The deep nature of the Mg acceptor will have important implications for the performance of high-speed GaN-based bipolar devices. In this work, the effect of the deep acceptor on the band bending within the depletion region is examined in detail. The width of the transition region, which separates the mobile holes from the space-charge edge, is carefully investigated. High-frequency modulation of the depletion region is discussed for both the large- and small-signal cases. For the small-signal case, calculated results are compared to experimental measurements of frequency-dependent capacitance which have been performed on Mg-doped GaN samples.