The charge distribution in high speed transistors

A self-consistent scheme of subpicocoulomb charge measurement and two-dimensional, transport analysis has been used to calculate the distribution of charge in high speed (3-7 GHz) Ge and Si transistors. This scheme carefully measures total stored charge and calculates the mobile charge stored in the two-dimensional base and collector space charge regions. Storage in the emitter space charge layer is obtained by subtraction of calculated charge from measured charge. The conclusions of this study are" (1) The majority (as much as 80%) of the mobile charge is stored in the emitter; (2) 30-50% of the remaining charge is injected through the emitter side-wall and stored in the remote base region; (3) overall transit time is dominated by low field mobility regions -- thus Johnson\´s fundamental limitations do not apply, and the transit time of state-of-the-art devices continues to reflect the low-field mobility of the material ( Ge ∼ 3 × Si).