InP HBT with 55-nm-wide emitter and relationship between emitter width and current density

We fabricated an InP heterojunction bipolar transistor (HBT) with a 55-nm-wide emitter. For an emitter width of 55 nm and that greater than 300 nm, the maximum gain was around 15 and around 120, respectively. To confirm the relationship between the acceptable current density and the emitter width, we measured the current density when the current gain was half its maximum value. The measured current density J_half increased with a decrease in the emitter width. The highest observed current density was approximately 5 MA/cm² and was nearly equal to the highest reported current density of InP HBTs.