This paper presents a broadband THz power detector with on-chip antenna implemented in $0.13\mu \mathrm{m}$ SiGe HBT technology. The detector has a center frequency of 280 GHz, bandwidth of 64 GHz and responsivity of about 1 A/W. The nonlinear response of the power detector is analyzed and un-desirable linearization effects are discussed. High DC current gain and low-impedance base biasing are shown to improve the conversion gain, but the dominant gain-reducing effect is the feedback from the emitter resistance. The dynamic range and noise sources are also explained.