Semiconductor detectors thermal noise

Noise from the detector is mainly thermal in origin it arises from the thermal fluctuation of electrons which carry signals. An FT-IR spectrometer uses either a thermal detector or a semiconductor (or quantum) detector. The former has considerable thermal noise as it is used at room temperature. By contrast, the latter, used at low temperatures, most often that of liquid nitrogen, has little thermal noise. 

Interferometers work with single detectors. For the NIR range InGaAs or Ge semiconductor detectors are used. They have to be extremely sensitive, since the intensity of the Raman lines decreases with the fourth power of its absolute frequency (the factor). In order to reduce their thermal noise they are cooled by Peltier elements or liquid nitrogen. 

Noise arises in semiconductor detectors from several mechanisms. Johnson noise is found in all resistive elements. It has already been discussed in coimection with thermal detectors [see Subsection 5.1 l.b and Eq. (5.11.20)]. If the load resistance in the circuit is larger than the detector resistance, the Johnson noise of the detector element dominates because load and detector act electrically in parallel as far as the noise properties are concerned. 

A photoconductive detector is a semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band. Photovoltaic detectors contain pn junctions, across which an electric field exists. Absorption of infrared radiation creates electrons and holes, which are attracted to opposite sides of the junction and which change the voltage across the junction. Mercury cadmium telluride (Hg,. Cd/Te, 0 < x < 1) is a detector material whose sensitivity to different wavelengths is affected by the stoichiome-try coefficient, x. Photoconductive and photovoltaic devices can be cooled to 77 K (liquid nitrogen temperature) to reduce thermal electric noise by more than an order of magnitude. 

It was clearly demonstrated that the composite BN semiconductor polycrystalline bulk detectors with BN grains embedded in a polymer matrix operate as an effective detector of thermal neutrons even if they contain natural boron only (Uher et al. 2007). A reasonable signal-to-noise ratio was achieved with detector thickness of about 1 mm. A Monte Carlo simulation of neutron thermal reactions in the BN detector was done to estimate the detection efficiency and compare with widely used He-based detectors to prove advantages of BN detectors. They are found to be promising for neutron imaging and for large area sensors. 

Consider first the simple extrinsic photoconductor. Here the sample is a semiconductor containing a single impurity level, the source of the free electrons (or holes) present in the sample. Thus the fluctuation in the number of the free carriers arises from the fluctuation in the generation and recombination rates through that level. If it is assumed that the temperature is so low that very few of the extrinsic centers are thermally ionized (which is valid for most extrinsic cooled photoconductive infrared detectors), then the short circuit g—r noise current and the open circuit g — r noise voltage which appear only in the presence of a bias current Ig, are given by... 

Generation-recombination (G-R) noise is found in PC detectors, and is caused by random fluctuations of charge carriers. These fluctuations can be due to thermal excitation within the semiconductor. Sometimes G-R noise is also defined to include random arrival of photons at the detector. PC detectors are normally operated at temperatures low enough to reduce the thermal generation of carriers well below that of all other noise sources. The residual G-R noise is then the photon noise already discussed in Subsection 5.1 l.b [see Eq. (5.11.33)]. Photon noise can also be understood in terms of the Poisson probability of n photons arriving during a given time interval. 

---