Nanomaterial devices have been proposed for high performance electronics. Some interesting nanomaterials are nanocarbons such as graphene and CNT due to high mobility and unique band structure. Especially, Dirac cone band structure is of great interest to elicit new functions. This study proposes new band engineering device and fabrication process to realize low contact field-effect transistor (FET) using a unique band structure of graphene contacted to semiconducting carbon nanotube network film on Si/SiO2 substrate. Effect of CNT/graphene junction was evaluated by comparing mobility and ON/OFF current ratio with and without graphene junctions for source and drain. Although the effect of this junction is unique and interesting phenomena, there are few researches on the junction analysis. It is found that CNT network FETs with graphene contacts show higher performance than ones without graphene.
To evaluate band engineered CNT/graphene junction, two types of devices with/without graphene junction were prepared. The mechanism to enhance the performance is due to valence band contact for p-type CNT-FET using the graphene dirac cone depending on the gate bias. Without graphene, work function of metal mainly determines the contact resistance of FET regardless state. On the other hand, having the Dirac cone band structure, by applying negative bias, Fermi level of graphene is closer to valence band of CNT, resulting in that contact resistance decreases. To realize this structure, we developed a unique fabrication process. CNT/graphene distributions were confirmed by Raman spectroscopy. By monitoring RBM peak (~160 cm-1) for CNT network and 2D peak for graphene, it is found that the unique structure is successfully fabricated. Electrical switching characteristics with and without graphene were measured to compare the effect of graphene contacts to CNT channel. The results clearly show that FETs with graphene junction have higher on-current while off-current is almost the same level. ON/OFF current ratio extracted is ~2×105 and ~105 for with and without graphene junction, respectively. This difference is due to on-current difference. The mobility for CNT/graphene structure is ~1.56±0.43 cm2/Vs while the one for CNT/metal structure is ~1.09±0.22 cm2/Vs. Based on the results, by having graphene contact, the mobility was enhanced up to ~50 %, which improvement is significant for the FET applications although the base mobility of CNT-FET regardless graphene junction is still low. This result may be explained by low contact resistance between CNT and graphene compared with the one between CNT and Cr/Au as explained above. Furthermore, another possibility is the injection difference between them, which is the future topic to confirm the phenomena.
In conclusion, by proposing new structure and fabrication process, we confirmed that FET performance can be boosted up for future low power and high speed electronics.