Optimization of gate geometry towards high-sensitivity AlGaN/GaN pH sensor

Open-gated AlGaN/GaN high-electron-mobility transistor (HEMT) based sensor can inherently deliver a high current sensitivity (S-I) in response to the pH change. However, it remains a challenge to further improve the performance of the packaged AlGaN/GaN-based sensor, due to a lack of investigation on the device design optimization. In this paper, the influence of the gate geometry on the device sensitivity is investigated through theoretical analysis and experiments. It has been found that the key factor limiting the current sensitivity is the series resistance (R-S) of the packaged sensor. There are two cases: (1) when the aspect ratio of the gate structure (W/L) is small, the channel resistance dominates the total resistance and the current sensitivity increases with W/L; (2) when W/L is large, the R-S dominates the total resistance, the sensitivity decreases with W/L. Therefore, there is an optimal W/L which can be approximately reached when W/L = rho(2DEG)/R-S. Based on the guidelines, the current sensitivity of the AlGaN/GaN sensor with an optimized geometry in our experiment can reach 157 mu A/pH, which is the highest value among the packaged AlGaN/GaN-based pH sensors in literature, to our best knowledge. The comparison with the Si-based ISFET and the impact of the gate membrane on the sensitivity of AlGaN/GaN-based sensor have also been analyzed and discussed.