The systematic design process using numerical simulations of the novel gallium nitride (GaN) enhancement-mode vertical superjunction high electron mobility transistor (HEMT) with breakdown voltage (BV) in the range of 5–20 kV is presented. The GaN superjunction pillar structure in the drift region of the vertical HEMT is first optimized using a simpler GaN superjunction diode structure, and the optimum half-pillar charge dosage is obtained to be , which is consistent with the value estimated from the Gauss's Law. The GaN vertical superjunction HEMT is then simulated and optimized, and the -BV tradeoff curves in the range of 5–20 kV are obtained by varying the epi thickness. The -BV tradeoff is found to improve with smaller pillar width as in silicon superjunction MOSFETs, and the best of 4.2 with BV of 12.4 kV is projected with half-pillar width of 3 . The robustness of the superjunction HEMT is also examined using structure with half-pillar width of 8 , and compared with the GaN vertical HEMT with conventional drift layer and same dimensions. The simulated on-state BV of the GaN vertical superjunction HEMT shows a 4.5% drop from the off-state BV and is only slightly higher than the 1.7% drop of the conventional GaN vertical HEMT.