Technical Insight
Graded base doping aids InGaAsN HBTs (research review)
Improvements in the material quality of InGaAsN has motivated the development of HBTs for low power electronic applications. InGaAsN is a low band-gap material (EG = 1.2 eV) that can be used as the base layer to achieve greater efficiency at lower bias conditions. In work first discussed this year in Electronic Letters 37 on p198, C Monier et al. from Sandia National Laboratories (Albuquerque, NM) demonstrate the enhanced performance of pnp AlGaAs/ InGaAsN single HBTs by employing a graded base doping to aid hole transport across the base. The band alignment is especially suitable for pnp HBTs with a change in conduction band energy (EC) of 0.5 eV that suppresses electron back injection, while an EV 0.15 eV facilitates hole injection into the base. The introduction of the quasi-electric field in the base not only improves the base transport factor to produce a better current gain ( > 60) compared to a device with a uniform base doping, but reduces the base transit time to allow a cut-off frequency of fT > 15 GHz. The turn-on voltage is 0.29 V lower than that of pnp AlGaAs/GaAs HBTs, which indicates the potential of InGaAsN HBTs for reducing power dissipation in GaAs-based transistors.
