Electrical Characteristics and pH Response of a Parylene-H Sensing Membrane in a Si-Nanonet Ion-Sensitive Field-Effect Transistor

Abstract

We report the electrical characteristics and pH responses of a Si-nanonet ion-sensitive field-effect transistor with ultra-thin parylene-H as a gate sensing membrane. The fabricated device shows excellent DC characteristics: a low subthreshold swing of 85 mV/dec, a high current on/off ratio of ~10⁷ and a low gate leakage current of ~10^-10 A. The low interface trap density of 1.04 × 10¹² cm^-2 and high field-effect mobility of 510 cm²V^-1s^-1 were obtained. The pH responses of the devices were evaluated in various pH buffer solutions. A high pH sensitivity of 48.1 ± 0.5 mV/pH with a device-to-device variation of ~6.1% was achieved. From the low-frequency noise characterization, the signal-to-noise ratio was extracted as high as ~3400 A/A with the lowest noise equivalent pH value of ~0.002 pH. These excellent intrinsic electrical and pH sensing performances suggest that parylene-H can be promising as a sensing membrane in an ISFET-based biosensor platform.

Keywords: ion-sensitive field-effect transistor (ISFET); pH response; parylene-H.

Figure 1

(a) Schematics and top-view SEM images of the fabricated Si-nanonet ISFET with parylene-H as the gate insulator. (b) Illustration of the parylene-H deposition process. (c) AFM images of the parylene-H nanolayer with a 7-nm thickness.

Figure 2

Electrical characteristics of the fabricated parylene-H (p-H) ISFET in 0.01× PBS buffer solution. (a) Typical I_D-V_D output characteristics in the linear scale. (b) Typical I_D-V_G transfer characteristics and I_G-V_G gate leakage current characteristics at V_D = 0.1 V in log scale indicating a typical n-type behavior. (c) I_G-V_G characteristics in log and linear scale. (d) Normalized C-V characteristics of the p-H ISFET.

Figure 3

pH-dependent filed-effect characteristics. (a) I_D-V_G transfer characteristics at V_D = 0.1 V in pH 4, 7, 10 buffer solutions for a p-H ISFET. (b) V_TH shift indicating a linear pH response: the sensitivity of 48.1 ± 0.5 mV/pH and device-to-device variation of ~6.1% for p-H ISFETs; the sensitivity of 34 ± 1.9 mV/pH for Ox ISFETs.

Figure 4

(a) Low frequency drain current noise characteristics of the p-H ISFET at various gate biases at V_D = 0.1 V. (b) SNR and noise equivalent pH vs. gate bias.