Material considerations for the design of 2D/3D hot electron transistors

Abstract

Combining two- and three-dimensional (2D/3D) materials provides a unique route to enabling next-generation hot electron transistors (HETs)—a vertical ballistic device, promising for high-frequency applications since they are not limited by electron velocity saturation, fabrication limitations, or short channel effects. The early demonstrations of HETs suffered from poor material and interface qualities and thick device components. The revival of the HET, with a cut-off predicted frequency above 1 THz, can be correlated with the arrival of 2D materials. Here, we discuss HET operating principles, examine HET material architectures with and without tunneling barriers, and review heterostructure considerations. We discuss material and interface properties that control barrier and base performance and critically review recent 2D/3D HETs for tunneling efficiency, output current density, current gain, and output conductance. Finally, we provide an overview of 2D and 3D semiconductors that form Schottky barriers with graphene that may be utilized as a collector while considering the device physics and growth issues.

2D Oxides Realized via Confinement Heteroepitaxy

“…On the other hand, when a disordered EG buffer is used, the CHet process leads to the formation of 2D metal oxides (GaO x , InO x , and SnO x , precise stoichiometry unknown) at the QFEG/SiC interface, confirmed by the absence of a 2D metal Raman MLFF (Figure 2a) and the metal oxide peaks in the Ga 3d (20.1 eV), In 3d (445.0 eV), Sn 3d (486.9 eV), and O 1s (531.0 ± 0.2 eV) regions (Figure 2b; Figure S5c-e, Supporting Information). Compositional analysis performed via XPS also confirms oxygen at the interface, where QFEG/GaO x exhibits [11][12][13][14][15][16][17][18][19][20] of SiC and corresponding EELS maps. Bilayer Ga or GaO x forms when Ga is intercalated through OB or DB, respectively.…”

“…For simplicity, we focus on the positive bias regime because, in the negative bias regime (electron flow from n‐SiC to graphene), more than one conduction mechanism (e.g., tunneling and TE) is likely to contribute to the current flow at high electric fields. Charge transport across tunneling junctions is mainly governed by direct tunneling (DT) at a low electric field through a trapezoidal barrier and followed by Fowler‐Nordheim tunneling (FNT) through a triangular barrier at a high electric field (see methods) [ 11 ] . The low bias conductance across QFEG/Ga 2 O 3 /SiC, which should be governed by the DT model, is not measurable in the forward bias.…”

“…Charge transport across tunneling junctions is mainly governed by direct tunneling (DT) at a low electric field through a trapezoidal barrier and followed by Fowler‐Nordheim tunneling (FNT) through a triangular barrier at a high electric field [ 11 ]…”

“…The increased η compared to literature [54] may be due to deviations from the Schottky-Mott theory or the non-idealities at the n-QFEG/Ga/SiC junction. [11] These non-idealities may arise from: 1) barrier inhomogeneities, where electrons can flow through the local shallow barriers leading to underestimation of the ϕ SB , 2) mixture of TE and thermionic field emission and/or field emission, 3) tunable E F of QFEG due to low density of states (DOS) near Dirac point, leading to enhanced charge transport at high voltages over the SB. [11] 2D Ga 2 O 3 intercalation at the QFEG/SiC interface significantly modulates the junction properties by transforming it from an Ohmic to a tunnel junction, reducing the current flow substantially across the junction.…”

“…The formation of 2D gallium oxide may offer a unique advancement for gate dielectrics and tunnel barriers in 2D electronics, [ 11,12 ] due to its large bandgap and breakdown electric field. [ 13 ] Although a few reports claim the absence of quantum confinement effects in 2D Ga 2 O 3 , [ 14,15 ] density functional theory (DFT) [ 16 ] predicts that 2D Ga 2 O 3 exhibits a thickness‐dependent bandgap (6.42 eV for monolayer and 5.54 eV for bilayer).…”