Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level





Gregory J. Salamo

Committee Member

Donald K. Roper

Second Committee Member

Jin Hu

Third Committee Member

Huaxiang Fu

Fourth Committee Member

Rick L. Wise


AlGaAs/GaAs Heterostructures, Deep Level Noise Spectroscopy, High-frequency Applications, Hot-electron Transport, Negative Differential Resistance, Real-space Charge Transfer


A systematic investigation of negative differential resistance due to real-space and k-space electron transfer was conducted in two material systems based on selectively doped AlxGa1-xAs/GaAs heterostructures (x = 0.3) with uniform doping and delta doping. The material systems have potential for integration into low-power low-noise amplifiers and oscillators.

The conduction band diagram of each system was simulated with nextnano3 software to determine the main conducting channels and electron spatial distribution. The heterostructures were grown by molecular beam epitaxy and fabricated into devices using standard photolithography and device processing techniques. The devices were electrically characterized by temperature-dependent Hall effect and Hall sensitivity measurements, deep level noise spectroscopy, and pulsed dark current-voltage measurements with and without preliminary photoexcitation.

In the uniformly doped system, electron Hall mobilities of 4817 cm2/Vs (290 K) and 58339 cm2/Vs (80 K), and sheet electron densities of 5.73x1011 cm-2 and 1.55x1011 cm-2, respectively, were measured. Electron Hall mobilities of 4382 cm2/Vs (290 K) and 6981 cm2/Vs (80 K), and corresponding sheet electron densities of 9.25x1011 cm-2 and 7.25x1011 cm-2 were measured in the delta-doped system. Both systems displayed real-space transfer but no electron trapping in deep levels at electric fields less than 0.7 kV/cm. In the delta-doped system, the threshold electric field for real-space transfer exhibited a second-order polynomial dependence on temperature. Similar threshold electric field values for real-space transfer-induced negative differential resistance of 2.3 kV/cm for the uniformly doped system and 2.2 kV/cm for the delta-doped system were observed at 85 K. At electric fields up to 5.3 kV/cm, the delta-doped system displayed larger peak-to-valley ratio within a wide temperature range, negative differential resistance due to k-space transfer, and self-sustained oscillating current instabilities. The differences in the current output were attributed to different energy level distributions, dominant scattering mechanisms, and trap activity. Configuration coordinate models based on large lattice relaxation were proposed to explain the trap-carrier kinetics. Finally, electron capture by deep traps was observed as a low-field current collapse after repeated exposure to high electric fields, and electron emission by the traps was demonstrated by restoring the original current-voltage characteristics with thermal and light treatment.

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