Electrical Characteristics of the Concentric-Shape Carbon Nanotube Network Device in pH Buffer Solution

A carbon nanotube network device having concentric-shaped electrodes (source and drain) is analyzed to examine the "self-gating" effect of various pH buffer solutions on its electrical properties. Using the 2-D homogeneous percolation theory, current-voltage characteristics of the devices are described as the classical MOSFET formula, in which the device is modeled as the p-type transistor with positive threshold voltage and the gate is tied with the drain (at positive bias) or source (at negative bias). To determine the apparent threshold voltage change due to the corresponding pH value, the ∂V_D/∂I_D (V_D:drain voltage; I_D:drain current) curve is extracted from the measured current-voltage characteristics to find V_D_SAT(= -V_TH, V_QS = 0). A threshold voltage shift according to the pH value is observed without an external gate electrode, showing the possibility of relaxing the requirement of the external gate electrode. By grafting protonation/deprotonation which occurs in the carboxylated single-walled carbon nanotubes, the decaying current as pH increases is explained. Better sensitivity according to the operation regime is examined by the device modeling.