Effect of Mechanical Stress on AlGaN/GaN High Electron Mobility Transistor (HEMT) Device Characteristics and Reliability


AlGaN/GaN HEMTs are regarded as promising candidate for RF and high power electronics applications due to unique material properties of GaN, such as, wide band gap, high breakdown field, high carrier mobility, and large saturation velocity. Other advantageous characteristics, such as, piezoelectricity and spontaneous polarization within AlGaN and GaN layers result in high 2D electron gas densities. However the wide deployment of the AlGaN/GaN HEMT technology is currently hindered due to its limited electrical reliability. Achieving high-level of reliability concurrently with high power operation remains an important challenge for this technology. Improvements in the reliability of these devices require a thorough understanding of the failure mechanisms that degrade the device performance.

Studies show that AlGaN/GaN HEMTs degrade significantly under typical device operation. Degradation in these devices has been hypothesized to occur due to charge trapping, hot electron effects, and crystallographic defect formation due to inverse-piezoelectric effect. GaN HEMTs have high internal stresses resulting from lattice mismatch between GaN and AlGaN layers and generated during device operation due to inverse piezoelectric effect. Mechanical stress impacts the device performance by affecting the carrier mobility, polarization, band-gap, trap energy levels and trap generation and hence influences the reliability of these devices. The goal of this project is to investigate the effect of stress, bias and temperature on device characteristics and understand the fundamental physics governing the device operation; and hence the failure mechanisms that degrade the device performance. Four-point mechanical wafer bending is used to study the effect of stress on AlGaN/GaN HEMT channel resistance and gate current to provide insight on the role of stress in device reliability.

On the Flow Physics of Effectively Controlled Open Cavity Flows

Flow-induced cavity oscillation exists in many practical environments, such as sunroofs in automobiles, landing gear bays and weapon bays on aircraft.  The cavity pressure fluctuations can exceed 170 dB (ref 20 μPa) and can potentially cause fatigue failure of the cavity and its contents.  In addition, the cavity pressure fluctuations can increase the drag on the aircraft.  These pressure fluctuations contain tonal and broad band components which both need to be altered for many modern applications.  Attempts at controlling the flow, so as to reduce the surface pressure fluctuations have been challenges to researchers. This study is concentrated on understanding the effects of “open loop” and “closed loop” on the flow field so that a better understanding of the most efficient way to control the flow can be developed. 

Development of Advanced Zero-Net Mass-Flux Actuators for Active Flow Control Applications

Active flow control is the foundation of future innovative aerodynamic concepts for improved vehicle efficiency.  However, the active flow control community generally lacks efficient, powerful actuators and, more importantly, corresponding validated design methodologies capable of demonstrating potential benefits at realistic vehicle flight Mach numbers and Reynolds numbers.  This research brings together a multidisciplinary team with expertise in electromechanical transducers, lumped element modeling, CFD, and experimental fluid dynamics to model, design, fabricate, and test advanced ZNMF actuators that use different transduction schemes:  including piezoelectric and electrodynamic.  The outcome of this research is a validated constrained design optimization tool that enables a class of robust ZNMF actuators for deployment on a larger scale.

An Experimental Investigation of Wing Tip Vortex Attenuation

A detailed study of the effect of blowing from a rounded wing tip on the attenuation of wing tip vortices generated by a NACA0012 semi-span airfoil is being conducted in two wind tunnels at IMG.  The first, the UF Closed Circuit Wind Tunnel, will be used for measuring lift and drag of the airfoil and characterizing the wing tip flowfield using one or more flow measurement techniques such as particle image velocimetry, laser Doppler velocimetry and a fast-response Cobra probe.  The second facility, the UF Aeroacoustic Flow Facility (UFAFF), is an open jet test section wind tunnel inside an anechoic chamber that will be primarily used to characterize the acoustics of the airfoil with far-field microphones and phased acoustic arrays.  Lift and drag will also be measured in the UFAFF to determine the effect, if any, of the differing wind tunnel configurations on aerodynamic performance.

An Integrated Study of Separation Control: Flow Physics, Nonlinear Dynamics and Effective Control Strategies


  • Separated flow past an airfoil is characterized by multiple natural frequencies associated with various instabilities.

  • Traditional separation control forces at a single frequency and often neglects the influence of actuator dynamics (resonance).

Questions & Objectives

  • Is the effectiveness of a control strategy related to the natural flow instabilities and/or interactions?

Objective 1: Identify and analyze the various global dynamics in a separated flow configuration

  • Can we detect and leverage natural instabilities to understand, model, and control separated flow in a more efficient and systematic manner?

Objective 2: Investigate the effectiveness and efficiency of targeting the instabilities in open-loop experiments

Objective 3: Apply observations to design and implement effective closed-loop control strategies

Challenges & Approach

1.     Traditional experimental methods for detection of flow frequency content are limited

-  Localized dynamic sensors (location sensitive)

-  High-rate global measurements ($$$)

Approach: Use modal analysis methods amenable to experimental techniques to obtain low-order, global estimates

2.     Characteristics of 2D flow separation on airfoils is highly dependent on angle of attack and surface curvature

Approach: A canonical flat plate model retains the essential separation characteristics, eliminates curvature effects, and is amenable to both simulations and experiments








Experimental Objectives

1.     Estimate global flow dynamics (time-resolved velocity fields)

2.     Identify characteristic frequencies/instabilities

3.     Perform open-loop control

Target characteristic frequencies by modulating actuator resonance. The extent and height of the time-averaged separation bubble is reduced during control.

Forcing at or near the natural shear layer frequency "feeds" the characteristic roll-up of the shear layer, which increases mixing between the high-momentum freestream and the low-momentum separated flow. This ultimately reattaches the flow further upstream with little control effort, or cost, described by the coefficient of momentum Cμ.


  • DMD extracts modal structures of the characteristic shear layer and wake components and results reveal interaction between the wake and separation bubble dynamics

  • Most efficient OL control scheme targets multiple flow instabilities rather than a single instability

  • CL control reattaches the flow (time average) by reducing the controlled shear layer oscillations

Wingless Electromagnetic Air Vehicle (WEAV)

The WEAV is a vehicle which is propelled by a plasma (ionized gas) discharge, requires no moving parts and provides near-instantaneous response.  The vehicle operates in atmospheric pressure and is capable of 360° maneuverability.  The key to the success of the project is to reduce the ratio of power to vehicle weight while significantly increasing the thrust generation capabilities of the plasma discharge. 

An Experimental Study of Aircraft Landing Gear Noise

A series of aerodynamic and acoustic wind tunnel experiments are being performed on a 1/4-scale high fidelity replica of a Gulfstream G550 aircraft nose landing gear at the University of Florida Aeroacoustic Flow Facility (UFAFF). Experiments consist of the analysis of model surface pressure measurements and flowfield velocity via laser Doppler velocimetry (LDV), noise source localization maps via beamforming using a large-aperture microphone array, and the determination of far-field noise level spectra using a linear array of free-field microphones. The objective of this study is to determine the mechanisms of nose landing gear noise and to explore potential noise reduction schemes.

Development of Fully-Integrated Micromagnetic Actuator Technologies

The objective of the effort is to transition high-performance micromagnets into fully-integrated, batch-fabricated micromagnetic actuators for applications such as micro adaptive flow control.


Magnetically-based electromechanical actuation schemes are ubiquitous in macroscale systems such as audio speakers, relays, solenoids, and electrical motors.  However, implementation of these transduction schemes at the microscale is nearly nonexistent because of certain design and fabrication challenges—primarily the inability to integrate high-performance, permanent-magnet (magnetically-hard) films within more complex micromachined structures.  As a result, most microfabricated transducers rely on other transduction mechanisms (e.g. electrostatic, piezoelectric, thermoelastic).  However, these mechanisms limit the actuation force, stroke (displacement), power density, and efficiency necessary for certain applications. 

The proposed actuator uses an adaptable and scalable actuation scheme, in that the device structure can be tailored for a wide range of applications.  Examples include micro pumps/valves for microfluidic (gas or liquid) systems; high power density motors/actuators for microrobotics; or low-profile acoustic radiators for navigations/sensing.  The evolution of fully-integrated micromagnetic transducers will enable performance improvements for existing applications and opportunities to explore new transformative technologies. 


(1) to validate and characterize the integrability (chemical stability, temperature stability, mechanical reliability, magnetization methods etc.) of the permanent magnet materials in complex microfabrication process flows

(2) to model and optimize a multi-magnet out-of-plane microactuation scheme

(3) to fabricate and characterize a fully-integrated out-of-plane piston-type actuator that can be directly implemented as a micro aerodynamic control surface or adapted as a synthetic jet actuator 




Figure 1: CAD drawings of the proposed micro-electrodynamic synthetic jet actuator actuator: (a) Actuator assembly comprising two dies made from silicon substrate, separated by a separator. The top die is covered with a PDMS diaphragm and has a bonded powder magnet in the center. (b) Exploded view of the bottom wafer.