This paper investigates the effects of n-type doping in the emitter-base heterojunction vicinity on the DC and high-frequency characteristics of InP/InGaAs heterojunction bipolar transistors (HBTs). The n-type doping is shown to be very effective for enhancing the tunneling-injection current from the emitter and thus for reducing the collector-current turn-on voltage. However, it is also revealed that an unnecessary increase in the doping level only degrades the current gain, especially in the low-current region. A higher doping level also increases the emitter junction capacitance. The optimized HBT structures with a 0.5-µm-wide emitter exhibit turn-on voltage as low as 0.78V and current gain of around 80 at J_C =1mA/µm². They also provide a current-gain cutoff frequency, f_t, of 280GHz and a maximum oscillation frequency, f_max, of 385GHz at V_CE =1V and J_C =3mA/µm². These results indicate that the proposed HBTs are very useful for high-speed and low-power IC applications.