By employing nonequilibrium Green's functions in combination with density functional theory, we have examined the electronic and transport properties of p-type doped, undoped, and n-type doped MXene heterojunctions [M2CTx-M2CO2 (M = Ti, Zr, or Hf; T=F, OH; x = 0 or 2)]. The geometries and electronic band structures are all obtained and the current voltage characteristics are predicted. We found that M2CF2-M2CO2 (M = Ti, Zr) heterojunctions have better electrical conductivity than M2C-M2CO2 and M2C(OH)(2) M2CO2, and Hf2C(OH)(2)-Hf2CO2 shows the best conductivity compared with the cases of other terminations studied herein. Rectification behaviors are observed as important characteristics from some of these devices. Moderate n-type doping is found to be effective in enhancing rectification for Hf2C(OH)(2)-Hf2CO2, and the currents at the intermediate positive bias show an excellent rectification ratio. Moreover, high n-type doping may generate a negative differential resistance (NDR) effect in the Hf2C(OH)(2)-Hf2CO2 heterojunction at a high voltage with a wide bias range, and the high doping concentration of both n- and p-types are found to generate high electrical conductivity. The mechanism of rectification and NDR effects is elaborated in detail from the electronic structure level. These findings not only help us to make appropriate choices in surface groups, doped carrier types, and concentration to improve the performance of MXene heterojunctions, but also provide new insight for guiding the design of novel MXene nanoelectronic devices
