Noise Mechanisms

The detectivity of a detector is limited by noise mechanisms. Shot noise is the fundamental mechanism in photovoltaic detectors. The shot noise current is given by [4.4] 

We have let = (4.10) because in any practical application the photon flux 

Generation-recombination (gr) noise and Johnson noise are the fundamental mechanisms in photoconductive detectors. The total noise current is given by 

The first two terms within the parentheses in (4.12) represent gr noise due to the background photon flux and to thermal equilibrium carriers in the semiconductor, respectively these terms are derived in Appendix A from the more familiar expression for gr noise. The third term within the parentheses in (4.12) represents the Johnson noise, where R is the photoconductor resistance, which in this case can depend upon the background photon flux density. The photoconductive gain G in (4.12) is generally a function of the bias voltage applied to the photoconductor. At high bias voltages carrier sweepout effects occur which will be discussed later, and the gr noise terms must be multiplied by a numerical factor a, where l/2 a 1, but this is only a minor modification. 

The expressions (4.10) and (4.12) for the noise currents of photovoltaic and photoconductive detectors are both of the form 

We have let 0 = (p in (4.10) because in any practical application the photon flux contributing to the detector noise will be almost entirely from the background. When = 0 and K = 0, (4.10) reduces to the well-known expression for the Johnson noise current. 

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The Occupational Safety and Health Act, 29 U.S.C. 651 et seq. (1970) Employers must provide a place of employment free from recognized hazards to safety and health, such as exposure to toxic chemicals, excessive noise, mechanical dangers, heat or cold stress, or unsanitary conditions. Employers must provide personal protective equipment and training, including communication of hazards. Eacilities must undergo hazard analysis. The Occupational Safety and Health Administration (OSHA) is established to promote best practices, inspect facilities, set standards, and enforce the law. 

Additionally active or passive insulation systems are integrated to the instrument in order to reduce external noise (mechanical and acoustic vibrations, electrical and optical noise). A computer and software interface finally is used to drive the system and to process, display, and analyze produced data. 

There is a certain imbalance in the level of presentation of the several sections which is due to the nature of the following chapters. The discussion of the photon, thermal, and wave interaction effects is at an introductory level, the purpose of which is to present an overview of the many effects without a detailed analysis of them. Subsequent chapters will treat in depth the important ones. For a similar reason the discussion of noise mechanisms is also at an introductory level. On the other hand, the derivation of the fundamental limits has been reserved to this chapter, since it is common to the later chapters. Thus Section 2.4 is of a much more mathematical nature than the other sections. 

As was true for that of photoeffects, the objective of this discussion of noise mechanisms is to acquaint the reader with the broad concepts of noise in detectors without deriving in great detail the appropriate equations. See Van Vliet [2.141] for a detailed treatment. Nevertheless, it will be necessary to present certain equations which describe the dependence of noise upon internal material parameters and external system parameters. The discussion will consider initially noise in semiconductor detectors, followed by noise in photoemissive devices. 

A recent review article by Law et al. [8.83] provides some insight into the design criteria of group III-V alloy heterostructure avalanche photodiodes in terms of their speed of response, noise mechanisms and gain. A useful direct comparison of Ga AlSb, GaAl AsSb, and InGaAsP APDs is given in terms of their basic operational parameters. 

For low-level light detection, the question of noise mechanisms in photomultipliers is of fundamental importance [4.129]. There are three main sources of noise ... 

The factors of the physical work environment—microclimate, noise, mechanical vibration, lighting, radioactive radiation, etc.—often have a direct effect on the safety and health of the working person. They are... 

Owing to its nature, the arrangement is inherently very sensitive to mechanical and electrical noise. Mechanical vibrations can be avoided by mounting the balance on a very heavy support. Electrical disturbances must be minimized by careful earthing. Temperature gradients within or around the apparatus can seriously disturb the system and therefore should also be minimized. 

Let us first shortly consider velocity fluctuation noise. This is the only noise mechanism that exists without electric bias. It is variably denoted as Johnson noise, thermal noise, Nyquist noise, Johnson-Nyquist noise, resistance noise [59, 60]. It is a consequence of stochastic motion of charge carriers within material with finite resistance, and it represents a mechanism to maintain thermal equilibrium in semiconductor [61]. In a general case the spectral density of thermal noise voltage is 5v(co) = 4J (hv/2 + hv/(c " / - l) = 2/flivch(hv/ i,7 ), which in the case hv becomes Sy = Ak TR, i.e., in this case the squared current due to this noise mechanism is... 

For our present consideration g-r noise is the most important noise mechanism. The g-r noise spectmm is flat (white) up until the cutoff frequency, approximately given as the reciprocal value of free carriers lifetime. For the component of this noise not connected with illumination in a case of an ideal photoconductive detector one may write [7]... 

In this consideration we neglect all other noise mechanisms, e.g., noise caused by nonuniformity of impurity distribution within detector ( pattern noise), all avalanche-related processes, burst ( popcorn ) noise, etc. [65, 66]. 

Since all of the considered noise mechanisms are independent, the squared values of all of them are summed, giving the square of total detector noise. 

Elhott et al. [339] analyzed noise mechanisms in MWIR and LWIR infrared detectors operating in the range from 3-13 om and proved that there is no fundamental obstacle that would prevent room temperature operation of photodetectors in background hmited performance, even if the field of view is reduced. 

Noise comes about because the carrier generation rate N is not absolutely constant - it fluctuates slightly. There are numerous sources of noise, and the total noise (expressed as a voltage or current) is the RSS (root of the sum of the squares) of the contributing noises. For most applications, only a few noise sources need be considered - the others are negligible. The ultimate limit to noise - the noise mechanism we cannot eliminate - is the noise due to the arriving photons. 

The noise mechanisms (and their names) are different for PC and PV detectors, but the causes and resulting formulas are very similar, and only a few equations are needed to describe most cases of interest. 

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