A. Sasikumar

Spatially-resolved spectroscopic measurements of Ec − 0.57 eV traps in AlGaN/GaN high electron mobility transistors

Simultaneous temperature-dependent measurements of resistance transients (RTs) and spatially resolved surface potential transients were made after bias switching on AlGaN/GaN high electron mobility transistors (HEMTs). We find an Ec − 0.57 eV trap, previously correlated with HEMT degradation, located in the GaN buffer and not in the AlGaN barrier or at the AlGaN surface. The amplitude of the Ec − 0.57 eV trap in RTs depends strongly on the Fe-concentration in the GaN buffer. Filling of this trap occurs only under bias conditions where electric fields penetrate into the GaN buffer.

Direct comparison of traps in InAlN/GaN and AlGaN/GaN high electron mobility transistors using constant drain current deep level transient spectroscopy

Traps in InAlN/GaN and AlGaN/GaN high electron mobility transistors (HEMTs) are identified and compared using constant drain-current deep level transient spectroscopy (CID-DLTS). For both structures with different barrier materials, the same drain-access electron trap at EC−0.57 eV dominates the drain-controlled CID-DLTS trap spectrum. This suggests that the physical source of this trap, previously associated with drain-lag, is not present in the barrier but instead is likely to reside in the GaN-buffer. Gate-controlled CID-DLS measurements, which are preferentially sensitive to the barrier under the gate, reveal different trap spectra for the two HEMTs, showing that choice of barrier materials can influence under-gate trap signatures.

Access-Region Defect Spectroscopy of DC-Stressed N-Polar GaN MIS-HEMTs

Distinct trap levels in the drain access regions of an N-polar GaN MIS-HEMT are investigated before and after semi-on dc stressing by thermal and optical trap spectroscopies. The most prominent dc stress effect was an increase in concentration of a pre-existing electron trap with an activation energy of 0.54 eV, accompanied by a decrease in concentration of an electron trap with a 0.65-eV activation energy. These distinct states had similar concentrations before stressing, with the 0.54-eV trap concentration dominating (by 6×) after stress. Deeper states revealed via optical measurements showed a mild ~20% increase in total concentration after stressing.

Spatially-discriminating trap characterization methods for HEMTs and their application to RF-stressed AlGaN/GaN HEMTs

New constant drain-current deep level optical/transient spectroscopy (CID-DLTS/DLOS) methods to quantify trap energies and concentrations in AlGaN/GaN high electron mobility transistors (HEMTs) are described. These methods are applied to RF stressed HEMTs to characterize the impact of stressing on traps and identified a significant increase in virtual gate related levels.

Direct correlation between specific trap formation and electric stress-induced degradation in MBE-grown AlGaN/GaN HEMTs

DC stressing of molecular beam epitaxy (MBE)-grown high electron mobility transistors (HEMTs) is found to degrade device performance primarily due to increased defect/trap formation. Using constant drain-current deep level optical/transient spectroscopy (CID-DLOS/DLTS) methods, a specific virtual-gate related electron trap with energy EC-0.45 eV was observed to increase in concentration following DC stressing. The enhanced formation of this defect correlates quantitatively with a stress-induced knee-walkout (performance-degradation) of the HEMT that manifests itself in the pulsed I-V measurements.

Correlation of proton irradiation induced threshold voltage shifts to deep level traps in AlGaN/GaN heterostructures

The impact of proton irradiation on the threshold voltage (VT) of AlGaN/GaN heterostructures is systematically investigated to enhance the understanding of a primary component of the degradation of irradiated high electron mobility transistors. The value of VT was found to increase monotonically as a function of 1.8 MeV proton fluence in a sub-linear manner reaching 0.63 V at a fluence of 1 × 1014 cm−2. Silvaco Atlas simulations of VT shifts caused by GaN buffer traps using experimentally measured introduction rates, and energy levels closely match the experimental results. Different buffer designs lead to different VT dependences on proton irradiation, confirming that deep, acceptor-like defects in the GaN buffer are primarily responsible for the observed VT shifts. The proton irradiation induced VT shifts are found to depend on the barrier thickness in a linear fashion; thus, scaling the barrier thickness could be an effective way to reduce such degradation.

Defects in GaN based transistors

Electrically active defects in AlGaN/GaN high electron mobility transistors (HEMTs) are the source of intense study due to their linkage to the mechanisms for GaN HEMT degradation upon a variety of stress conditions. The ability to directly characterize traps and identify their sources in GaN HEMTs is challenging, however, and this is due to a combination of the large bandgap of these materials and the complex electrostatics present in these device structures. Furthermore, the targeted extreme operating conditions intended for GaN HEMTs, whether designed for RF or power applications, greatly exacerbate the ability to identify those defects that are most relevant to device degradation under actual operation. Over the past few years, however, we have developed several electronic defect characterization methods based on deep level optical spectroscopy (DLOS) and thermally-based deep level transient spectroscopy (DLTS), which have been adapted from basic studies of defects in GaN and AlGaN to become applicable to working HEMTs. These so-called constant drain current (CID) DLOS/DLTS methods are able to directly provide individual trap concentrations and energy levels for traps that may exist throughout the AlGaN/GaN HEMT bandgap, and can discern between traps under the gate or in the transistor access regions. This talk will first review the CID-DLOS/DLTS methodology and then will focus on the application of these methods to stressed and un-stressed AlGaN/GaN HEMTs. Direct correlations of the formation of several specific traps are made with a variety of HEMT degradation mechanisms induced by stresses that include RF, DC and also proton irradiation

Toward a physical understanding of the reliability-limiting E C -0.57 eV trap in GaN HEMTs

Recent reliability testing campaigns on AlGaN/GaN HEMTs have consistently revealed a critical EC-0.57 eV trap that is responsible for drain-lag, RF output power degradation, and current-collapse among other non-idealities. In this work, a comprehensive set of data obtained from a range of measurements and specialized test structures are combined and presented to reveal compelling evidence that this nearly ubiquitous degradation-causing defect in GaN HEMTs is physically located in the GaN buffer.