Silicon Mach-Zehnder interferometric (MZI) waveguide modulator incorporating the n-channel junction field-effect transistor (JFET) as a signal modulation unit was designed, fabricated, and analyzed. The proposed MZI with JFET was designed to operate based on the plasma dispersion effect in the infrared wavelength of 1550nm. The three different modulation lengths (ML) of 500, 1000, and 2000µm while keeping the overall MZI length constant at 1.5cm were set as a general design rule for these 10µm-wide MZIs under study. When the JFET was operated in an active mode by injecting approximately 50mA current (I_s) to achieve a π phase shift, the modulation efficiency of the device was measured to be η =π/(I_s·L) $\\simeq$ 40π/A-mm. The temporal and frequency response measurements also demonstrate that the respectively rise and fall times measured using a high-speed photoreceiver were in the neighborhood of 8.5 and 7.5µsec and the 3dB roll-off frequency (f_3dB) measured was in the excess of ∼400kHz.