Heterojunction bipolar transistors fabricated from the III-V nitrides offer improved linearity for high-power and high-frequency amplifiers. We report the results of two-dimensional simulations of the characteristics of AlGaN/GaN heterojunction bipolar transistors (HBTs) using the MEDICI(R) simulator. The simulated performance of single-heterojunction devices predict that it is possible to realize an amplification coefficient beta = 1130, an emitter-base turn-on voltage V-BCO = 3 V, a corrector saturation current I-Csat = 3.5 kA/cm(2), and a collector breakdown voltage BVCEO = 55 V. The simulated frequency response of these devices indicate a maximum cutoff frequency f(T) = Is GHz.