Noise performance of MMIC HBTs as photodetectors

The radio-frequency (RF) responsivity and noise performance of HBTh used as photodetectors in the microwave/millimeter-wave band are reported for the first time. The RF responsivity for a common base HBT (CBHBT) shows a 3-dB bandwidth (f-3dB) of more than 20 GHz. The CBHBT with floating emitter operate as a base-collector junction photodiode. The noise performance of a CBHBT-FET receiver depends on the noise characteristics of the FET amplifier that follows the CBHBT. A common emitter HBT (CEHBT) operate as a transimpedance amplifier with a base-collector junction photodiode. This device enables a lower noise floor than that of the PIN-FET receiver in a 50-¿ system. For a CEHBT used as a photodetector, equivalent input noise currents of 12 and 20 PA/¿Hz are obtained at 10 and 20 GHz, respectively, by noise floor measurements and an equivalent circuit model.