Dual-mode operation of 2D material-base hot electron transistors

Vertical hot electron transistors incorporating atomically-thin 2D materials, such as graphene or MoS2, in the base region have been proposed and demonstrated in the development of electronic and optoelectronic applications. To the best of our knowledge, all previous 2D material-base hot electron transistors only considered applying a positive collector-base potential (VCB > 0) as is necessary for the typical unipolar hot-electron transistor behavior. Here we demonstrate a novel functionality, specifically a dual-mode operation, in our 2D material-base hot electron transistors (e.g. with either graphene or MoS2 in the base region) with the application of a negative collector-base potential (VCB < 0). That is, our 2D material-base hot electron transistors can operate in either a hot-electron or a reverse-current dominating mode depending upon the particular polarity of VCB. Furthermore, these devices operate at room temperature and their current gains can be dynamically tuned by varying VCB. We anticipate our multi-functional dual-mode transistors will pave the way towards the realization of novel flexible 2D material-based high-density and low-energy hot-carrier electronic applications.

The 2D-HETs rely upon the vertical (e.g. out-of-plane) emission of hot electrons through an atomic-scale base region and the subsequent filtering of these hot electrons by a built-in potential energy barrier near the base-collector junction. However, in spite of these accomplishments, there is a dearth of insight into the actual out-of-plane transport (e.g. the dominant scattering mechanisms and the actual potential energy landscape) experienced by the hot electrons in these 2D-HETs [27][28][29] . As far as we know, all previous 2D-HETs operated under the application of a positive collector-base potential (V CB > 0) and thus were limited to a single functionality, namely the typical unipolar hot electron transistor behavior 20,21 . To augment the functionality of electronics such as multi-level cells for low footprint vertical transport-based memory applications, here we introduce an alternative and peculiar conduction mode of operation which we refer to as a dual-mode operation in our 2D-HETs upon application of either a positive collector-base potential (V CB > 0) or a negative collector-base potential (V CB < 0). Thus, our 2D-HETs can operate in either a hot-electron or a reverse-current dominating mode depending upon the particular bias configuration. The 2D-HETs operate at room temperature and their current gains can be dynamically tuned by varying V CB . Furthermore, we surmise that the current saturation-like behavior in the transfer characteristics of the MoS 2 -HETs when operated in the reverse-current dominating mode (V CB < 0) could serve as a multi-level cell (e.g. data storage) in future multi-functional 2D material-based high-density and low-energy hot-carrier electronic (e.g. vertical transport based logic and memory) applications.

Results
We demonstrate vertical transport 2D-HETs which exhibit a novel dual-mode operation by incorporating either monolayer MoS 2 (MoS 2 -HET) or monolayer graphene (G-HET) in the base region. The device structure of the 2D-HET is presented in Fig. 1a and a top-view optical micrograph of an actual MoS 2 -HET is shown in Fig. 1b. The three-terminal device consists of a degenerately-doped n ++ silicon substrate (N D ~ 10 19 cm −3 ) as the emitter (E), a monolayer of chemical vapor deposition (CVD) grown 2D material (e.g. either MoS 2 or graphene) as the base (B), and sputtered (~45 nm) ITO as the collector (C). A thermally grown thin (~3 nm) SiO 2 tunnel barrier separates the emitter and base terminals, whereas an atomic-layer deposited (~55 nm) HfO 2 separates the base and collector and serves as the filtering barrier. The detailed fabrication process is described in the Methods and in our previous work 22 . In this particular study, a common-base configuration was employed during the electrical measurements. Note that both of the base contacts are grounded during the electrical measurements in order to achieve a uniform potential distribution across the MoS 2 base region.
We first focus on describing the two modes of operation for the 2D-HETs using energy band diagrams in order to clearly understand the physics governing the device transport. The 2D-HET with monolayer MoS 2 as the base (MoS 2 -HET) will serve as an example. Figure 1c shows the energy band diagram for the off-state and the on-state conditions of the MoS 2 -HETs. In the absence of an applied V CB , most of the hot-electrons injected through the tunnel oxide have insufficient kinetic energy to overcome the filter barrier at the collector-base junction and do not reach the collector. Instead, they back-scatter and thermalize into the MoS 2 base region. However, the situation drastically changes with the application of a large V CB . There are two possible cases for the on-state condition of the MoS 2 -HETs, depending upon the polarity of the applied V CB . The first case describes the typical hot-electron injection behavior and occurs for V CB > 0. In this scenario, hot-electrons tunneling through the emitter-base tunnel oxide have sufficient kinetic energy to overcome the filter barrier, reach the collector, and contribute to the collector current (I C ). The second case describes a reverse-current behavior, which is a novel feature and mode of operation enabled by our 2D-HETs, and occurs for V CB < 0. In this scenario, the injected hot-electrons tunneling from the emitter do not have sufficient kinetic energy to surpass the raised filter barrier. Subsequently, these electrons are back-scattered and accumulate within the 2D material-base region which serves to suppress the base-collector reverse-current (I C ). Interestingly, Δ I C , which denotes the amount of change in the base-collector current due to the hot electron injection from the emitter, can be tuned with the applied V BE in this mode of operation. Figure 1d shows the common-base output characteristics of one of our MoS 2 -HETs. The collector current (I C ) is shown as a function of V CB at various V BE . It is evident that I C increases at a large positive V CB whereas I C is suppressed at a large negative V CB . Thus, by adjusting the polarity of V CB , it is possible to operate the 2D-HETs such that their collector current is mainly contributed by either hot-electrons originating from the emitter or electrons originating from the collector. Since Δ I C is the change in the collector current caused by the injection of the hot electron input current (e.g. an increasing magnitude of V BE > 0 modulates Δ I C ), it will be used instead of I C for the discussion of the dual-mode operation for the remainder of this report.
Hot-electron dominating mode of operation in the MoS 2 -HETs. We first characterize the MoS 2 -HET in the hot-electron dominating mode of operation by applying positive V CB . Figure 2a shows the energy band diagram depicting the conduction and valence band edges at the collector-base junction with a positive V CB applied. In this mode of operation, once hot-electrons tunneling through the emitter-base tunnel barrier have sufficient kinetic energy, they can vertically transport through the MoS 2 base region, surpass the filter barrier at the collector-base junction, and reach the collector. Consequently, an increasingly positive V CB will continue to effectively make the filter potential barrier thinner and promote hot-electrons reaching the collector due to an increase in their transmission probability. This qualitative behavior is exhibited in the input and transfer characteristics of the MoS 2 -HETs. The input characteristics (I E -V BE ) correspond to how the emitter current depends on V BE , whereas the transfer characteristics (I C -V BE ) correspond to the manner in which the collector current varies  with V BE . Figure 2b shows the input and transfer characteristics for one of MoS 2 -HETs. The emitter current (I E ) and the collector current (I C ) are shown as a function of V BE (V BE was swept from 0 to + 3 V) at a V CB of + 2 V. Both currents rapidly increase at larger V BE , as is typical for HETs. From the input and transfer characteristics, the common-base current gain (α ) of this device can be determined, which is a figure of merit for HETs and is defined as α = I C /I E . For this particular device and biasing condition of V CB = + 2 V and V BE = + 3 V, α is about 0.81, which implies that at least 80% of the injected hot-electrons ballistically traverse the single-layer MoS 2 base region at room temperature. Further details concerning the hot-electron dominating mode of operation in the MoS 2 -HETs is mentioned in our previous work 22 .
Reverse-current mode of operation in the MoS 2 -HETs. Shifting from the hot-electron dominating mode of the MoS 2 -HET, we next investigate the device characteristics operating under the reverse-current dominating condition. Figure 3a shows the energy band diagram depicting the reverse-current mode of operation for the MoS 2 -HET. Specifically, Fig. 3a shows the conduction and valence band edges at the collector-base junction with a negative V CB applied. In this mode of operation, the increasingly negative V CB drives more and more electrons to flow from the degenerately n-doped ITO conduction band, past the filter barrier and into the base region, thus forming the reverse-current. With the injection of hot-electrons from the emitter, the continuously increasing filter barrier (V CB < 0) causes these hot-electrons to have insufficient kinetic energy to reach the collector and thus they back-scatter into the MoS 2 base region. Consequently, these back-scattered electrons build up in the MoS 2 base region which cause a deficiency in the available density of states in the MoS 2 and thereby decrease or suppress the reverse-current flowing into the base region from the degenerately n-doped ITO conduction band. The change or modulation in the reverse base-collector current (I C ) caused by the injected hot-electrons from the emitter into the MoS 2 base region is denoted as Δ I C . This reverse base-collector current (Δ I C ) can be modulated with V BE by tuning the amount of hot-electrons that are injected from the emitter and which eventually build up in the MoS 2 base region. In the reverse-current mode of operation (V CB < 0), we define the effective current gain: α * = |Δ I C |/|I E | as the ratio of the measured reverse base-collector current to the injected hot-electron emitter  Figure 3b shows the input and transfer characteristics for one of the MoS 2 -HETs biased at three different V CB (V CB = 0, − 8, and − 10 V). It is evident that both the emitter and collector currents increase with larger negative V CB . Similarly, Fig. 3c shows a family of transfer characteristics, with the suppressed reverse base-collector current (Δ I C ) as a function of V BE shown for various negative V CB . The transfer characteristics of the MoS 2 -HET when operated in the reverse-current mode (V CB < 0) are peculiar in that the reverse base-collector current tends to saturate with increasing V BE . Additionally, the reverse-current magnitude increases with larger negative V CB bias. We speculate that this novel current saturation-like behavior could serve as a multi-level cell for low footprint vertical transport-based memory [30][31][32][33][34][35] applications in the future. As an example, consider biasing the 2D-HET at V BE = + 3 V (e.g. the highest hot electron injection current to avoid dielectric breakdown of the tunnel barrier). We can vary the steady-state reverse-current (Δ I C ) by setting V CB < 0 to various values. Based on Fig. 3c, we can address distinguishable (Δ I C ) charge states for V CB from − 6 V to − 10 V and thus encode at least 4 states for a minimum of a 2-bit memory cell. Multi-level cells are memory units capable of storing more than one bit of information and thus can result in lower cost per unit of storage and higher data storage density. Furthermore, it was recently shown that cheaper multi-level cell flash drives used in practice are just as reliable as more expensive single-level cells 36 . Thus, our dual-mode 2D-HETs may find opportunities as ultra-dense multi-functional logic/memory units. From the input and transfer characteristics, we can next ascertain the effective current gain (α *) of this device for the reverse-current dominating mode of operation. Figure 3d shows α * for this MoS 2 -HET as a function of V BE at V CB = − 10 V. Such a large negative V CB significantly raises the filter barrier height for the injected hot-electrons originating from the emitter, which causes them to back-scatter into the MoS 2 base region and build up, leading to the effective suppression of the reverse base-collector current. Similar to the hot-electron dominating mode of operation in our previous paper 22 , it is evident that α * exhibits a nearly constant characteristic at all V BE with a value of at least 90% for this particular MoS 2 -HET biased at V CB = − 10 V. The inset of Fig. 3d shows a family of α * characteristics as a function of V BE at several negative V CB (V CB = − 4, − 6, − 8, − 9, and − 10 V). The effective current gain, α *, increases with negative V CB and exhibits a nearly constant characteristic throughout the entire V BE range with a magnitude of about 94% at V CB = − 10 V.

Output characteristics and tunable current gain in the MoS 2 -HETs.
With the analysis of the input and transfer characteristics complete, we now investigate the common-base output characteristics of the MoS 2 -HETs, which correspond to how the output collector current depends on V CB . In order to clearly present the dual-mode operation of our MoS 2 -HETs, the base-collector leakage current when V BE = 0 was subtracted from the measured collector current. Figure 4a shows the common-base output characteristics for one of the MoS 2 -HETs. The collector current is shown as a function of V CB at three positive V BE biases. The dual-mode operation is evident as the device is biased in either the hot-electron (V CB > 0) or the reverse-current (V CB < 0) dominating mode of operation. Above a critical electric field across the HfO 2 , the collector current is quite sensitive to modulation and rapidly increases with a further increase in V CB for both cases of V CB > 0 and V CB < 0. Based on Fig. 4a, the on-off current ratio (I ON /I OFF ) is about 140 when V CB = − 10 V and V BE = + 3 V, whereas I ON /I OFF ~ 125 when V CB = + 10 V and V BE = + 3 V. In order to convey the robust and dual-mode operation of our MoS 2 -HETs, Fig. 4b shows a semi-log plot of the current gain as a function of V CB at positive V BE = + 3 V, which is biased in both the hot-electron (α ; V CB > 0) and the reverse-current (α *; V CB < 0) dominating modes of operation. The effective current gain, α *, increases with larger negative V CB as a result of a suppression in the reverse-current and reaches a very high-current gain with a value of about 90% at V CB = − 9 V. It is evident that α * can be tuned around two orders of magnitude by varying V CB . A similar dependence of α on V CB for the hot-electron dominating case exists as shown in the right portion of Fig. 4b. In the hot-electron dominating mode of operation (V CB > 0), α increases with an increasingly positive V CB and can be tuned over an order of magnitude since this lowers the filter potential barrier experienced by the hot-electrons and allows them to reach the collector.
Output characteristics and tunable current gain in the graphene-HETs. Furthermore, in order to demonstrate the novel dual-mode operation enabled by our 2D material-base hot electron transistors, we shall now investigate our G-HETs. Figure 5a shows the common-base output characteristics in one of our G-HETs, which is biased in both modes of operation. The collector current is shown as a function of V CB at two positive V BE biases. It is evident that a dual-mode operation is also observed in the G-HETs as the devices are biased in either the hot-electron (V CB > 0) or the reverse-current (V CB < 0) dominating modes of operation. Based on Fig. 5a, the on-off current ratio (I ON /I OFF ) is about 3 when V CB = − 10 V and V BE = + 2 V, whereas I ON /I OFF ~ 2 when V CB = + 10 V and V BE = + 2 V. The current gain (α ) increases with larger positive V CB as a result of a reduction of the filter barrier in the hot-electron dominating process, whereas the effective current gain (α *) increases with larger negative V CB due to suppression in the reverse base-collector current. Additionally, we observed that the current gain can be tuned by varying V CB as shown in Fig. 5b. Hence, by biasing either the MoS 2 -HETs or G-HETs in the common-base configurations, we have explicitly shown the existence of a dual-mode operation and a tunable current gain in our new class of 2D material-base hot electron transistors. Nevertheless, the profiles of both the collector current and the current gain in the output characteristics of the MoS 2 -HET and the G-HET are quite different. At this time, not much is known of the actual out-of-plane transport (e.g. the dominant scattering mechanisms and the actual potential energy landscape) experienced by the hot electrons in these 2D-HETs [27][28][29] . What we do know is that these are two very different materials (e.g. feature different conduction band offsets, etc.). Monolayer graphene lacks a bandgap and features a linear dispersion relation, whereas monolayer MoS 2 has a direct bandgap and features a parabolic dispersion relation at the K and K' points in the Brillouin zone. Furthermore, the effective mass of the electrons travelling perpendicular to the graphene was predicted to be ~ 25-30 m o in a seminal paper 37 . A few reasons for the particularly low current gain in the G-HET compared to the MoS 2 -HET, may be due to the fact that the graphene-HfO 2 interface features a much higher filter barrier height (e.g. 2.05 eV) compared to that of the MoS 2 -HfO 2 interface (e.g. 1.52 eV) as well as the possibility of more prevalent acoustic phonon scattering near the base-collector junction for graphene than for MoS 2 . Clearly, further investigations into the out-of-plane transport among different 2D materials and their contact with bulk dielectrics will greatly benefit future device optimization. In the mean time, further improvement in the device performance of the 2D-HETs will be directed towards increasing the injected tunneling current density to a more suitable level for practical applications. This can be achieved via fine tuning of the thickness, barrier height, and uniformity of the tunnel barrier, implementation of bilayer insulator tunnel barrier 38 , as well as lowering the contact resistance between the 2D material and the metallic contact leads (e.g. via chemical doping 39 or 1D edge contact to 2D materials 40 ).

Summary
In conclusion, we have demonstrated a novel vertical dual-mode 2D material-base hot-electron transistor (2D-HET) incorporating either monolayer MoS 2 (MoS 2 -HET) or monolayer graphene (G-HET) in the base region. This new class of 2D-HETs can operate in either a hot-electron or a reverse-current dominating mode depending upon the particular bias configuration. For the hot-electron dominating mode of operation (V CB > 0), once the hot-electrons tunneling through the emitter-base tunnel barrier have sufficient kinetic energy, they can vertically transport through the 2D material base region, surpass the filter barrier at the collector-base junction, and reach the collector. For the reverse base-collector current dominating mode of operation (V CB < 0), the continuously increasing filter barrier precludes the injected hot-electrons from having sufficient kinetic energy to reach the collector, hence they back-scatter into the 2D material base region. Consequently, these back-scattered electrons build up in the 2D material base region, which induces a deficiency in the available density of states in the 2D material, thereby reducing the reverse base-collector current. Furthermore, these 2D-HETs operate at room temperature and their current gains can be dynamically tuned by varying V CB . This dual functionality is enabled by incorporating 2D materials in the base region of the HET structure and by varying the polarity of V CB . We anticipate our transistors will pave the way towards the realization of novel flexible 2D material-based high-density and low-energy hot-carrier electronic applications.

Methods
In this work, we commenced the fabrication process with a 100 mm degenerately-doped n ++ (N D ~ 1 × 10 19 cm −3 ) silicon wafer and performed a standard LOCal Oxidation of Silicon (LOCOS) procedure in order to define arrays of active areas for the 2D material-HETs, which were isolated from each other by a 300 nm thick SiO 2 field oxide. With the silicon surface of the active areas exposed, we then thermally grew a thin ~3 nm SiO 2 tunnel oxide. Afterwards, we transferred either a large-area CVD monolayer MoS 2 or graphene on top of the substrate (e.g. SiO 2 field oxide) so that the particular 2D material covered several arrays of active areas of the 2D material-HETs. The large-area monolayers of MoS 2 and graphene were grown using CVD methods 41,42 and transferred onto the substrates using PMMA transfer methods 43,44 . Subsequently, a photolithography step was performed to mask circular regions of the 2D material covering the active areas. The 2D material outside of the active regions was etched in order to isolate the various devices. The 2D material area of each device is about 8 × 10 4 μ m 2 . A second photolithography step was performed in order to pattern and deposit the base contacts (20 nm thick Ti/100 nm thick Au for MoS 2 -HETs or 20 nm thick Cr/100 nm thick Au for G-HETs). A 1 nm thick Ti seed layer was evaporated on top of the 2D material and naturally oxidized in air, followed by atomic layer deposition (ALD) of a 55 nm thick HfO 2 as the filtering barrier. A third photolithography step was performed in order to define a central circular top-gate (e.g. collector) region which encompasses the entire active area of the 2D material-HETs. We then RF sputtered 45 nm of ITO at room temperature into this circular region followed by lift-off. Finally, a fourth photolithography step was performed in order to pattern and deposit the side electrodes (150 nm thick Al/50 nm thick Au) on top of the filtering barrier dielectric. These metallic side electrodes intimately contact the central ITO collector region and allow for easy probing and biasing of the 2D material-HETs. Electrical measurements were performed with a Keithley 4200 Semiconductor Characterization System. All measurements were performed in air and at 300 K. The leakage current was subtracted for all of the data presented in the main text. Specifically, the base-collector leakage current when I E = 0 was subtracted from the measured collector current when biased in the common-base configuration.