Current collapse and device degradation in AlGaN/GaN heterostructure field effect transistors

Balaz, Daniel (2011) Current collapse and device degradation in AlGaN/GaN heterostructure field effect transistors. PhD thesis, University of Glasgow.

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A spectrum of phenomena related to the reliability of AlGaN/GaN HEMTs are investigated
in this thesis using numerical simulations. The focus is on trap related phenomena that lead
to decrease in the power output and failure of devices, i.e. the current collapse and the
device degradation. The current collapse phenomenon has been largely suppressed using
SiN passivation, but there are gaps in the understanding of the process leading to this
effect. Device degradation, on the other side, is a pending problem of current devices and
an obstacle to wide penetration of the market.
Calibration of I-V measurements of two devices is performed with high accuracy to
provide a trustworthy starting point for modelling the phenomena of interest. Traditionally,
in simulations of nitride based HEMTs, only direct piezoelectric effect is taken into
account and the resulting interface charge is thence independent of the electric field. In this
work, the impact of the electric field via the converse piezoelectric effect is taken into
account and its impact on the bound charge and the drain current is studied, as a refinement
of the simulation methodology.
It is widely believed that the current collapse is caused by a virtual gate, i.e. electrons
leaked to the surface of the device. We have found a charge distribution that reproduced
the I-V measurement that shows current collapse, hence validating the concept of the
virtual gate. While it was previously shown that the virtual gate has a similar impact on the
I-V curve as is observed during the current collapse, we believe that this is for the first time
that a wide range of gate and drain voltages was calibrated.
High gate/drain voltage leading to permanent degradation was also investigated. The
hypothesis that stress induced defects and dislocations might be responsible for the
degradation was tested but not fully confirmed.
Finally, the leakage of electrons thought to be responsible for formation of the virtual gate
and the current collapse due to the Poole-Frenkel emission, is simulated in order to explain
the surface charge distribution responsible for the current collapse and deduced in
Chapter 5.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: AlGaN, GaN, HFET, HEMT, converse piezoelectric effect, stress, strain, current collaspe, device degradation, Poole-Frenkel
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Colleges/Schools: College of Science and Engineering > School of Engineering
Supervisor's Name: Asenov, Prof. Asen
Date of Award: 2011
Depositing User: Daniel Balaz
Unique ID: glathesis:2011-2676
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 07 Jun 2011
Last Modified: 10 Dec 2012 13:58

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