Extremely high saturation current-bandwidth product performance of a near-ballistic uni-traveling-carrier photodiode with a flip-chip bonding structure

In this study, we demonstrate near-ballistic uni-traveling carrier photodiodes (NBUTC-PDs) with an optimized flip-chip bonding structure, wide 3-dB optical-to-electrical (O-E) bandwidth (> 110 GHz), and extremely high saturation current-bandwidth product performance (37 mA, >110 GHz, >4070 mAċ GHz). NBUTC-PDs with different active areas (28-144 μ²) are fabricated and flip-chip bonded with coplanar waveguides onto an AlN-based pedestal. The overshoot drift velocity of the electrons in the collector layer of the NBUTC-PD means that both the thicknesses of the collector layer and active areas of our device can be increased to reduce the density of the output photocurrent, compared to that of the traditional UTC-PD. This improves the high power performance without seriously sacrificing the speed performance. According to the measured O-E frequency responses, devices with even a large active area (144 μ²) can still have a flat O-E frequency response, from near dc to 110 GHz. A three-port equivalent circuit model for accurately extracting the 3-dB bandwidth of the devices is established. The extracted 3-dB O-E bandwidth of a device with a small active area (28 μm²) can be as high as 280 GHz under a load of 25 Ω. In addition, the saturation current measurement results indicate that after inserting a center bonding pad on the pedestal (located below the p-metal of the NBUTC-PD for good heat sinking), the saturation current performance of the device becomes much higher than that of the control device (without the center bonding pad), especially for the device with a small active area (28 μm²; ). The measurement and modeling results indicate that a device with a 144 μm² active area and optimized flip-chip bonding pedestal can achieve an extremely high saturation current-bandwidth product (6660 mA-GHz, 37 mA, 180 GHz).