An experimental study of distributed effects in a microwave bipolar transistor

Conventional microwave transistors are normally considered to operate as a single lumped circuit element. That this is only an approximation has been accommodated in the past by the use of an equivalent circuit including some inductance to correct for the finite size of the contact metallization. The effect of this inductance can drastically change the properties of the transistor when the size of the device becomes a significant part of an electrical wavelength. To study this form of behavior, a transistor with an electrical length on the order of one eighth of a wavelength was constructed. When tested in a suitable circuit, feedback effects involving the metallization inductance caused the transistor to produce four times more stable power gain than would be expected from a conventional transistor having the same area. Amplifiers designed using the new transistor and an equivalent conventional transistor had equal gain-bandwidth products. Other circuit properties of the transistor were observed to be significantly modified, the emitter lead inductance becoming an insensitive parameter while the base lead inductance became very critical. The measured properties of the transistor were found to be in good agreement with theoretical predictions.