Bipolar Junction Transistors in Two-Dimensional WSe2 with Large

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Letter pubs.acs.org/NanoLett

Bipolar Junction Transistors in Two-Dimensional WSe2 with Large Current and Photocurrent Gains Pratik Agnihotri, Prathamesh Dhakras, and Ji Ung Lee* Colleges of Nanoscale Science and Engineering, SUNY-Polytechnic Institute, Albany, New York 12203, United States S Supporting Information *

ABSTRACT: In the development of semiconductor devices, the bipolar junction transistor (BJT) features prominently as being the first solid state transistor that helped to usher in the digital revolution. For any new semiconductor, therefore, the fabrication and characterization of the BJT are important for both technological importance and historical significance. Here, we demonstrate a BJT device in exfoliated TMD semiconductor WSe2. We use buried gates to electrostatically create doped regions with back-to-back p−n junctions. We demonstrate two central characteristics of a bipolar device: current gain when operated as a BJT and a photocurrent gain when operated as a phototransistor. We demonstrate a current gain of 1000 and photocurrent gain of 40 and describe features that enhance these properties due to the doping technique that we employ. KEYWORDS: Bipolar junction transistor, transition metal dichalcogenides, two-dimensional semiconductors, current gain, photocurrent gain

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The technique we employ allows reconfigurability to create either an n−p−n or a p−n−p BJT device, with variable doping density that is difficult to achieve in bulk processing techniques. Here, we achieve doping by using electrostatic gating techniques. The principle of electrostatic doping was first demonstrated in a carbon nanotube to create a reconfigurable p−n diode along individual nanotubes.15 There, two gates in a coplanar configuration that are biased with opposite voltages, to create a p-type region on one side and an n-type region on the other side of the device. This technique has been extended to other nanostructured materials, including graphene16,17 and TMD semiconductors.11,18 To create a BJT device, we incorporate a third gate, as we show in Figure 1A, to create back-to-back diodes. The details of the gate and device fabrication are discussed in Supporting Information (SI). The source (S) and drain (D) make contacts to the two ends of the device, while a narrow base (B) makes contact with the side of WSe2 to prevent shunting of the base current. The S and D terminals are equivalent to the emitter and collector in a standard BJT terminology, respectively. When we operate the device as a phototransistor, the base is left floating. In addition to the contacts to the WSe2, three gates labeled G1, G2, and G3 define the three doped regions of a BJT. The polarity of gate voltage determines whether an n- or p-type region forms. The width of the base contact ranges from 50 to 70 nm and is made narrower than the width of G2, which is 100 nm. We show in

he invention of the bipolar junction transistor (BJT) as the first solid state transistor helped to usher in the digital revolution.1 It is a three-terminal device formed by connecting two p−n junction diodes in a back-to-back configuration and is a critical component in many analog, digital, and sensor applications. The central feature of a BJT is current gain.2 When the center contact is absent, one can achieve photocurrent gain where the light replaces the function of the middle contact. Transition metal dichalcogenides (TMDs) have attracted great attention because they can be naturally thin,3 an important consideration in modern transistors,4,5 and several novel devices have already emerged.6−8 In the single-layer form they are direct bandgap materials, which are necessary for optoelectronic devices.9−12 Here, we demonstrate the BJT device in the exfoliated TMD semiconductor WSe2 with both current and photocurrent gains. In addition, we discuss specific features that enhance these gains from the doping technique we employ. Nearly all semiconductor devices rely on the p−n junction. One of the most difficult techniques to achieve with any new semiconductor is doping. In bulk semiconductors, p−n junctions are formed by carefully incorporating substitutional impurities. It is a critical part of any device fabrication that imparts a semiconductor to be either electron-conducting (ntype) or hole-conducting (p-type). Such techniques, however, are difficult to implement in nanostructured semiconductors, dating back to the discovery of carbon nanotubes.13,14 Here, we demonstrate the fundamental principles of the BJT device in the absence of such doping techniques. © XXXX American Chemical Society

Received: April 6, 2016 Revised: June 20, 2016

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Figure 1. BJT device construction and ambipolar conduction. (A) Schematic and (B) false-colored SEM image of the BJT device. G1, G2, and G3 are gates used to dope the device electrostatically to create either an n−p−n or p−n−p BJT. Source and drain electrodes contact the two ends of WSe2, while a narrow base electrode makes contact with the side to prevent shunting of the base current. In B, the three buried gates are visible under the metal contacts. The thickness of the flake is between 3 and 6 nm, as measured by AFM (see SI Figure S1). (C) Transfer curve (IDS vs VG [VG1 = VG2 = VG3]) with floating base shows ambipolar conduction.. The inset shows the diode I−V curves (IDS vs VDS) for a p−p−n (VG1 = VG2 = −2 V, VG3 = 2 V) and an n−p−p (VG1 = 2 V, VG2 = VG3 = −2 V) configurations.

Figure 1B a false-colored scanning electron micrograph (SEM) image of the fabricated device. There, one can observe the faint images of the buried gates. Below, we show representative data from over dozen devices. The critical feature of electrostatic doping is ambipolar conduction. In an ambipolar conduction, a contact injects either electrons or holes, depending on the polarity of the gate. We demonstrate ambipolar conduction between S−D contacts by plotting the transfer curve in Figure 1C. Large currents flow when all the gates are biased identically to allow electron conduction (VG1,2,3 > 0) or hole conduction (VG1,2,3 < 0). With the ambipolar conduction, we show two diode characteristics in the inset of Figure 1C by biasing the gates to create p−p−n and n−p−p diodes. The rectifying characteristics are consistent with the two diode polarities, confirming that Schottky tunnel barriers are small. In Supporting Information (Figure S2), we demonstrate the individual diode characteristics that make up an n−p−n BJT by measuring the current−voltage (I−V) characteristics of S−B and B−D electrodes. These diodes can be described by a diode equation with an ideality factor n > 1, which indicates that defect states mediate some of the recombination and generation processes. Despite having nonideal diodes, we demonstrate substantial gains in our devices. We begin by discussing the BJT. The band diagram labeled “dark” in Figure 2A results when gates are biased to form an n− p−n BJT. From here on, Junction 1 (J1) refers to the junction that forms between S−B terminals, and Junction 2 (J2) refers to the junction that forms between B−D terminals. In our analysis, we keep the base contact at ground, which is similar to

Figure 2. Band structure of an n−p−n bipolar junction and a phototransistor that achieves current gain and photocurrent gain, respectively. (A) Energy band diagram of an n−p−n (VG1 > 0 VG2 < 0 VG3 > 0) configuration with VSD = 0 in dark (black) and under illumination (red) with base floating. Holes generated by light (green open circles) collect in the base region and lower the potential energy barrier by ΔE. (B) The device in A operated as a BJT with J2 reverse biased (VDB > 0) and J1 forward biased (VSB < 0). (C) The device in A operated as a phototransistor with base floating, which results in a virtual ground. Left band diagram is for VD < 0, and J2 is forward biased. Right band diagram is for VD > 0, and neither J1 nor J2 is forward biased.

how the device operates as a phototransistor when the base is left to float and allows a direct comparison between the two devices. The central feature of a BJT is current gain. The current gain is defined as β = ID/IB.2 It is a measure of the efficiency of injected carriers from the forward bias of J1 to cross the base without recombining. These injected carriers become minority B

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Figure 3. Bipolar junction characteristics. (A) I−V characteristics showing currents from all three terminals (|ID|, |IS|, and |IB|) for an n+−p−−n BJT (VG1 = 2.5 V, VG2 = −0.2 V, VG3 = 1.5 V). VS = 0 V (black), VS = −0.1 V (blue), and VS = −0.3 V (red). For VS = 0 V, IB changes sign at VD = 0. For VS < 0, IB is positive throughout as we enter a different regime of transport due to a steady injection of holes. (B) Map of BJT current gain β with varying VS and VG2 determined at VD = 0.2 V. (C) Taking the mean of data from B (black triangles), we show β ∝ 1/|VG2|3 (red line) with a = 0.11 and b = 0.031. This dependence on VG2 is unique to our device construction. The inset shows the linearity in ID vs IB at VG2 = −0.2 V.

carriers in the base. To maximize β, the doping on S has to be large, and the base width has to be narrower than the minority carrier diffusion length. To demonstrate gain, we forward bias J1 and reverse bias J2, which maximizes the collection of carriers that are injected into the base. With the base at ground, the corresponding bias configurations are VS < 0 and VD > 0 for an n−p−n BJT. In Figure 2B, we show the resulting band diagram under these bias conditions. With J1 under forward bias, electrons from S are injected into the base, becoming minority carriers. In an ideal BJT, all of the minority carriers in the base diffuse through it and are collected at the D terminal. In reality, some of the minority carriers recombine and give rise to a recombination current in the base, as we show in Figure 2B. In a properly designed BJT, this is the dominant contribution to IB. To better assess the working principles of our devices, we sweep VD while maintaining a forward bias of J1. We show in Figure 3A the currents at all three terminals for several values of VS, as we sweep VD. The gate leakage characteristics are below our measurement limit and do not contribute to the terminal currents. The device is configured as an n+−p−−n BJT, which we achieved by using VG1 = 2.5 V, VG2 = −0.2 V, and VG3 = 1.5 V. We start by discussing the case when VS = 0, noting that the base is also at ground. We observe that all three currents (IS, ID, and IB) follow a diode-like behavior as a function of VD. This is expected since J2 determines the I−V characteristics as VD is swept from reverse (VD > 0) to forward (VD < 0) bias. IB changes sign at VD = 0, as signified by a dip there in the log plot. Here, IB and ID are due to the standard recombination (VD < 0) and generation (VD > 0) currents of a p−n diode, except that not all the recombination and generation occur in the base since it is narrow. On the other hand, with VS < 0, IB maintains the same sign throughout the bias range as we enter a different regime of

transport. With VS = 0, we already observe some of the features that indicate large current gain. Over the entire sweep range, we observe that most of the current flow between the S and D terminals. Under a forward bias of J1, with VS progressively stepped to more negative voltages, the currents at all three terminals increase. However, the sign of IB remains positive throughout the entire range of VD, as signified by the lack of pronounced current drop seen in IB when VS = 0. For VS < 0, the dominant effect in the base is recombination from the injection of electrons from S, creating a steady injection of holes into the base from terminal B. We measure this as a positive current. Two other components contribute to the base current which can reduce gain. Since J1 is forward biased, an additional hole current can flow from base to source. We make this component small by intentionally making the base doping small (VG2 = −0.2 V). On the other hand, the S doping is made large (VG1 = 2.5 V) to maximize the injection of electrons into the base. The other component is the generation current from the reverse bias of J2. The important contribution to the reverse bias current is the injection of holes from drain to base. As this component cannot be controlled by the base, it does not contribute to current gain and is undesirable. This component can be made small by using a large doping (VG3 = 1.5 V) on D. To further demonstrate the utility of electrostatic doping, we plot in Figure 3B the gain β at VD = 0.2 V, as we vary both VG2 and VS. The value for VD was chosen because all the currents have saturated and allow us to draw meaningful conclusions. As Figure 3B shows, we observe a substantial increase in β as the magnitude of VG2 reduces (lower base doping), while it is weakly dependent on VS. For the gate biases used in Figure 3, IB is a measure of the recombination current. The reduction in the base doping furnishes two features that enhance β, one of which is unique to our device construction. C

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Figure 4. n+−p−n phototransistor. (A) I−V characteristics of a phototransistor in dark and under illumination at different intensities (VG1 = 2.5 V, + VG2 = −0.2 V, VG3 = 1.5 V). (Inset) Ilight O vs intensity showing a linear dependence. (B) ID vs VDS for a n −p−p (VG1 = 2.5 V, VG2 = −0.2 V, VG3 = −0.2 V) photodiode showing the increase in short-circuit current and open circuit voltage with intensity. (C) ID vs VDS for an n+−p−n phototransistor (blue) and n+−p−p photodiode (red) for varying VG2 at λ = 590 nm (170 mW/cm2). Dotted curves on the bottom are the VOC values for the corresponding configurations. (D) Optical gain with varying VDS and VG2 showing that J2 becomes forward biased with VD < 0. Maximum gain is ∼40.

To proceed further, we note that β = τn/τt, the ratio of the lifetime τn to the transit time τt. This is simply restating the earlier definition of β in terms of the respective charges in the base. In the base, τt ∝ W2B (see SI), where WB is the effective width of the base. In a bulk device, WB would be defined by doping. Since we define doping electrostatically, WB becomes dependent on the bias on all three gates and VD. We performed finite-element modeling and show in Figure S3 that for the bias configurations we examined, WB ∝ |VG2|. In addition, τn can depend inversely with doping (∝1/|VG2|).19 The combined effect results in β ∝ 1/|VG2|3, which we demonstrate in Figure 3C by fitting (solid curve) our data. Since β is weakly dependent on VS, we took the mean across VS in Figure 3B to plot Figure 3C. The β that we can achieve is over 103, which is substantial considering that β < 102 in Si.20 In Supporting Information (see section titled Figure-of-Merit), we calculate important figure-of-merits for WSe2, including τn ∼ 5 × 10−9 s and diffusion length Ln ∼ 1 μm at low doping regimes, using measured mobility values.21,22 A similar lifetime of 2 ns is found for Si BJTs.23 Although not directly related, this lifetime is also similar to the photoluminescence lifetimes in TMD materials.24,25 The lifetime that we calculate would proportionally be lower by the gain at higher doping. The long diffusion length compared to the base width is consistent with the large

gain that we achieve. The large gain that we achieve is due, in part, to the decrease in the base width with reduced doping, which is unique to our device construction. Finally, we show in the inset of Figure 3C the linearity of our device (ID proportional to IB), which is an important aspect of a properly functioning BJT. To illustrate this differently, we provide Gummel plots in Figure S4 in both common base and common source (emitter) configurations, showing that both ID and IB depend exponentially on the forward bias voltage of J1. We now discuss the characteristics of the BJT when we operate it as a phototransistor. As discussed earlier, the essence of gain in BJT comes from the forward bias characteristics of one of the junctions. To operate as a phototransistor, we float the base and use light instead to control the base potential. The resulting band diagram under illumination for an n−p−n BJT is depicted in Figure 2A. The illumination has effectively forward biased both junctions, but the currents cancel. The gain of the phototransistor is defined as βphoto = IDS/IPN, at a given illumination.26 With the base floating, IDS is the two-terminal current measured between electrodes D and S. The gain here is in reference to the photocurrent generated in one of the p−n diodes (IPN), which we can measure. We show in Figure 4A the phototransistor characteristics (IDS) using the same doping configuration as the BJT in Figure 3 (n+−p−−n). We show the D

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can operate also as a phototransistor by using light to modulate the base potential. The electrostatic gating technique that we employ provides new mechanisms for large current and photocurrent gains.

corresponding photodiode (IPN) characteristics in Figure 4B by switching the polarity of VG3 while keeping the other gates the same, to create an n+−p−−p− junction. Both data set were taken under the same illumination conditions. The diode characteristics in Figure 4B show all the features of a p−n diode operating as a photovoltaic device. We observe clearly both the open-circuit voltage and short-circuit current that increase with intensity. The phototransistor characteristics, on the other hand, are noticeably different from the diode characteristics and show little or no open-circuit voltage, which suggests that both junctions contribute roughly equal photogenerated currents. Figure 4C shows an expanded comparison between the two devices as we vary both VD and VG2, to show more clearly the region where gain occurs. It is clear from this figure that large photocurrent gain occurs is when VDS < 0. We plot in Figure 4D the photocurrent gain βphoto of our device. Typically, in bulk phototransistors large gain would be observed when VD > 0, which comes from forward biasing of J1. In our device, we observe substantially less gain than when the same device is operated as a BJT. Our analysis provided in Figure S5 shows that when the base is floating, capacitive coupling pins the base potential. This pinning of the base potential, which we model as a virtual ground in Figure 2C, prevents J1 from being forward biased when VD > 0 (Figure 2C, right), and we typically measure βphoto < 5 for this polarity, as we show in Figure 4D. The lowest curves in Figure 4A and Figure 3A confirm that the phototransistor operates in a similar manner as the BJT in the common base configuration. On the other hand, a large gain is possible when VD < 0, which forward biases J2 (Figure 2C, left). Indeed, this is observed in Figure 4A and C. The virtual ground results in a diode-like behavior, similar to the BJT characteristics seen in Figure 3, except here the saturation current depends on intensity. The diode-like characteristics is unique to our device −qVDS/nkT construction. It can be modeled using IDS = Ilight − O (e ⎛ p⎞ light 1), where IO = I0⎜ p ⎟, as described in SI (see section titled ⎝ 0⎠ Phototransistor), and results in the asymmetry in the I−V characteristics seen in Figure 4A. This asymmetry is also seen in a symmetrically doped device (see SI Figure S6) and in a p−n− p phototransistor (see SI Figure S7). Here, n is the ideality factor, k is Boltzmann’s constant, and T is absolute temperature. p0 and I0 are the hole density in the base and the diode leakage current of J2 in the dark, respectively. p is the steady-state photogenerated hole density in the base and results in the ΔE shown in Figure 2A. It is directly proportional to the illumination intensity. The solid curves in Figure 4A are fits to our model using n = 2, focusing on the region VDS < 0. The with inset there shows the linearity in the prefactor Ilight O intensity. The maximum gain (∼40) is less than that of the same device operated as BJT and is due to n > 1. We can ⎛I τ⎞ explicitly define βphoto = ⎜ qp0 ′ ⎟|e−qVDS/ nkT − 1| (see SI section ⎝ 0⎠ titled Phototransistor), where p′0 is the total hole carrier in the base. This expression shows that by fabricating ideal diodes with n = 1, the device has the potential to achieve βphoto that is substantially larger than what we realized in Figure 4D. Based on the calculated lifetime, the phototransistor should be able to operate with a bandwidth in the sub-GHz range. In summary, we show one of the most important solid state devices, the BJT, in the recently discovered TMD material WSe2. The device relies on electrostatic gating techniques and

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b01444. BJT fabrication, J1 and J2 diode characteristics, finite element simulations, Gummel plots, floating base model, p−n−p phototransistor, figure of merit calculations, and phototransistor model (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support provided by the U.S. Naval Research Laboratory (grant number N00173-14-1G017).

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