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 beta= 15 is found at a collector current density of 2.5 kA/cm(2) and the gain is maintained at this value up to 4.1 kA/cm(2). 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 x 10(16) cm(-3). The current gain varied from 22 to 6 when the base doping was increased from 5 x 10(17) to 2 x 10(18) cm(-3). At the same time, the maximum oscillation frequency f(MAX) increased from 3 to 6 GHz. A severe degradation of the current gain, f(T), and f(MAX) was observed for HBT designs with a base thickness wider than 1000 Angstrom. By optimizing the transistor design and bias, an f(T) of 44 GHz and an f(MAX) 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.