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Quantitative spectroscopy of reliability limiting traps in operational gallium nitride based transistors using thermal and optical methods

Sasikumar, Anup
http://rave.ohiolink.edu/etdc/view?acc_num=osu1415298691

Abstract Details

2014, Doctor of Philosophy, Ohio State University, Electrical and Computer Engineering.
Gallium nitride (GaN) based high electron mobility transistors (HEMTs) have shown a lot of promise in high voltage, high power, and high radiation applications. However the full realization of the III-nitride potential and large scale adoption of this technology has been hindered by the existence of electrically active defects that manifest as deep levels in the energy bandgap. These deep levels can potentially act as charge trapping centers limiting device performance and long term reliability. It is therefore imperative to monitor these traps in operational GaN HEMTs as close as possible to their real world operational conditions. With that goal in mind, in this dissertation, a suite of advanced thermal and optical based trap spectroscopy methods and models collectively known as constant drain current deep level (thermal) transient spectroscopy and deep level optical spectroscopy (CID-DLTS/DLOS) were developed and expanded upon to directly probe and track traps in three terminal operational GaN HEMTs. These techniques have allowed an unprecedented ability to quantitatively track trap levels throughout the wide bandgap of operational GaN devices. Depending on their mode of switching (gate-controlled versus drain-controlled) the techniques are able to distinguish between under gate and access region defects irrespective of device design and/or operational history. The devices studied here were subjected to a range of different stressors and very different trap induced degradation mechanisms were identified that further confirms the need for such high resolution defect spectroscopic studies in GaN HEMTs. Specifically the GaN HEMTs studied here were subjected to three very different kinds of stressors, i) high frequency moderate drain voltage (<50 V) accelerated lifetime stressor were applied to GaN HEMTs optimized for radio frequency (RF) applications, ii) very high off-state drain voltage (up to 600 V) stressors were applied to GaN-on-Si MISHEMTs optimized for power switching applications, and iii) high energy particle irradiation (in this case 1.8 MeV protons) stressor applied to high frequency GaN HEMTs targeted for RF space applications. In the case of the RF accelerated electrical life testing, the GaN HEMTs over an array of different suppliers (mostly commercial) showed the signature of a EC–0.57 eV trap that was was identified as occurring almost ubiquitously. This trap was determined to be causing knee-walkout degradation, drain-lag and linked directly to RF output power loss through its trapping/detrapping activity in the drain access region. This level was unambiguously located in the GaN buffer using a combination of CID-DLTS, and supporting nano-scale DLTS/DLOS approaches. It was observed that the detection of this buffer trap was observed to be highly dependent on the reverse gate leakage of the GaN HEMTs and an empirical leakage based filling model was proposed to describe the electron capture process in HEMTs with leakage (>10-7 A/mm). In contrast, for GaN HEMTs with very low reverse gate leakage (<10-7 A/mm), a broad distribution of mid gap states between (EC–1.6 to 2.5 eV) was found to be directly responsible for the RF output power loss through a persistent increase in on-resistance. The same CID-DLTS/DLOS methods were adapted and expanded further to ensure applicability to high voltage GaN-on-Si power MISHEMTs up to very high drain voltages (up to 600V). On applying these advanced methods on commercial power switching GaN MISHEMTs, a deep trap at EC–2 eV was directly found responsible for very large current collapse effects after high voltage switching GaN-on-Si power HEMTs. This trap too was unambiguously located in the GaN buffer using a consensus of experimental results from constant capacitance DLOS and nanoscale DLOS on simple two terminal Schottky gate structures and past reports of a similar trap in carbon doped semi-insulating GaN. Lastly, proton irradiation effects on traps in GaN HEMTs targeted for space applications were studied and two major threshold instability mechanisms were identified. For proton fluences of up to 1014 cm-2 a large persistent VT shift (~0.59 V) and a smaller switching dependent VT dispersion (~100-200 mV) were attributed to traps. Using Atlas Silvaco based simulations, the large and persistent VT shift was linked to very deep traps at EC–1.3 and 3.28 eV both occurring in the GaN buffer. The VT dispersion increase was attributed to an EC–0.72 eV trap concentration increase in the GaN buffer under the gate. Critical comparison of electrical stressor and irradiation stressors revealed that trap formation/activation in the electrically stressed devices occurred mostly in the drain access regions where the fields were highest. However in the GaN HEMTs exposed to particle irradiation, trap formation/activation occurred uniformly in the III-nitride material system due to uniform displacement damage occurring under the gate terminal and in the access regions. However, it was concluded that for the same damage i.e. for the same concentration of defects created, the impact of increased traps under the gate was far more severe on the device performance and reliability than the additional traps formed in the access regions. Through such a deeper understanding of trap assisted degradation mechanisms, more effective predictive reliability models can be developed that will ultimately contribute to better growth/design strategies, performance, and reliability enhancement. Such improvements will eventually result in th large-scale adoption of GaN electronics in RF and power applications.
Steve Ringel, Prof. (Advisor)
Siddharth Rajan, Prof. (Committee Member)
Aaron Arehart, Prof. (Committee Member)
Patrick Roblin, Prof. (Committee Member)
215 p.
Electrical Engineering; Materials Science
Gallium Nitride, GaN, high electron mobility transistors, HEMTs, traps, defects, reliability, virtual gate, dynamic on-resistance, degradation, deep level, deep level transient spectroscopy, DLTS, deep level optical spectroscopy, DLOS, drain-lag, gate-lag

Recommended Citations

Citations

  • Sasikumar, A. (2014). Quantitative spectroscopy of reliability limiting traps in operational gallium nitride based transistors using thermal and optical methods [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1415298691

    APA Style (7th edition)

  • Sasikumar, Anup. Quantitative spectroscopy of reliability limiting traps in operational gallium nitride based transistors using thermal and optical methods. 2014. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1415298691.

    MLA Style (8th edition)

  • Sasikumar, Anup. "Quantitative spectroscopy of reliability limiting traps in operational gallium nitride based transistors using thermal and optical methods." Doctoral dissertation, Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1415298691

    Chicago Manual of Style (17th edition)

osu1415298691
1,412
© 2014, some rights reserved.Creative Commons License Quantitative spectroscopy of reliability limiting traps in operational gallium nitride based transistors using thermal and optical methods by Anup Sasikumar is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by The Ohio State University and OhioLINK.