A modulation‐doped junction field‐effect transistor incorporating an optical waveguide under the gate modulates light by the carrier band‐filling effect (two‐dimensional Moss–Burstein effect) in a single quantum well, achieving a 5:1 extinction ratio in a 250‐&mgr;m‐long waveguide for 4 V reverse gate‐source biasVgsswing and 0 V drain‐source biasVds. Similar performance is obtained over a 16 nm spectral range. A novel band‐edge transparency effect is observed forVds>0 allowing an extinction ratio of 10:1, corresponding to a change in absorption of 92 cm−1to be obtained through band‐gap dilation by hot electrons at biases ofVds=8 V. Below‐band‐gap refractive index modulation of 1.6×10−3is obtained for aVgsswing of 2.4 V. The novel junction field‐effect transistor optical modulator also functions as a photovoltaic or photoconductive optical detector, a transistor, and a light‐emitting diode.