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The base transport characteristics of InP n-p-n DHBTs grown using metal-organic vapor phase epitaxy (MOVPE) were examined in both constant base and graded base structures. Constant-base InP structures demonstrated an inverse square dependence of DC current gain on base thickness, in contrast to the linear inverse dependence measured for GaAs HBTs. The different gain dependence in the InP DHBTs is attributed to a more diffusive, compared to a quasi-ballistic, nature of the electron transport in the base. The use of a compositionally graded InGaAs base increased the DC current gain by ≈ 1.6 times compared to constant base InP DHBTs. The higher DC gain is attributed to reduced neutral base recombination resulting from the grading-induced increase in base electron velocity. The RF cutoff frequency increased by 9% at high current density in the graded-base HBTs compared to the constant base, due to the reduced base transit time.