Physical limitations on frequency and power parameters of transistors

Abstract

A simple analysis shows that the ultimate performance limits of a transistor are set by the product <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ev_{s}/2\pi</tex> , where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> is the semiconductor's dielectric breakdown strength and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v_{s}</tex> is its minority carrier saturated drift velocity. This product, having a value of about <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 \times 10^{11}</tex> volts/ second for silicon, emphasizes that a semiconductor material has a maximum capability for energizing the electric charges that process a signal. If the device operating frequency is high, the frequency time period is short and only a small amount of energy can be given to a charge carrier. Consequently, the power and power amplification must be relatively low. At low frequencies the inverse is true. That is, device physics demands an inverse relation between frequency and power parameters that is independent of the thermal dissipation arguments commonly given to explain the trade-off between these parameters. The analysis leads to an effective means for making comparisons between existing devices. This is illustrated.