Paper BI-MoA4
High Sensitivity Recessed AlGaN/GaN HFET Protein Sensors

Monday, October 31, 2011, 3:00 pm, Room 108

We have demonstrated the improvement of sensitivity by biasing the AlGaN/GaN HFET biosensors into the subthreshold regime. To bias the channel into the subthreshold regime, V_g=~-4 V was. The gate voltage causes ions in the physiological buffer solution to move and result in measurement noise. To avoid side effects from a high gate voltage, we adopted the gate recess process to shift the subthreshold gate voltage to zero/near zero volt and retain high sensitivity.

The AlGaN/GaN heterostructure used in this study has a 23 nm thick undoped AlGaN barrier. The recession of AlGaN barrier was conducted with an Oxford Plasmalab 100 system. A two-step recession process was used. The first step uses BCl₃ to etch the AlGaN barrier and the second step uses a combination of Cl₂/N₂/O₂ to passivate the etched surface. To achieve zero/near-zero subthreshold gate voltage, we recessed devices with the first step duration to be 50, 55, 60, 65, and 70 sec respectively. We also fabricated diodes with the Schottky area recessed with the same conditions to check the material property changes. The measured C-V characteristics show that the threshold voltage shifts along the positive direction with the increase of etching time. With 70 sec etching time, the threshold voltage of Ni diodes achieve subthreshold regime at Va=0 V. Extracted from C-V curves, the etched depths are 12.0, 11.6, 11.2, 9.7, and 8.8 nm respectively. AFM measurements of both recessed and original AlGaN surfaces show that the surface smoothness is improved after recession processes.

The fabricated AlGaN/GaN HFET biosensors have a recessed gate of 100 um (L) × 2 mm (W). To detect streptavidin, the AlGaN surface was modified by silanization and biotinylation. The measured Id-Vg curves of AlGaN/GaN HFETs with (a) recession time of 65 sec, and without any recession. There are two major differences: 1) the threshold voltage of recessed device shifted to around 0 V; and 2) the off-state current of recessed device is 1-2 orders lower. The threshold voltage promises high sensitivity when the gate is floating or biased at a very low voltage. The decrease of the off-state current indicates that signal-to-noise ratio is improved. The difference between the threshold voltage of diodes and HFETs mainly because of 1) the diode Schottky metal is Ni and the HFET gate is PBS and 2) the HFET is treated with oxygen plasma for surface modifications. The detection of 25 pg/ml (473 fM) SA solution in 1X PBS shows that the drain current is decreased by 22.7% because SA carry negative charges in PBS solution. The sensitivity is increased by one order compared to our previously published results with non-recessed devices.