It is shown in our experiments that the threshold current of the lasers decreases with the lowering of temperature, and finally enters saturation region. This is essentially consistent with Hwang's result in the range of 300-10 K. But further lowering in temperature will increase the threshold current of the lasers. We suppose that this iucreasement is caused by the leakage of carriers. When the temperature is in the range of 2.8-6 K, the threshold current, the voltage at the junction and the dynamic electric resistance all exhibit regular structures. If the electron-hole liquid has been generated in this process, it may be explained qualitatively by Landau's phase diagram. However, we also can not exclude the possibility that this structure is due to the impurity energy band, which could play an important role in the above mentioned process.