage suppression and contact engineering in nitride heterostructures

We present a self-consistent approach to examine current flow in a general metal–polar heterostructure junction. The approach is applied to examine properties of three classes of junctions that are important in devices: (i) GaN/AlGaN structures that are used in nitride heterojunction field effect transistors; (ii) GaN/AlGaN/high-κ insulator structures for potential application in very small gate devices to suppress gate tunneling current; and (iii) GaN/AlGaN/polar insulator junctions with practical application for low source resistance regions. The physical parameters used for high-κ dielectrics and polarization charges reflect values typically found in ferroelectric materials. Our studies indicate that tailoring of junction properties is possible if a dielectric thicknesses of ∼20 Å∼20 Å can be achieved. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69840/2/JAPIAU-94-9-5826-1.pd

Study of carrier dynamics and radiative efficiency in InGaN/GaN LEDs with Monte Carlo method

In this paper, we have applied the Monte Carlo method to study carrier dynamics in InGaN quantum well. Vertical and lateral transport and its impact on device radiative efficiency is studied for different In compositions, dislocation densities, temperatures, and carrier densities. Our results show that the non‐radiative recombination caused by the defect trapping plays a dominating role for higher indium composition and this limits the internal quantum efficiency (IQE). For lower indium composition cases, carrier leakage plays some role in the mid to high injection conditions and carrier leakage is strong in very high carrier density in all cases. Our results suggest that reducing the trap density and QCSE are still the key factors to improve the IQE. The paper examines the relative roles of leakage and non‐radiative processes on IQE. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87087/1/2393_ftp.pd

Localization landscape theory of disorder in semiconductors. III. Application to carrier transport and recombination in light emitting diodes

This paper introduces a novel method to account for quantum disorder effects into the classical drift-diffusion model of semiconductor transport through the localization landscape theory. Quantum confinement and quantum tunneling in the disordered system change dramatically the energy barriers acting on the perpendicular transport of heterostructures. In addition they lead to percolative transport through paths of minimal energy in the 2D landscape of disordered energies of multiple 2D quantum wells. This model solves the carrier dynamics with quantum effects self-consistently and provides a computationally much faster solver when compared with the Schr\"odinger equation resolution. The theory also provides a good approximation to the density of states for the disordered system over the full range of energies required to account for transport at room-temperature. The current-voltage characteristics modeled by 3-D simulation of a full nitride-based light-emitting diode (LED) structure with compositional material fluctuations closely match the experimental behavior of high quality blue LEDs. The model allows also a fine analysis of the quantum effects involved in carrier transport through such complex heterostructures. Finally, details of carrier population and recombination in the different quantum wells are given.Comment: 14 pages, 16 figures, 6 table

The 2020 UV emitter roadmap

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments

Transport issues and multi-functional devices based on nitrides and other polar heterostructures.

This thesis addresses the theoretical and computational methods to examine the transport and multi-functional devices based on nitrides and other polar heterostructures like BaTiO3 and LiNbO3. Nitrides have a large polarization charge in the growth [0001¯] direction and are also wide bandgap materials, which make them important materials for high power electronics. Ferroelectrics, such as BaTiO3, not only have very large polarization like nitrides, but also exhibit greater piezoelectricity and pyroelectricity, which make them sensitive to environmental changes, such as stress and temperature. Therefore, they are very good candidates for designing sensor applications. We present a self-consistent approach to examine tunneling current and current flow in a general ferroelectric heterostructure junction. We examined the tunneling current of junctions with high kappa/high polar materials. These junctions can be designed to produce highly tailorable I-V characteristics. We have also developed self-consistent Monte Carlo and 2D Poisson drift-diffusion simulation techniques to simulate the behavior of nitride based transistors. We have applied our tools in studying the transconductance behavior and scaling issues in nitride FETs, We have examined transconductance collapse behavior in nitride devices and determined the possible sources of this collapse from a theoretical and computational standpoint. We also examined scaling issues for nitride HEMTs, where device operation frequency does not scale with gate length, We found that large effective gate length (in comparison to lithographic gate length) is the main factor limiting the operation frequency. GaN based devices are useful for high power applications where considerable heat is dissipated. We find that heating time constants of nitride FETs are around a few nano seconds so that short pulse measurements can not completely remove self-heating effects. We also discuss possible solutions to these problems and examine the practicality of each solution. We also address the potential of devices based on heterostructures made from highly polar ferroelectric materials and semiconductors. We have examined the feasibility of making a sensor-FET. In our calculation, we have identified optimum performance under different parameters. Our results predict that the device sensitivity could be one order of magnitude higher than the traditional sensors.Ph.D.Applied SciencesElectrical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126327/2/3238117.pd