A small-signal theory of avalanche noise in IMPATT diodes

Abstract

A general small-signal theory of the avalanche noise in IMPATT diodes is presented. The theory is applicable to structures of arbitrary doping profile and uses realistic ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha \neq \beta</tex> in Si) ionization coefficients. The theory accounts in a self-consistent manner for space-charge feedback effects in the avalanche and drift regions. Two single-diffused <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n-p</tex> diodes of identical doping profile, one of germanium and the other of silicon, are analyzed in detail. For description of the noise of the diodes as small-signal amplifiers the noise measure <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</tex> is used. Values for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</tex> of 20 dB are obtained in germanium from effects in the depletion region only, i.e., when parasitic end region resistance is neglected. Inclusion of an assumed parasitic end resistance of one ohm for a diode of area 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> produces the following noise measure at an input power of 5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and at optimum frequency: germanium 25 dB, silicon 31 dB. For comparison, a noise figure of 30 dB has been reported [1] for a germanium structure of the same doping profile as used in the calculations. Measurements of silicon diodes of the same doping profile are not available, but typically silicon diodes give 6-8 dB higher noise figures than germanium diodes of comparable doping profile.