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Microwave amplification in piezoelectric semiconductors in a transverse magnetic field
Başlık:
Microwave amplification in piezoelectric semiconductors in a transverse magnetic field
Yazar:
Livingstone, John, author.
ISBN:
9780438057708
Yazar Ek Girişi:
Fiziksel Tanımlama:
1 electronic resource (197 pages)
Genel Not:
Source: Dissertation Abstracts International, Volume: 76-08C.
Advisors: W. Duncan.
Özet:
Amplification of shear mode, microwave frequency sound waves in the n-type piezoelectric semiconductor indium antimonide, using crossed electric and magnetic fields, at 77°K, has been examined from a number of aspects in order to assess fully its potential as an acousto-electric travelling wave amplifier. High acoustic gain has been achieved, generally about 50 dB/cm, with a maximum for an 18 mm. long amplifier of 87 dB acoustic gain. Evaporated cadmium sulphide transducers were used to generate the sound waves and these typically had tuned insertion losses in the region of 30 dB, so that the maximum net or terminal gain recorded was 6 dB. The bandwidth of the best InSb amplifier was > 50%, but superimposed on this was the bandwidth of the transducers, which in fact limited it to 20-40%. As with the conventional acousto-electric amplifier, where no transverse magnetic field is used, the electrical, elastic and piezoelectrical parameters of the material determine the frequency of optimum gain, its magnitude, and the bandwidth of the device, for given basic conditions. With a high mobility semiconductor such as n-type InSb, a transverse magnetic field normal to the sound wave propagating parallel to the long axis acts via the rf mobility of the electrons and reduces the drift velocity required for optimum gain by up to three orders of magnitude, so that the power requirements of the amplifier are drastically reduced. However, in the crossed fields configuration, the cross-sectional geometry and length of the device play in important role in gain behaviour. This was first observed when the magnetic field, B, was rotated in the transverse plane, so that B was normal to a varying length to width ratio. In those experiments, the gain was seen to vary markedly from high gain for large length/width ratios to low gain for low length/width ratios. This is due to the large area end plate contacts tending to short out the dc Hall field. The shorting causes gross electric field distortion and non-uniform current flow in the region of the plated ends, and this tends to dominate gain features in such regions. A method of conformal mapping showed the distortion existing in crystals of various length to width ratios; these theoretical studies were later confirmed experimentally by probing crystal faces with an array of fine tungsten points and so obtaining equipotential lines in these faces. It was also shown that the optimum length/width ratio would have to be much greater than 25/1 in order to reduce the non-uniform current flow regions below 10% along the gain path. The electric field distortion was also thought to contribute to the incoherent noise phenomenon which was observed under gain conditions. Very high fields were observed in regions near the plated ends, and these were thought to generate collision-induced avalanching, with its resultant electromagnetic emission. Amplification of the thermal phonon flux via acousto-electric interaction also contributed to the noise generated and it has been concluded that this was the major component in the microwave noise emission. This conclusion followed a delay-rod experiment which separated out in time the two different components of the noise, acoustic and electromagnetic.
Notlar:
School code: 0547
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Yer Numarası | Demirbaş Numarası | Shelf Location | Lokasyon / Statüsü / İade Tarihi |
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XX(684629.1) | 684629-1001 | Proquest E-Tez Koleksiyonu | Arıyor... |
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