Simulation of Radiation Damage in GaN HEMT Structures

GaN based HEMT structures are of interest for a variety of high performance applications.  Of natural interest is their tolerance to radiation damage.  Devices damaged with protons exhibit several interesting features.  First, there is little change in the threshold voltage.  This implies that there can be little change in the charge under the gate stack.  Additional charges there would shift the threshold voltage.  Second, there is a change in the drain voltage for increased gate leakage.  The nominal theory for this is that the peak field increases the mechanical strain and induces new leakage paths.  For a given drain to gate voltage, the peak field can be decreased by charging along the top of the buffer layer adjacent to the gate.  To simultaneously account for both features, the uniform irradiation of protons must produce localized charges between the gate and drain, above the channel.

To further complicate the issue, significant degradation of mobility and transconductance are observed.  Normally, this would be accounted for by increased impurity scattering near the channel.  However, we do not observe an increase in charges in the gate stack.  Additionally, back side irradiation where the damage is contained below the channel, exhibit much the same behavior as front side irradiation.  Are the defects migrating?  Is it electronic charging?  

Simulation can help answer these types of questions.  We will describe our simulation approach and discuss results that can help separate and identify the mechanisms responsible for these results.