Neutrons pep up PHEMT performance
French researchers have discovered that neutrons can actually improve some aspects of PHEMT performance, such as knee voltages and leakage currents.
“We were very surprised by this result”, commented team member Bertrand Boudart from the University of Caen Lower Normandy. Previous studies by other researchers revealed that the electrical performance of PHEMT deteriorates at doses in excess of 1 x 1015 neutrons/cm2.
Understanding how PHEMT characteristics are influenced by neutron bombardment is important, because these devices are widely used in the military, nuclear and space industries.
MBE growth created the epitaxial structure (see figure), and Ni/AuGe/Ni/Au deposition formed the n- and p- contacts. The cap was then selectively etched away, before electron-beam lithography defined a Al/Ni/Au gate. A SiNx dielectric passivated the devices.
After irradiation of 1.2 x 1010 neutrons/cm2, the maximum drain-source current (IDS) increased from 121 to 128mA at a drain source voltage (VDS) of 2.5V and gate-source voltage (VGS) of 0.4V. The “knee voltage” (Vk) which is the value of VDS at the end of the saturation zone, decreased from 0.6 to 0.46V.
The researchers claim that these gains to PHEMT performance resulted from improvements in the ordering of structural impurities at the metal-semiconductor interface.
Further radiation doses of up to 3.3 x 1010 neutrons/cm2 had little effect on the peak drain-sourcecurrent, the knee voltage and the leakage current.
Coating the devices with a 1mm-thick sheet of lead ensured that gamma rays did not influence electrical performance of the devices.
Further details on this research are in the original publication which is available online:
‘Electronics Letters ‘, Volume 46, Issue 9, 2010, p650-652.
In a separate effort, this French team has investigated how gamma rays affect PHEMT characteristics; themaximum output power density was improved by a considerable 18 %.
GaN transistors have also been studied by the group. Boudart was tight-lipped, but he did describe the results as “interesting”.