Transient Characteristics of InGaP/GaAs/AlGaAs Double
Heterojunction Bipolar Transistors
C.R. Lutz, R.E. Welser, N. Pan
695 Myles Standish Blvd., Taunton, MA 02780
K.M. Lau and C.F. Musante
ecE Department, University of Massachusetts, Amherst, MA 01003
Both the electrical characteristics and optical properties of the base layer in InGaP/c-GaAs/AlGaAs double heterojunction bipolar transistors (DHBT's) are investigated to facilitate a better understanding of the mechanisms governing the short term transient electrical performance observed in HBT devices. The double heterojunction design is highly attractive for photoluminescence measurements of the base layer since electrons can be better confined. Differences in the burn-in related transient of the dc-current gain are observed and lead to dramatic increases when collector current densities approach 200-500 A/cm2 in large area (AE=75 x 75 mm2) devices measured at room temperature. In general, samples possessing high dc current gain-to-base sheet resistance (b/Rsbase) ratios tend to exhibit the largest degree of dc current gain shift. We also observe a marked dependence in the 77oK photoluminescence (PL) intensity originating from the base layers of these samples. The intensity dependence is strongly correlated to the dc-current gain performance in large area devices. Samples with the highest gain-to-base sheet resistance rations demonstrated PL intensities that were 25% higher than identical structures exhibiting significantly lower gains. Furthermore, the hydrogen concentration in the base layer - incorporated during growth and usually implicated in the origin of the burn-in phenomena - remains constant in spite of the observed differences in the burn-in characteristics. This result suggests that the underlying physical mechanisms of the burn-in effect occurring during this process may be more complex than originally recognized.