A four-color quantum well infrared photodetector (QWIP) has been demonstrated in this work. Four stacks of quantum well structures with four different detection wavelengths are sandwiched among three highly doped contact layers. The peak wavelengths of the four colors are centered at 4.7, 8.5, 9, and 12.3 μm. The 4.7 and 8.5 μm stacks are separated from the 9 and 12.3 μm stacks by a middle ohmic contact layer, and the change of peak detection wavelengths within the two-stack QWIPs is achieved by varying the bias voltage. Four different combinations of two-color simultaneous reading can be obtained. The detector could achieve simultaneous reading of four colors by adding two extra contact layers to the design with appropriate readout circuitry. By using a small number of quantum wells, we are able to use all four stacks for voltage-tunable detection with two terminals. In spite of using InGaAs/AlGaAs and GaAs/AlGaAs materials in the four stacks, the device shows excellent material quality and performance characteristics. © 1999 American Institute of Physics.