Schottky noise

Noise due to carrier number fluctuation is connected with detector bias and is denoted as shot noise or Schottky noise [5]. It is also denoted as the excess noise, but this expression is also sometimes used for 1//noise [62]. It is a consequence of carriers passing through energy barriers, i.e., it appears as a result of the statistic nature of interband transitions and transitions band-impurity level, and in the final instance it is a consequence of the discrete nature of carriers [63]. When carrier number fluctuations are caused by g-r processes, this noise is also denoted as generation-recombination (g-r) noise. 

Ainong the first TFIz mixers to be constructed were those based on room-temperature Schottky diodes [11]. Over the past decade, new mixers based on superconducting tunnel junctions have been developed that have effective noise levels only a few tunes the quantum limit of [12]. Flowever, certain conditions... 

More recently, Schottky-barrier diodes and backward diodes have been used as detectors. These do not require as much power to bias the diode to its optimum output and thus permit observation of EPR at lower incident power levels. They also have a much lower 1/f noise characteristic so that modulation frequencies between 6 and 25 kHz (equivalent to 200- to 900-pT sidebands) can yield the same sensitivity that 100 kHz provides with silicon diodes. 

The field is modulated at some audio- or radio-frequency (usually -100 kHz), and this produces signals at the crystal detector whose output can be amplified at the modulation frequency. In general, sensitivity is higher at high-modulation frequencies because of the linear variation of excess crystal noise with the inverse of frequency. However, the recently developed Schottky diode makes it possible to obtain good sensitivity at relatively low frequency. 

The smaller the current and voltage applied, the smaller the Schottky shot noise. 

Open-channel noise levels in excess of the Schottky formula were also reported in several papers on the same subject (35-39). Eisenberg et al. (39) obtained a 1/f type spectrum for a cation channel of frog lens epithelium. The intensity of the spectrum is orders of magnitude higher than expected for corresponding shot noise. An approximately flat noise spectrum of about 3 X 10 30 A2/Hz from a potassium channel of lobster sarcoplasmic reticulum was reported by Eisenberg et al. (40), who claimed that it was close to the level of the channel Johnson noise. 

The Schottky diode is the most practically useful and affordable solid state detector at present and can be obtained for frequencies up to 325 GHz. In terms of frequency range and noise performance the nearest contender, the Si point-contact diode mixer, has a noise figure of 11-14 dB over the range 60-80 GHz, its maximum operating frequency. Compared with 5-6 dB for a Schottky device... 

Cryogenically cooled detectors employ the low-noise GaAs Schottky barrier Mott diodes. Between 140 and 220 GHz they exhibit 400 K noise equivalent temperature at a lower limit junction temperature of 20 K, below which the performance degrades. The noise temperature is around 1000 K at 300 K junction temperature. Sensitivity of a Schottky barrier mixer diode ranges from about 2.75 VmW" to 1 VmW over the range 90-325 GHz.In comparison the helium-cooled InSb bolometer used by the present authors (Section 3.4.1) can provide double sideband noise temperatures of 200-300 K in the region 100-300 GHz and sensitivity of 5-6 VmW . ... 

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