by Michael D’Agati

The development of classic silicon-based transistors at the nanoscale level has become more challenging, especially within the past decade. Scientists have thus turned towards producing transistors with new materials and geometries. Black phosphorus (BP), a material discovered a few years ago, could potentially provide a breakthrough towards making smaller transistors. It has grown in popularity in electronic device production because of its unique anisotropic (having different properties in different directions) atomic structure, high mobility, and a thickness dependent band gap energy that varies between 0.3 eV (bulk) and 1-2 eV (monolayer).

Dr. Nazila Haratipour and Dr. Steven Koester at the University of Minnesota investigated the on/off current ratio performance of BP-based metal oxide semiconductor field effect transistors (MOSFETs). This ratio is a standard marker for how well a transistor performs. The off-current measures how much leakage occurs when the transistor is off. The lower this number is, the higher the ratio, indicating a better performance transistor. The same is true for a higher on-current. The on-current measures the rate of flow of electric charge when the transistor is on. As this parameter increases, the ratio increases as well, indicating better performance.

The MOSFET devices were constructed by embedding a back gate in a silicon oxide substrate, followed by atomic layer deposition of HfO2 as a gate dielectric. The BP was aligned and transferred on top of the dielectric with the source and drain contacts patterned on top of the BP flake. The BP flakes were varied in thickness from 4 nm to 14 nm and the on-current and off-currents were quantified. The voltage (VDS) across the drain to source was also varied, and the results showed p-type behavior with improvements in the on-current up to 100 uA/um as the BP thickness increased up to 7 nm. Yet for BP with thickness greater than 7 nm, excessive gate induced drain leakage of the device caused an increase in the off current, which decreased the overall performance of the transistor.

The quantification of the trade-offs between the on- and off-currents in BP p-MOSFETS based on BP thickness and varying VDS demonstrates that black phosphorus can be used in transistors at certain thicknesses. Increasing BP thickness can increase the on-current, but at thicknesses greater than 7nm, the BP may also cause unwanted leakage current. A happy medium in BP thickness is required to create a transistor that is incredibly small, but still has high performance. This study shows that materials like black phosphorus can possibly be used to create transistors that are smaller and more powerful in the future.

References:

1. N. Haratipour, S.J. Koester, Quantifying the impact of thickness and drain bias on black phosphorus field effect transistor performance. 2016 IEEE Silicon Nanoelectronics Workshop (SNW), 62-63 (2016). 10.1109/SNW.2016.7577986.
2. Image retrieved from: https://upload.wikimedia.org/wikipedia/commons/6/61/IGFET_N-Ch_Enh_Labelled_simplified.svg