Electrical performance and characterization of deep levels in vertical GaN P-i-N diodes grown on low threading dislocation density (~10<
sup>
4<
/sup>
?10<
sup>
6<
/sup>
cm<
sup>
?2<
/sup>
) bulk GaN substrates are investigated. The lightly doped n drift region of these devices is observed to be highly compensated by several prominent deep levels detected using deep level optical spectroscopy at E<
sub>
c<
/sub>
-2.13, 2.92, and 3.2 eV. A combination of steady-state photocapacitance and lighted capacitance-voltage profiling indicates the concentrations of these deep levels to be N<
sub>
t<
/sub>
= 3 � 10<
sup>
12<
/sup>
, 2 � 10<
sup>
15<
/sup>
, and 5 � 10<
sup>
14<
/sup>
cm<
sup>
?3<
/sup>
, respectively. The E<
sub>
c<
/sub>
-2.92 eV level is observed to be the primary compensating defect in as-grown n-type metal-organic chemical vapor deposition GaN, indicating this level acts as a limiting factor for achieving controllably low doping. The device blocking voltage should increase if compensating defects reduce the free carrier concentration of the n drift region. Understanding the incorporation of as-grown and native defects in thick n-GaN is essential for enabling large V<
sub>
BD<
/sub>
in the next-generation wide-bandgap power semiconductor devices. Furthermore, controlling the as-grown defects induced by epitaxial growth conditions is critical to achieve blocking voltage capability above 5 kV.