A 6-20 GHz compact multi-bit digital attenuator using InP/InGaAs PIN Diodes

A broadband (6-20 GHz) 3-bit digital attenuator MMIC using InP/InGaAs PIN diodes has been proposed and fabricated. The proposed digital attenuator is composed of T-resistive networks for non-reflective step attenuation and fabricated by using a benzocyclobutene(BCB)-based multi-layer MMIC technology. The multi-layer technique and meandered thin film resistor lines significantly reduce the chip size of the 3-bit digital attenuator to 1.47times0.99 mm². By using the InP/InGaAs PIN diodes having a high cutoff frequency for digital switching, the proposed digital attenuator has shown the low insertion loss and excellent return loss characteristics. The cut-off frequency which is a figure of merit of the InP/InGaAs PIN diode is obtained to be 5.17 THz. The 3-bit digital attenuator has a 4 dB attenuation step and a 28 dB attenuation range. The minimum insertion loss is 4 dB and the input/output return loss is greater than 10 dB over all attenuation states and frequencies.