The DC and high frequency performance of AlGaN/GaN Heterojunction Bipolar Transistors (HBTs) is analyzed using an enhanced drift diffusion model and GaN/AlGaN material parameters, which were previously verified by modeling experimental device characteristics. The emitter-base diode turn-on voltage is as high as 2.7 V while the collector and base ideality factors are 1.16 and 1.46 respectively. A DC current gain of β = 15 is found at a collector current density of 2.5 kA/cm2 and the gain is maintained at this value up to 4.1 kA/cm2. The devices show a small offset voltage of 0.5 V. A forward breakdown voltage BVCEO of 70 V is found for designs with collector doping of 5 × 1016 cm-3. The current gain varied from 22 to 6 when the base doping was increased from 5 × 1017 to 2 × 1018 cm-3. At the same time, the maximum oscillation frequency fMAX increased from 3 to 6 GHz. A severe degradation of the current gain, fT, and fMAX was observed for HBT designs with a base thickness wider than 1000 angstrom. By optimizing the transistor design and bias, an fT of 44 GHz and an fMAX of 24 GHz are predicted. Results of a large-signal analysis are also presented to evaluate the power capability and efficiency of AlGaN/GaN HBTs.