In this paper, we start with the calculation on subbands of the undoped AlGaAs-GaAs triple-well structure by using transfer matrix approach. In order to obtain more accurate solutions, we have considered the influence of the different electron mass within quantum wells, and potential barriers. .,The studied structure consists of a n+-GaAs, a 30-A GaAs, a 20-A Alo.45Qao.55As, a 50-A GaAs, a 40-A Alo.3Gao.7As, a 30-A GaAs, a 40-A Alo.3Gao.7As, a 110-A GaAs, a 20-A Alo.45Gao.55As, and a n+-GaAs layers. The band diagram of this structure is shown in Fig.l. There are three subbands (Eiq, En, E12) in Wi, one subband (E20) in W2, and one subband (E30) in W3, respectively. The illustration of the transmission coefficient T∗T versus the longitudinal electron energy Ez at zero bias is shown in Fig.2(a). Subband Eto operates as an injection level of electrons from the cathode layer. So only the electrons with energy close to E10 are injected to Wi. The transmission probability of this level approaches 100% when subband E20 or E30 resonates with this injection level E10 under external bias. Furthermore, when an external bias is applied, the voltage drop is expected to take place not in Wi, but mostly in W2 and W3, because the electric field applied to Wi is screened by the injected electrons. Subband E30 resonates with the injection level Eio when an external bias V. of 80mV is applied; and, subband E20 resonates with subband Eto when V. of 400mV is applied. The transmission A. resonance spectra (T∗T-EZ) for 80mV and 400mV external bias are shown in Fig.2(b) and (c). We have also fabricated a GaAs bipolar transistor with a triple-well structure as emitter by molecular beam epitaxy (MBE). The current-voltage characteristics at 140K are shown in Fig.3. Only a negative-differential-resistance (NDR) phenomenon is observed. This is perhaps produced by the large energy difference existing between E20 and E10. The large number of the thermionic electrons, resulted from the higher external electric field, give a significant injection current and lead to a negligible resonant-tunneling effect in the transport properties. With a suitable parameter adjustment, this transistor will be more attractive in the design of quantum-functional devices.