GaAs bipolar transistors with a Ga0.5In0.5P hole barrier layer and carbon-doped base grown by MOVPE

GaAs bipolar transistors with a 50-Å-thick lattice matched Ga_0.5In_0.5P layer between the emitter and base acting as a hole repelling potential barrier in the valence band were fabricated from films grown by metalorganic vapor phase epitaxy (MOVPE). The 1000-Å-thick base was doped with carbon to 2×10¹⁹ cm^-3, resulting in a base sheet resistance of 250 Ω/□. Carbon has been chosen because of its low diffusivity. Using the barrier layer as an etch stop the authors fabricated mesa-type broad-area devices. The output characteristics of the devices are ideal with very small offset voltages and infinite Early voltages. Common emitter current gains of up to 70 at 10⁴ A/cm² collector current density were obtained. The current gain is clearly higher than the one calculated for a bipolar junction transistor with the same doping profile because the base-emitter hole current is suppressed by the Ga_0.5In_0.5P potential barrier in the valence band