Noise Limitations in Solid State Photodetectors

Abstract

The first part of this paper deals with general concepts of noise and response in photodetectors. The noise in photodetectors is of a fivefold nature: (i) noise produced by the blackbody photon field, J_b; (ii) noise produced by the ambient photon field J_a; (iii) noise associated with the signal J_s; (iv) spontaneous noise characteristic for the device and not associated with J_a; (v) noise associated with the circuitry or amplifiers. Concepts for the characterization of the noise are—the noise equivalent powers P′_eq,λ[λ,f,Δf,A], P_eq,λ[λ,f,Δf,A] P′_eq,T[T,f,Δf,A], and P_eq,T[T,f,Δf,A]; the photon limited noise equivalent powers ℘_eq,λ[λ,f,Δf,A], ℘_eq,T[T,f,Δf,A]; various detectivities D_λ*[λ,f], D_T*[T,f]; the photon limited detectivities ${{\mathcal{D}}_T}^{*}${T_s,Ω_s}, ${{\mathcal{D}}_{\mathrm{\lambda }}}^{*}${T_s,Ω_s}, ${{\mathcal{D}}_{\mathrm{\lambda }}}^{*}${T_e,Ω_e}, and ${\mathcal{D}}_{\mathrm{\lambda }}$† giving the ultimate limit attainable with a detector in radiative equilibrium; the noise figure F and the signal-to-noise ratio σ/N. The merits and limitations of photon limited behavior are discussed and the theoretical detectivities are calculated for various circumstances. If, in the absence of a signal, the noise stems mainly from the circuitry (class B), a characterization by P_eq or D is largely arbitrary. Such is the case for photoemissive detectors and photoconductive insulators in the absence of bias light. In the event that the device operates and produces the main noise in the absence of a signal (class A), concepts like P_eq, D, F, are meaningful (semiconductors, p-n junctions, bolometers, PEM cells, etc.). Special attention has been given to introducing a consistent notation. I the second part of this paper we discuss two topics: some aspects of the photodetective conversion processes and the fluctuations of the photon field. The photodetective processes in emission diodes, junction diodes, photovoltaic cells, and avalanche diodes are simple. The noise is mainly of (amplified) shot noise nature. In photoconductors and PEM cells we must consider the collective carrier processes (unless the electrodes are blocking as in detectors in the thirties). Such processes do not fit too well a gain mechanism as has often been suggested as is testified by ambipolar sweep out and by the non-Poisson nature of generation–recombination (g–r) and transport fluctuations. The noise in a blackbody field is stated and the effect of imperfect absorbers is considered, involving stimulated emission. The commonly applied variance theorem leads to a degrading of the Boson factor. The noise of nonthermal photon fields is discussed following Mandel–Wolf, Alkemade, and Enns. In a nonthermal equilibrium state the effects of coherence and wave-interaction noise are manifest in the detector-output noise. In the final part the detectivity is derived for various devices. We first consider briefly photoemission devices, junction photodiodes, both in short-circuited and photovoltaic operation, and avalanche diodes. Avalanche multiplication decreases the detectivity by a factor G^½, where G is the gain. A new derivation of McIntyre’s formula—including the effect of a non-Poissonian Boson field is presented. The remainder is devoted to photoconductive detectors. Four classes are distinguished: intrinsic, minority trapping models (like PbS), two center models (like CdS), extrinsic semiconductors (like Ge: Au). The main theoretical results are stated and illustrative experimental results are discussed. Some remarks are made on hot electron photoconductivity and Landau level transitions in magnetically tuned InSb detectors.