Integrated HBT/QWIP structure for dual color imaging

Abstract

An integrated two-terminal structure of quantum well infrared photodetector (QWIP) and heterojunction bipolar phototransistor (HBT) for dual band imaging is presented. The integrated device functions as a voltage switchable dual band detector for long wavelength infrared (LWIR) and near infrared (NIR) imaging. The device consists of series integration of floating base HBT and a QWIP in a two terminal configuration. Switching between the two spectral bands is achieved by changing the applied bias voltage. At a low bias voltage the HBT operates in the saturation mode and, due to its higher impedance, it is the only portion of the device that responds to NIR radiation. With the increasing bias voltage the HBT enter the breakdown region to activate the operation of the QWIP as LWIR detector. In order to achieve NIR response up to about 1.064 μm, strained layers of InGaAs/GaAs quantum wells were implemented in the sub-collector of the HBT. The NIR photocurrent acts as the base current of the HBT and is amplified inherently by the transistor gain. Furthermore, the breakdown of the HBT is designed to follow the punch-through mode so that one can engineer the breakdown voltage to be of the order of a few volts. In addition, this mode allows fast, reliable and nondestructive switching of the integrated device. Standard design of GaAs/AlGaAs QWIPs layers have been used to achieve LWIR detection. The integrated device, which is based on the GaAs technology, allows fabrication of large focal plane array (FPA) that can be operated using a commercial two-terminal readout integrated circuit (ROIC). Such FPA configuration would allow multispectral applications such as simultaneous imaging of a NIR laser spot superimposed on a thermal imaging scene (SEE SPOT).