Realization of High Current Gain for Van der Waals MoS₂/WSe₂/MoS₂ Bipolar Junction Transistor

Abstract

Two-dimensional (2D) materials have attracted great attention in the past few years and offer new opportunities for the development of high-performance and multifunctional bipolar junction transistors (BJTs). Here, a van der Waals BJT based on vertically stacked n⁺-MoS₂/WSe₂/MoS₂ was demonstrated. The electrical performance of the device was investigated under common-base and common-emitter configurations, which show relatively large current gains of α ≈ 0.98 and β ≈ 225. In addition, the breakdown characteristics of the vertically stacked n⁺-MoS₂/WSe₂/MoS₂ BJT were investigated. An open-emitter base-collector breakdown voltage (BV_CBO) of 52.9 V and an open-base collector-emitter breakdown voltage (BV_CEO) of 40.3 V were observed under a room-temperature condition. With the increase in the operating temperature, both BV_CBO and BV_CEO increased. This study demonstrates a promising way to obtain 2D-material-based BJT with high current gains and provides a deep insight into the breakdown characteristics of the device, which may promote the applications of van der Waals BJTs in the fields of integrated circuits.

1. Introduction

Since the discovery of graphene in 2004, a large number of two-dimensional (2D) atomic crystals, such as black phosphorus (BP), transition metal dichalcogenides (TMDs), and hexagonal boron nitride (h-BN), have been synthesized and extensively studied owing to their unique physical and chemical properties [1,2,3,4,5,6]. 2D materials have atomically thin thicknesses, which can be used to fabricate ultrathin and lightweight devices. Moreover, their dangling-bond-free surfaces make 2D layered materials very suitable to fabricate van der Waals heterojunctions as well as more complex layered architectures without considering the crystal lattice mismatch [7,8]. These fascinating properties make 2D materials, as well as their van der Waals heterostructures, have great application potential in high-performance photodetectors [9,10], memorizer [11,12], biosensors [13], gas sensors [14], and synaptic devices [15].

A bipolar junction transistor (BJT) is a two-junction, three-terminal semiconductor device that is widely used for signal amplification [16]. It is composed of three separately doped regions, namely, the emitter, base, and collector. The emergence of two-dimensional materials provides a platform for studying the electrical properties of BJTs with ultra-thin base regions, as the thickness of the base region has a great influence on device performance. In the past few years, several BJTs fabricated from 2D materials have been demonstrated with n-p-n [17] and p-n-p [18,19] configurations. Their electrical performance, as well as their potential applications such as photodetection [20,21] and biosensing [22], have been extensively studied. However, further improving the current gain and investigating the device breakdown behavior are still worthy of further research. A high-quality BJT usually consists of several components, such as asymmetric doping concentration, ultrathin base thickness, and high-quality contacts between semiconductors and metal electrodes. By systematically optimizing the structural design of the van der Waals BJT, there is still room to further improve the electrical performance of the device.

In this study, we present a vertically stacked n⁺-MoS₂/WSe₂/MoS₂ (n⁺-p-n) BJT. A re-doped MoS₂ flake and an undoped MoS₂ sheet were employed as the emitter and collector materials, respectively, to improve the current gain [22] and the breakdown voltage [23,24] of the van der Waals BJT. Also, we adopted appropriate n⁺-MoS₂ and MoS₂ thicknesses to minimize the impact of the Schottky barrier at the interface between 2D materials and metal electrodes. Relatively large current gains of α ≈ 0.98 and β ≈ 225 can be observed under common-base and common-emitter configurations. Additionally, the open-emitter base-collector breakdown voltage (BV_CBO) and the open-base collector-emitter breakdown voltage (BV_CEO) of the vertically stacked n⁺-MoS₂/WSe₂/MoS₂ BJT under different temperatures were systematically explored, in which a positive temperature coefficient with the breakdown voltage can be observed.

2. Materials and Methods

2.1. Materials

The pristine MoS₂ and WSe₂ bulk crystals were purchased from HQ Graphene Company (Groningen, The Netherlands). The re-doped MoS₂ bulk crystal was purchased from 2D Semiconductors (Scottsdale, AZ, USA).

2.2. Device Fabrication

The fabrication procedure of the vertically stacked n⁺-MoS₂/WSe₂/MoS₂ BJT is illustrated in Figure 1. Firstly, a multilayer MoS₂ flake was mechanically exfoliated onto the SiO₂/Si (thickness of 300 nm and 525 μm, respectively) substrate by using scotch tape. Secondly, with the help of the micro-manipulator, a multilayer WSe₂ flake stamped on polydimethylsiloxane (PDMS) film was transferred onto the previously prepared MoS₂ flake to form a MoS₂/WSe₂ heterostructure at the overlapped region. Thirdly, through the same dry transfer process, a re-doped multilayer MoS₂ flake was transferred onto the MoS₂/WSe₂ heterostructure to create a n⁺-MoS₂/WSe₂/MoS₂ vertically stacked configuration. Then, the contact patterns were precisely defined using mask-less lithography, after which the Cr/Au electrodes (10 nm/100 nm) were deposited on the top of the TMD materials by thermal evaporation. Finally, annealing was performed at a temperature of 573 K for 2 h to remove the residual organic matter attached to the material interfaces.

2.3. Characterization

The composition and height profile of the vertically stacked n⁺-MoS₂/WSe₂/MoS₂ BJT were characterized by Raman spectroscopy (In Via Reflex, Renishaw, Wotton-under-Edge, Gloucestershire, UK) with a 532 nm laser and atomic force microscopy (AFM) (NTEGRA Spectra, NT-MDT, Moscow, Russia), respectively. The temperature-dependent electrical characteristics of the device were carried out using a thermal system (ETC 200L, ESPEC, Osaka, Japan) and a semiconductor parameter analyzer (B1500A, Agilent Technologies, Santa Clara, CA, USA).

3. Results and Discussion

The vertically stacked n⁺-MoS₂/WSe₂/MoS₂ BJT was fabricated by a controlled multistep dry transfer process. Here, the bottom MoS₂ (top n⁺-MoS₂) flake was determined as the collector (emitter) material, and the middle WSe₂ flake was used as the base material. Figure 2a illustrates the Raman spectra of the individual materials and the heterojunction area. The blue line shows the Raman spectrum of the bottom MoS₂ flake, in which the peaks at 384.3 cm⁻¹ and 409.1 cm⁻¹ can be observed [25]. For the WSe₂ flake, it presents two Raman peaks centered at 249.8 cm⁻¹ and 258.1 cm⁻¹, consistent with previous works [26,27,28]. In addition, the Raman peaks in the MoS₂/WSe₂ overlapped region are consistent with the individual MoS₂ and WSe₂ sheets, indicating the successful preparation of the high-quality van der Waals heterostructure [29,30]. It should be noted that an appropriate increase in the thickness of the MoS₂ and n⁺-MoS₂ can improve the contact quality between 2D materials and metal electrodes, as shown in Figures S1 and S2 of the Supplementary Materials. Therefore, relatively thick MoS₂ (≈16.1 nm) and n⁺-MoS₂ (≈17 nm) were employed as the collector and emitter regions, respectively. The thickness of WSe₂, about 7.4 nm, was also characterized using AFM, as shown in Figure 2b.

To confirm the n-type and p-type behavior of the individual 2D materials, the transfer characteristics of the MoS₂, WSe₂, and re-doped MoS₂ flakes were investigated, as shown in Figure S3 of the Supplementary Materials. The pristine MoS₂ and re-doped MoS₂ show n-type behaviors with threshold voltages of around −4 V and −22.5 V, respectively, indicating the electron density of around 3.1 $\times$ 10¹¹ cm⁻² and 1.73 $\times$ 10¹² cm⁻², respectively. A p-type behavior can be observed in the WSe₂ flake. Therefore, the formation of the n-p-n heterostructure is confirmed. The rectification properties of the base-emitter and base-collector junctions were also characterized, as shown in Figure S4 of the Supplementary Materials. Negative differential resistance (NDR) effects were observed in the I–V curves of the base-emitter and base-collector junctions, which may be attributed to band-to-band tunneling [31].

The electrical performance of the vertically stacked n⁺-MoS₂/WSe₂/MoS₂ BJT was first characterized under a common-base configuration. Here, the electrical measurements were performed in an ambient and dark environment. Figure 3a shows the schematic illustration of the electrical connection. The base-emitter junction was forward-biased, whereas the base-collector junction was reverse-biased. The base was connected to the ground. Figure 3b shows the input characteristics of the device, where the I–V curves of the emitter current (I_E) versus the base-emitter voltage (V_BE) at various fixed collector-emitter voltages (V_CB) are performed. As the V_BE increases, more electrons can inject from the emitter to the base region owing to the lower barrier height at the base-emitter junction, leading to an increase in I_E. Figure 3c shows the dependence of the collector current (I_C) on the V_CB at various fixed V_BE values, namely the output characteristics of the device. At a small value of V_CB (V_CB < 1 V), only a portion of the electrons injected from the emitter into the base region can be successfully transmitted to the collector region. The increase in V_CB promotes the collector’s ability to drain electrons in the base region. Therefore, I_C increases approximately linearly with V_CB. When V_CB is larger than 1 V, most of the electrons are injected into the collector region under the reverse bias of the base-collector junction, becoming the main component of the collector current. At this time, the value of I_C nearly becomes unaffected by changing V_CB. The V_BE becomes the main factor affecting the collector current. Figure 3d shows the I_C, I_E, and the corresponding common-base current gain (α) as a function of V_BE at V_CB = 5 V. Both I_C and I_E increase exponentially with the increase in V_BE. From the I–V curves of I_C and I_E, the corresponding gain α (α = I_C/I_E) can be calculated to be approximately 0.98.

Then the performance of the vertically stacked n⁺-MoS₂/WSe₂/MoS₂ BJT was investigated under a common-emitter configuration. Figure 4a shows the schematic illustration of the electrical connection. Figure 4b shows the input characteristics of the device, where the I–V curves of the base current (I_B) versus V_BE at various fixed collector-emitter voltages (V_CE) are performed. As the V_BE increases, the base current increases exponentially. Additionally, room-temperature NDR effects are observed in Figure 4b (red circle). As the V_BE increases, I_B initially increases but then decreases and finally gains a second increase in voltage, resulting in a negative slope on the current–voltage curve. This phenomenon may be attributed to lateral band-to-band tunneling, which is consistent with the previously reported performance of MoS₂/WSe₂ heterojunction [31]. The corresponding band diagram is shown in Figure S5 of the Supplementary Materials. Figure 4c shows the dependence of V_CE on the collector current (I_C) under several fixed V_BE values, which is called the output characteristic of the device. At a small value of V_CE (V_CE < 1.8 V), the collector current increases approximately linearly with V_CE, which is defined as the saturation mode of the BJT. Above the saturation region, I_C is mainly dependent on V_BE, indicating that the van der Waals BJT is operating in the active region. The difference between I_B and I_C with a fixed V_BE of 2 V is illustrated in Figure 4d. For a BJT operating under the common-emitter configuration, the current gain (β) can be calculated by taking the ratio of I_C and I_B [32]. A maximum common-emitter current gain of approximately 225 is obtained at V_CE = 6 V. The performance of the n-p-n BJTs fabricated by 2D materials is compared and listed in

Table 1. Summary of the performance of this work and recently reported BJT prepared by 2D materials [16,22,33,34,35].

Material	Structure	Type	α	β	Ref.
n+-MoS2/WSe2/MoS2	Vertical	npn	0.98	225	This work
MoTe2/GeSe/MoTe2	Vertical	npn	0.95	29.3	[22]
MoS2/WSe2/MoS2	Vertical	npn	0.97	12	[34]
WS2/GeSe/WS2	Vertical	npn	1.11	20.7	[35]
MoS2/WSe2/MoS2	Lateral	npn	/	3	[33]
MoS2/BP/MoS2	Lateral	npn	0.98	41	[16]

[16,22,33,34,35]. Our van der Waals BJT exhibits a relatively high common-emitter current gain.

Furthermore, based on the prepared van der Waals BJT with excellent characteristics, the breakdown characteristics of the device were investigated. Figure 5a shows the open-emitter base-collector breakdown characteristics of the device at room temperature. In this case, the collector was set to ground, a reverse bias was applied to the base-collector junction, and the emitter electrode was set to an open state. At a reverse bias of <25 V, only a small reverse dark current of around 0.14 nA exists. As the reverse bias is increased beyond 25 V, a breakdown occurs, leading to a rapid increase in I_CBO. The critical voltage (V_cr) is determined to be 25 V (the gray line) when the dark current begins to increase rapidly [36]. The multiplication factor (M) can be calculated by taking the ratio of I_CBO and I_s, where I_s represents the current at 25 V. Measurements of carrier multiplication M in junctions near breakdown lead to the following empirical equation [37]:

$$M=\frac{1}{1-{(V/V_b)}^n}$$

(1)

where n represents the ionization index and V_b is a fitting parameter. Equation (1) can be rewritten as follows:

$$\ln (1-1/M)=n\ln (V/V_b)$$

(2)

suggesting a linear dependence of ln(1−1/M) on ln(V) in the avalanche carrier multiplication effect. Figure 5b shows the relationship between ln(1−1/M) and ln(V), which directly indicates the occurrence of the avalanche breakdown. In addition, the avalanche breakdown voltage is sensitive to the temperature because a higher electric field is needed to compensate for the energy loss due to electron-phonon scattering as the temperature increases [38]. Therefore, a positive temperature coefficient of the breakdown voltage can be observed in the avalanche carrier multiplication effect [39,40]. Figure 5c shows the open-emitter base-collector breakdown characteristics of the device at different temperatures from 280 K to 340 K. The relationship between the BV_CBO (defined as the voltage at which an I_CBO of 0.1 μA is measured) and temperature is consistent with the theoretical expectation (Figure 5d).

The open-base collector-emitter breakdown characteristics of the vertically stacked n⁺-MoS₂/WSe₂/MoS₂ BJT under room temperature conditions were also investigated. The I–V curve and the schematic illustration of the electrical connection are shown in Figure 6a. The I_CEO increases slowly at first and then rapidly increases when the V_CE is beyond V_cr2 (=35 V), indicating that a breakdown occurs in the van der Waals BJT. The open-base collector-emitter breakdown characteristics of the van der Waals BJT at different temperatures from 280 K to 320 K were also investigated, as shown in Figure S6 of the Supplementary Materials. The BV_CEO (defined as the voltage at which an I_CEO of 5 μA is measured) increases with the operating temperature, indicating a positive temperature coefficient of the breakdown voltage, as shown in Figure 6b.

4. Conclusions

In summary, a vertically stacked n⁺-MoS₂/WSe₂/MoS₂ BJT was fabricated by a controlled multistep dry transfer process. The electrical characteristics of the device were investigated, which show relatively large current gains (α ≈ 0.98 and β ≈ 225) under common-base and common-emitter configurations. Additionally, to allow safe measurements at practical current densities, the temperature-dependent breakdown characteristics of the van der Waals BJT were investigated. A BV_CBO of 52.9 V and a BV_CEO of 40.3 V were obtained at room temperature. With the increase in the operating temperature, the breakdown voltage increases. This work provides a promising way to improve the electrical performance of the van der Waals BJT through proper architecture design and systematically demonstrates the temperature-dependence breakdown characteristics of the device, which might be helpful for the applications of the 2D-material-based BJT in the fields of integrated circuits.