GaAs Heterojunction Bipolar Transistor Emitter Design

We demonstrate that GaAs-based HBTs with very low base currents at both low and high injection levels can be achieved using either Al0.35Ga0.65As or InGaP in the emitter with the proper optimization of structure and growth. We observe an order of magnitude reduction in space charge recombination current as the Al composition, and hence the energy-gap, of the emitter increases from 25% (1.77 eV) to 35% (1.89 eV). AlGaAs/GaAs HBTs with approximately 35% Al have the same energy-gap as InGaP and exhibit comparable space charge recombination in large area devices (L = 75 x 75 μm). Moreover, this reduction in the space charge recombination in Al0.35Ga0.65As/GaAs HBTs can be achieved while maintaining a low turn-on voltage and high DC current gain over a wide range of current densities. Small area devices (L = 1.4 x 3 μm) fabricated with an Al0.35Ga0.65As emitter and a base sheet resistance of 330 Ω/† exhibit very high DC current gain at all bias levels, with a DC current gain exceeding 140 @ 25 A/cm and a peak DC current gain of 210 @ 26 kA/cm. The temperature dependence of the peak DC current gain is significantly improved over a similar structure with a 25% AlGaAs emitter. The RF performance of the 35% AlGaAs structure is also comparable to the 25% structure, with an ft of 34 GHz and an fmax of 55 GHz.