Two-Section Model of the Current Distribution in Strip Layouts of Bipolar and Hetero-Bipolar Microwave Transistors

An approximate analytical expression for the density of the emitter current under the emitter metallization path in strip layouts of bipolar and hetero-bipolar microwave transistors has been derived in the isothermal approximation with allowance for the voltage drop across the distributed resistances of the active base and conductive emitter metallization. It is shown that, when these factors of the nonuniform current distribution are taken into account separately, the position of the average current density line is independent of the total current and divides the layout into two unequal parts with current densities higher and lower than the average value. It is established that, when the nonuniform current distribution factors are taken into account jointly, the average current lines are ellipses that pass through characteristic points and have their foci in the middle of the emitter path edge. It is proposed to calculate the performances of the transistors and evaluate their operation limits by presenting a transistor in the form of two parallel-connected transistors, the currents through which are generally nonidentical and determined by the nonuniform current distribution factors.