Noise in single quantum well infrared photodetectors

The noise of the dark current and photocurrent is an important factor in operation of the quantum well infrared photodetectors ~QWIPs!. For typical operating conditions, the main source of fluctuations in QWIPs is generationrecombination noise associated with the excitation of carriers from the QWs into the continuum and their capture into the QWs. For applications the study of noise in QWIPs is important in order to obtain devices with high detectivity, and it also provides additional physical insight into these systems. Fluctuations are the natural sources of transient excitation, therefore the QWIP response to this excitation provides information on internal physical processes, and thus forms the basis for noise spectroscopy. Due to the numerous experimental and theoretical works on noise properties of QWIPs with multiple QWs ~see, e.g., Refs. 1,4–9!, the overall understanding of the QWIP noise characteristics is satisfactory. Some issues, however, are still controversial, such as the relation between the noise gain and the photocurrent ~optical! gain, the frequency dispersion of the noise spectral density, and the role of the injecting contact. In this letter we consider a single QW QWIP ~SQWIP!. SQWIP is especially attractive theoretically because its simple structure allows an accurate self-consistent calculation of the electric field, injection current, and charge accumulation in the QW, and hence a better understanding of the noise properties. In addition, SQWIPs are intrinsically fast devices promising for CO2-laser based high-speed applications, for which noise is an important consideration. Although SQWIPs have been studied by several research groups ~see, e.g., Refs. 13–16!, we are aware of only one theoretical paper dealing with the noise in SQWIPs. However, the result obtained in Ref. 17 requires a number of restrictive assumptions ~large photocurrent gain, absence of the electron transport from the QW to emitter, etc.!. In this letter we extend recent theoretical studies of SQWIPs to present a noise theory for SQWIPs. The frequency-dependent spectral density of current fluctuations is expressed in terms of the basic transport and injection parameters, and is applicable for SQWIPs with different design concepts. The SQWIP under consideration contains a single-level QW separated by undoped barriers from heavily doped con-