Two-dimensional analysis of emitter-size effect on current gain for GaAlAs/GaAs HBT's

The current-gain dependence on emitter size (emitter-size effect) is theoretically investigated for GaAlAs/GaAs heterojunction bipolar transistors with an ion-implanted extrinsic base region, using a two-dimensional numerical model. It is clarified that the current gain degrades with a decrease in emitter size, depending on the carrier lifetimes in the extrinsic base region. From the calculated results, it is shown that the emitter-size effect is suppressed for a very-thin-base or a graded-base structure. On the other hand, the effect is enhanced for a double-heterostructure. These results are explained by taking account of the electron-density profile in the extrinsic base region. The mechanism of a lateral diffusion of electrons into the extrinsic base region is also discussed.