Question

Briefly describe any two of Johnson’s power frequency limitation for conventional transistors at microwave frequency. Mention the assumptions and postulates with basic mathematical relations.

Answer 1

A simple analysis shows that the ultimate performance limits of a transistor are set by the product Evs/2?, where E is the semiconductor’s dielectric breakdown strength and vs is its minority carrier saturated drift velocity. This product, having a value of about 2 × 1011 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.