High temperature operation test stability of Ka-band transistors with different Al content in AlGaN layer
SEM photo of a typical Ka-band transistor used for HTO test
Fig. 1: Drain current measured on devices with 28% (red) and 30% (blue) of Al in AlGaN layer during HTO test
Fig. 2: Schottky barrier height measured before and after HTO test
Fig. 3: Threshold voltage measured before and after HTO test
In order to optimize the AlGaN/GaN epitaxial structure for short channel transistors (0.15 µm gate length), epitaxial stacks with varying Al content of the AlGaN layer were investigated by utilizing FBH's Ka-band GaN process line. For evaluation of principal device stability the high temperature operational test procedure (HTO) has been selected. HTO is one of the screening tests applied qualifying the devices for statistical lifetime evaluation. It estimates device stability during operation in a well-defined bias point.
Test conditions were as follows: chuck temperature 150°C, drain bias Vds = 28 V and Ids adjusted to obtain 2.5 W/mm DC dissipated power (Ids ~ 10% Idsmax); test duration 16 hours. Transistors with 150 nm gate length and AlGaN barriers containing 28% and 30% of Al were used for the test. Fig. 1 shows the evolution of drain current during testing. For devices with 28% of Al, drain current increases during the first 2 hours and then saturates till the end of the test at 20% - 30% above the initial value. At the same time, devices with 30% of Al show an increase of drain current by 70% - 80%. In order to find the root cause of this behavior, measurements of Schottky barrier height (SBH) and threshold voltage were performed before and after testing (Figs. 2 and 3). A clear correlation between change in drain current and SBH before and after the test can be noticed. The epitaxial structures with lower Al content show more stable SBH behavior (no noticeable change), while structures with 30% Al content in the barrier demonstrate a SBH decrease of about 0.1 eV. The reduced SBH of the latter devices manifests itself in a negative shift of threshold voltage which, in turn, results in a drain current increase at a given gate voltage.
In conclusion, HTO tests suggest that epitaxial structures with more than 28% Al in AlGaN layer may lead to device instabilities during the operation.