Surface Passivation Studies of AlGaN/GaN High Electron Mobility Transistors
Open Access
- Author:
- Meyer, David J.
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 26, 2008
- Committee Members:
- Professor Joseph R Flemish, Committee Chair/Co-Chair
Professor Joan M Redwing, Committee Chair/Co-Chair
Thomas Nelson Jackson, Committee Member
Suzanne E Mohney, Committee Member - Keywords:
- passivation
plasma
silicon nitride
SiNx
pulsed I-V
current recovery
XPS
RF - Abstract:
- GaN based transistors have recently emerged as contenders for replacing existing Si and GaAs RF power devices. Wide bandgap group III-N materials exhibit the benefits of high electric field breakdown strength and high saturated carrier velocity, which allow for high power and high frequency device operation. While the theoretical advantages of the AlGaN/GaN high electron mobility transistor (HEMT) are beginning to be realized, technological development is still inhibited by critical problems such as RF dispersion and off-state leakage current. Electrical (pulsed and dc I-V, Hall Effect, and small signal RF) and materials (XPS, AES, FTIR, PL, and thin film stress measurements) characterization techniques were used to attain a comprehensive viewpoint of how the HEMT surface treatment and PECVD SiN<sub>x</sub> passivation procedure affects device characteristics. Based on experimental and modeling results, the potential mechanisms responsible for reducing device virtual gating and increasing isolation current were discussed. In general, the pulsed I-V performance of HEMTs can be improved by using one of several plasma treatments, such as C<sub>2</sub>F<sub>6</sub>, Cl<sub>2</sub>, NH<sub>3</sub>, or O<sub>2</sub>, immediately prior to passivation. Isolation current degradation was found to be relatively independent of pre-passivation surface treatment, but instead showed five orders of magnitude variation when different SiN<sub>x</sub> passivation film types were used. Organic surface contamination that is present in as-processed, unpassivated devices was found to impede the mechanism by which SiN<sub>x</sub> deposition reduces virtual gating. XPS results show that surface treatments that reduce carbon concentration also lead to improved pulsed I-V performance after passivation. The passivation mechanism that reduces virtual gating is suspected to be related to chemical modification of the HEMT surface that reduces populations of electron trapping centers, or changes their characteristics. Several arguments support the hypothesis that oxygen could be directly involved in the passivation mechanism. Two-dimensional simulation of the isolation test structure suggested that the increase in isolation current could be explained by the incorporation of donors or donor type defects at the SiN<sub>x</sub>/HEMT interface. Fixed charge in the SiN<sub>x</sub> or strain-induced sheet charge at the interface was also shown by modeling to be capable of enhancing the conductivity of an electron inversion layer near the SiN<sub>x</sub>/HEMT interface in the semiconductor.