Our experimental results indicate that the 1/f noise spectra density of HBTs is strongly influenced by the emitter ledge potential. By biasing an on-ledge Schottky diode, we can externally control the ledge potential and alter HBT's 1/f noise spectra density and its dependence on the base current. When biasing the on-ledge diode at VL < VBE, HBT's 1/f noise spectra densities are found to increase dramatically (up to 30 dB) from the unbiased value and exhibit stronger dependence on IB (from IB1.42 to IB1.84). When biasing the on-ledge diode at VL > VBE, HBT's 1/f noise spectra density is found to decrease (about 10 dB) and exhibit virtually no dependence on IB (from IB1.42 to IB0.18). These findings help us identify the sources of the 1/f noise and create a novel four-terminal HBT (with an extra ledge electrode) for extremely low 1/f noise RF transceiver applications.