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

GaAs bipolar transistors with a 50-A-thick lattice-matched Gasub0.5Insub0.5P layer between emitter and base acting as a hole repelling potential barrier in the valence band have been fabricated from films grown by metalorganic vapor phase epitaxy (MOVPE). The 1000-A-thick base of these transistor structures was doped with carbon to 2 x 10high19 cmhighminus3 resulting in a base sheet resistance of 250 Omega. Carbon has been chosen because of its low diffusivity. Using the barrier layer as an etch stop we fabricated mesa-type broad-area devices. The output characteristics of the devices is ideal with very small offset voltages and infinite Early voltages. Common emitter current gains of up to 70 at 10high4 A/square centimetre collector current density have been 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 Gasub0.5Insub0.5P potential barrier in the valence band. It was found that the electron injection across the emitter-base junction is not affected because of the small band offset in the conduction band.