Phosphorene, a monolayer of black phosphorus, has emerged as a promising two-dimensional semiconductor due to its unique electronic properties. Unlike graphene, which lacks a bandgap, or transition metal dichalcogenides (TMDCs) with relatively low carrier mobility, phosphorene combines a tunable direct bandgap with high carrier mobility and strong anisotropy. These characteristics make it particularly attractive for transistor applications, where control over electronic transport and band structure is critical.

The electronic properties of phosphorene are heavily influenced by its anisotropic crystal structure. The puckered lattice arrangement results in different effective masses for charge carriers along the armchair and zigzag directions. The effective mass of electrons along the armchair direction is approximately 0.15m₀, while along the zigzag direction, it is around 0.30m₀. For holes, the effective masses are 0.18m₀ and 0.75m₀, respectively. This anisotropy leads to direction-dependent carrier mobility, with reported values reaching up to 1000 cm²/Vs for electrons and 600 cm²/Vs for holes along the armchair direction at room temperature. In contrast, mobility along the zigzag direction is significantly lower, highlighting the importance of crystal orientation in device design.

The bandgap of phosphorene is layer-dependent, transitioning from a direct bandgap of approximately 2.0 eV for a monolayer to 0.3 eV for bulk black phosphorus. This tunability allows for optimization based on application requirements. Unlike TMDCs, where the bandgap remains relatively large even in multilayer form, phosphorene’s bandgap can be adjusted across a wide range, making it suitable for both low-power and high-performance electronics. The direct bandgap nature also ensures efficient carrier injection, which is advantageous for transistor operation.

Strain engineering further enhances phosphorene’s electronic versatility. Applying uniaxial strain along the armchair or zigzag direction can modulate the bandgap by up to 300 meV per 1% strain. Biaxial strain, on the other hand, can induce a transition from a direct to an indirect bandgap. Strain can also alter the anisotropy of carrier mobility, providing additional degrees of freedom for device optimization. For instance, compressive strain along the armchair direction can enhance hole mobility, while tensile strain may improve electron transport.

Doping is another effective method to tailor phosphorene’s electronic properties. Substitutional doping with elements such as sulfur or selenium can introduce n-type or p-type conductivity, respectively. Electrostatic doping via gate voltage is particularly effective due to phosphorene’s high electrostatic control efficiency. Carrier concentrations can be modulated over a wide range, from 10¹² to 10¹³ cm⁻², without significant degradation in mobility. This makes phosphorene highly suitable for field-effect transistors (FETs), where precise control over carrier density is essential.

When compared to other 2D semiconductors, phosphorene offers distinct advantages. Graphene, while exhibiting ultra-high mobility, lacks a bandgap, limiting its utility in digital transistors. TMDCs like MoS₂ have a sizable bandgap but suffer from lower carrier mobility, typically below 200 cm²/Vs. Phosphorene bridges this gap by providing both a tunable bandgap and high mobility. Additionally, the anisotropic properties of phosphorene enable novel device geometries, such as orientation-dependent transport channels, which are not feasible with isotropic materials like graphene or TMDCs.

In transistor applications, phosphorene’s combination of high mobility and tunable bandgap translates to superior performance metrics. Simulations predict that phosphorene-based FETs could achieve ON-currents exceeding 1000 µA/µm at a supply voltage of 0.5 V, with subthreshold swings approaching the thermionic limit. The anisotropic conduction also allows for the design of devices with reduced short-channel effects, as carriers predominantly travel along the high-mobility armchair direction. Furthermore, the ability to strain-engineer the bandgap and mobility provides opportunities for adaptive transistors that can reconfigure their performance based on operational requirements.

Despite these advantages, challenges remain. Phosphorene is sensitive to environmental degradation, necessitating encapsulation for stable operation. The anisotropic properties, while beneficial, require precise alignment during device fabrication. Advances in transfer techniques and passivation methods are critical to harnessing phosphorene’s full potential in practical transistor applications.

In summary, phosphorene stands out among 2D semiconductors due to its anisotropic band structure, high carrier mobility, and tunable bandgap. The ability to modulate these properties through layer thickness, strain, and doping offers unparalleled flexibility for transistor design. While graphene and TMDCs have their respective strengths, phosphorene’s unique combination of attributes positions it as a leading candidate for next-generation electronic devices, particularly in scenarios demanding both high performance and energy efficiency. Continued research into material stability and fabrication techniques will be essential to realize its promise in real-world applications.