  Extended Floating Gate Field Effect Transistors (EGFETs) are CMOS-compatible
floating gate devices capable of detecting charges on their sensing area by the
relative shifts in current-voltage (I-V) characteristics. The I-V shifts are
generally computed by measuring the EGFET parameters in the strong inversion
region of operation. This could lead to errors in estimating the device
sensitivity because the simple I-V model ignores the mobility degradation and
series resistance effects in EGFETs. Our goal is to model these parasitic
effects and present methods to extract the key device parameters. We derive an
analytical I-V model for EGFETs in the linear region of transistor operation,
accounting for both the mobility degradation and series resistance effects.
Based on the analytical model, three methods are presented to estimate the key
parameters, namely the threshold voltage, series resistance, surface roughness
parameter, low-field mobility, and effective mobility from the I-V
characteristics, gate transconductance, and drain conductance. The peak
transconductance method is used as a benchmark for comparing the extracted
threshold voltages. Silicon-based EGFET devices are fabricated, and their I-V
characteristics are measured with deionized water and three polyelectrolytes.
From the I-V data, the parameter extraction methods are used to compute the
values of the key parameters, and the suitability of each method is discussed.
The gate transconductance methods show good agreement between the values for
the key parameters, while the drain transconductance method gives lower values
of the key parameters. There is scope to improve the presented methods by
incorporating the effects of substrate bias and asymmetric series resistance
for new extended-gate device architectures, including solution-based organic
field-effect transistors.
