  We present the electrical characterization of N-polar AlN layers grown by
metal-organic vapor phase epitaxy and the demonstration of N-polar AlN-channel
metal-semiconductor field-effect transistors (MESFETs). A high concentration of
silicon is unintentionally incorporated during the high-temperature growth of
N-polar AlN, causing high buffer leak current. The silicon concentration
reduces from 2x10^18 /cm^3 to 9x10^15 /cm^3 with decreasing growth temperature,
reducing the buffer leak current to 5.6 nA/mm at a 100 V bias. The N-polar AlN
MESFET exhibits an off-state drain current of 0.27 nA/mm and a transistor
on/off ratio of 4.6x10^4 due to the high-resistivity AlN buffer layers.
