A novel three terminal GaAsn+-i-p+-i-n+negative differential resistance device prepared by molecular beam epitaxy is demonstrated for the first time. The peak-to-valley current ratios can be modulated by the third external applied voltage which can be expressed asIp/Iv= 5.08 × 10−3exp [1.999VBE] at room temperature, whereVBEis in units of volts. It implies that large peak-to-valley current ratios (e.g.IP/Iv= 300 atVBE= 5.5 V) and large peak current densities can easily be obtained just by increasing theVBEbias. A phenomenological bipolar-unipolar transition model is proposed to interpret the observed behavior and confirmed by experiments.