Schottky barrier detectors

Figure 7. A beam-lead Schottky-barrier detector/mixer [15].

There is a continuing interest in metal silicide/silicon Schottky-barrier detectors. .. first... 

The ability to make ever smaller solid-state devices by improved lithography techniques has led to the development of so-called beam lead Schottky-barrier diode detectors and mixers in which diodes are fabricated by the same techniques used to make integrated circuits, and for this reason, they can be included in these circuits. Figure 7 shows such a beam-lead detector/mixer made by Virginia Diodes of Charlottesville, VA [15]. This same configuration is used in fabricating the varactor devices used for frequency multiplication discussed in the preceding section. 

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. 

Photodetector — Device used to detect photons. After a long period having only thermal photodetectors, quantum photodetectors based on photocurrent were developed and are used quite widely in applications such as photographic meters, flame detectors and lighting control. In the late 1950s the p-i-n photodiode, simply referred to as photodiode, was developed and now is one of the most common photodiodes. There are several types of photodetectors, the most adequate depending on the specific application, like photoconductors, p-i-n photodiodes, Schottky-barrier photodiodes, charge-coupled... 

The rectification properties of semiconductor interfaces are the most important electrical characteristic of semiconductor contacts. Certain types of devices, such as transistors, require both ohmic and rectifying contacts on a given semiconductor surface, whereas other devices, such as Schottky barriers, are based on the inherent rectification properties of semiconductor/metal Junctions. Numerous photonic devices, such as photon detectors and photovoltaic cells, require rectification at a semiconductor Junction, and light-emitting diodes require both ohmic contacts and rectifying Junctions in a well-defined geometry. Thus, successful fabrication of a desired device structure depends entirely on the electrical properties of the specific semiconductor contacts that are formed in the process. The principles described above allow the rational fabrication of contacts with the desired properties, and also describe the operation of the resulting devices within a simple, chemically intuitive, kinetic framework. 

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 . ... 

The peak absorption coefficient of OCS, 10 m", occurs at 462 GHz. This is by no means, however, the optimum working frequency due to the non-ideal behaviour of most MMW detectors. Commercial Schottky barrier mixer diode detectors show a quadratic roll-off in sensitivity at frequencies >100 GHz. If this is factored into Equation 6.1, the peak sample sensitivity occurs around 300 GHz, and the response is so flat that even at 100 GHz it has only fallen off by a factor of two. What is common to both curves is the dramatic fall-off in sample sensitivity at frequencies <100 GHz, reinforcing the point that the band 26-40 GHz is ill suited to high-sensitivity analytical spectroscopy. 

The Gunn oscillator frequency-lock became progressively more unstable as its slewing rate increased, whether caused by rapid frequency stepping or by the application of FM. We found ourselves limited in practice to a stepping time of 50-100 ms and an FM deviation of 10 MHz at a rate of 1 kHz. This was considered reasonably satisfactory in that a complete scan could be completed in a few seconds and that our signal detection rate 2 kHz was close to the frequency at which manufacturers, e.g. Millitech, specified their detector performance. The same FM rate was used whether our detector was a Schottky barrier mixer diode or the helium-cooled bolometer. 

Ruths, R, Ashok, S., Fonash, S. and Ruths, I, A study of Pd-Si MIS Schottky-barrier diode hydrogen detector , IEEE Trans. Electron. Devices, 1981, ED-28, 1003-9. 

It is assumed that the reader is broadly familiar with the properties of semiconductors, from which most detectors of importance are prepared, and with semiconductor technology which allows the preparation of interfaces such as p — junctions and Schottky barriers. For those who are not familiar with these concepts, many good books are available, including those by Sze [2.25], Grove [2.26], Kittel [2.27], and Moll [2.28]. It is also assumed that the reader is aware of the nature of electromagnetic radiation, including the properties of monochromatic and black body sources. For further information on this topic see the appropriate chapters in [2.1-7]. 

The second photon effect of general utility is the photovoltaic effect. Unlike the photoconductive effect, it requires an internal potential barrier with a built-in electric field to separate a photoexcited hole-electron pair. Although it is possible to have an extrinsic photovoltaic effect, see Ryvkin [2.32], almost all practical photovoltaic detectors employ the intrinsic photoeffect. Usually this occurs at a simple p — n junction. However, other structures employed include those of an avalanche, p—i — n, Schottky barrier and heterojunction photodiode. There is also a photovoltaic effect occuring in the bulk. Each will be discussed, with emphasis on the p—n junction photoeffect. 

Not all semiconductors can be prepared in both n- and p-types. Schottky barrier photodiodes are of special interest in those materials in which p — n junctions cannot be formed. They also find application as UV and visible radiation detectors, especially for laser receivers where their high frequency response (in the gigahertz range in many cases) is of particular usefulness. See Ahlstrom and Gartner [2.41], Schneider [2.42] and Sharpless [2.43] for more detailed descriptions. 

Another method of realizing a photovoltaic detector is with a Schottky barrier made by depositing a metal onto the surface of a semiconductor. The energy level diagram for a Schottky barrier is shown in Fig. 4.3. Its current-voltage characteristic is [4.10]... 

Schottky barrier if > E, a layer of the semiconductor next to the surface is inverted in type, and there is then a p-n junction within the material. Let us estimate RA with these limitations for a Schottky barrier photovoltaic detector operating at T=77 K we obtain RA <470ohm-cm from (4.29). This estimate represents the upper limit of RA achievable with a Schottky barrier. We should keep this result in mind for comparison later with the RA values calculated for p-n junction photovoltaic detectors. 

There exists a wide variety of approaches to the use of charge transfer devices in infrared focal planes. We shall discuss five high packing density, high quantum efficiency, approaches appropriate for series-parallel scan 1) IR sensitive CCD, 2) ctirect injection hybrid, 3) direct injection extrinsic silicon, 4) accumulation mode extrinsic silicon, and 5) infrared sensitive CID with silicon CCD signal processing. The reader is referred to a review article by Steckl et al. for a comprehensive discussion of a number of other approaches not discussed here which include indirect injection pyroelectric detectors and Schottky barrier photoemissive injection [6.1]. Three approaches in our list of five do not require... 

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