The carriers transport at the base-emitter interface of abrupt heterojunction bipolar transistors (HBTs) is controlled by thermionic emission and tunneling, which depends on the form and height of the energy barriers. The interface charges at the heterojunction disturb the energy barriers, thus bringing about the change of the electrical characteristics of HBT. Based on thermionic-field-diffusion model which combines the drift-diffusion transport in the bulk of the transistor with the thermionic emission and tunneling at the interface, a conclusion can be drawn that the positive interface charges can improve the electrical characteristics of abrupt InP/InGaAs HBT, while the negative interface charges deteriorate the devices.