Tribotronic bipolar junction transistor for mechanical frequency monitoring and use as touch switch

Tribotronics, a new field that involves the coupling of triboelectricity and semiconductors, has attracted great interest in the nanoenergy and nanoelectronics domains. This paper proposes a tribotronic bipolar junction transistor (TBJT) that incorporates a bipolar junction transistor and a triboelectric nanogenerator (TENG) in the single-electrode mode. When the mobile triboelectric layer slides on the device surface for electrification, a bias voltage is created and applied to the emitter junction, and then the base current from the TENG is amplified. Based on the fabricated TBJT, a mechanical frequency monitoring sensor with high sensitivity and excellent stability and a finger-triggered touch switch were developed. This work demonstrated for the first time a tribotronic device with simultaneously controlled voltage and current voltage/current simultaneously controlled tribotronic device, which has promising potential applications in micro/nano-sensors, human-machine interactions, intelligent instrumentation, wearable electronics, and other applications.


Introduction
In the last few decades, transistor scaling has followed the well-known Moore's law, resulting in an increase of two times in the transistor density every 2 years 1,2 , which is approaching the limitations of physical size and power 3,4 . Therefore, information technology (IT) in the post-Moore's law period will develop toward new directions including diversification, improved sensors, multifunctionality, and self-powering capacities [5][6][7][8][9][10] . As one of the basic transistors, the bipolar junction transistor (BJT) still plays an important role in the design of discrete and very-high-frequency circuits and is used as an amplifier or switch because of its high transconductance and output resistance compared to MOSFETs 11,12 . However, the traditional BJT is modulated by internal electrical signals and lacks the direct interaction mechanism between the external environment and electronics.
Recently, the invention of a triboelectric nanogenerator (TENG) has successfully provided an effective approach to convert ambient mechanical energy into electricity 13 . The working principle of the TENG is based on contact electrification and electrostatic induction, which have been widely used in micro/nano-energy [14][15][16][17] , selfpowered systems [18][19][20][21][22][23][24][25][26] , and blue energy [27][28][29] . Furthermore, by coupling the triboelectricity with semiconductors, a new field of tribotronics has been proposed [30][31][32] , which concerns research regarding the interaction between triboelectricity and semiconductors 33 using triboelectric potential controlling electrical transport and transformation in semiconductors for information sensing and active control [34][35][36] . To date, various tribotronic devices have been developed including contact-gated LEDs 37 , touch memory 38 , adjustable phototransistors 39,40 , sensing arrays 41 , tribotronic tuning diodes 42 , and flexible organic tribotronic transistor 43 . All these applications demonstrate the potential of tribotronics in active interaction electronics, offering a prospective strategy to design smart sensors with the advantages of low cost, simple mechanism, and excellent sensitivity.
In this study, we developed a tribotronic BJT (TBJT) by combining a triode with a TENG in the single-electrode mode. The collector current of the TBJT can be amplified and tuned by the base current from the TENG with the sliding motion of the mobile triboelectric layer. Based on the fabricated TBJT, a mechanical frequency monitoring sensor and a finger-triggered active smart touch switch have been developed. In contrast, from previous tribotronic transistors, the TBJT is controlled by the voltage and current created by the TENG simultaneously, which has potential applications in micro/nano-sensors, humanmachine interaction (HMI), intelligent instrumentation, and remote controls.

Structure of the TBJT
The basic structure of the TBJT comprises a flexible polyimide substrate, a Cu pad (25 mm × 25 mm), a freestanding fluorinated ethylene propylene (FEP) film, and an NPN type triode, as schematically illustrated in Fig. 1a. The Cu pad is deposited on the top surface of the flexible polyimide substrate, and the triode made of a siliconbased n-p-n junction is constructed on the bottom layer. The emitter, base and collector are electrically connected with the n + -type, the p-type, and the n-type region of the triode, respectively. Through the designed via hole, the Cu pad is also electrically connected to the base electrode, as shown in the inset of the cross-section. The mobile layer is assembled next to the Cu pad, which experiences vertical contact and separation from the Cu pad by the external force. The other inset shows the SEM image of the nanostructures on the surface of the mobile triboelectric layer made of FEP film, which is modified via inductive coupling plasma (ICP) to enhance the surface triboelectric charge density. A well-designed TBJT is presented in Fig. 1b (the detailed fabrication process is introduced in the Methods section). Figure 1c presents the equivalent circuit of the TBJT, which intuitively shows the interaction between the external force and the electronics.
Working mechanism of the TBJT The working mechanism of the TBJT is elaborated in Fig. 2, which demonstrates a working principle different from that of the conventional BJT configuration, based on the coupling of the NPN-BJT, triboelectrification and electrostatic induction. As demonstrated in Fig. 2a, the FEP film is purposely chosen as the mobile triboelectric layer owing to its high electronegativity according to the triboelectric series 44 . The collector of the TBJT is connected with a voltage source, whereas the emitter is grounded.
In the initial position, when the FEP film fully contacts the Cu pad, equal negative and positive charges are induced on the surfaces of the FEP film and the Cu pad, respectively, owing to their difference in triboelectric polarity. Because the positive triboelectric charges completely balance out the negative counterpart in this circumstance, there is no electrical potential difference applied to the base region and no charge transfer occurs from the TENG. The emitter-base junction is zero offset and the collector-base junction is reverse biased; therefore, the TBJT stays in the cut-off mode, which corresponds to a logical "off" state. Meanwhile, the depletion regions are generated in both the junctions where the electrons and holes recombine. Moreover, the Fermi levels of the base and emitter region in Fig. 2a are horizontally aligned, and are higher than those of the collector in its band diagram. Once the FEP film is gradually separated from the Cu surface as the external force is vertically applied, the equilibrium potential is disturbed, resulting in a positive electrical potential being applied to the triode (Fig. 2b). Meanwhile, the emitter-base junction is forward biased (the depletion layer width decreases) and the collector-base junction is reverse biased (the depletion layer width increases); thus, the TBJT shifts into the active mode. In the active mode, the electrons are injected from the forward biased n + -type emitter region into the ptype base where they diffuse as minority carriers to the reverse-biased n-type collector and are swept away by the electric field in the reverse-biased collector-base junction. Then, based on the law of charge conservation, the current from the TENG is amplified dramatically in the collector by the triode. The Fermi level of the base region in Fig. 2b decreases owing to the positive potential in the base. Next, as shown in Fig. 2c, when the movement of the FEP film continues, the inner electric field gradually increases as well. Therefore, with both junctions being forward-biased (widths of both depletion layers decrease), the TBJT enters the saturation mode, and the Fermi level of the base region decreases between the emitter and collector. This mode corresponds to a logical "on," or a closed switch, which means that the collector current has a high magnitude and is stable. Finally, the TBJT and its band diagram return to the cut-off mode when the FEP film slides backwards (Fig. 2d). However, in this process, the Fermi level of the base region increases because of the negative potential in the base. Figure S1 shows the theoretically calculated results (performed by the software Comsol Multiphysics) of the potential difference generated from the contactseparating and sliding mode TENG with the FEP and copper films, indicating that this triboelectric potential varies with the separation distance of the mobile triboelectric layer.

Electrical characterization of the TBJT
To better evaluate the performance of the TBJT, its electrical characterizations were systematically studied, as depicted in Fig. 3. The measured results demonstrated satisfactory performance of the BJT. Figure 3a plots the output characteristic of the TBJT, and the separation frequency of the FEP film can be accurately controlled using a linear motor. As shown in the graph, the I C rises as the separation frequency increases from 2 Hz to 12 Hz within a V CE of 0-10 V. The curves of I B and frequency were also studied, and are shown in Fig. 3b. When the separation frequency of the FEP film varies from 0 to 10 Hz, the I B increases from 50 to 153 nA. In addition, the curve of the collector current and base current at the frequency of 4 Hz and V CE = 0.5 V are shown in Fig. 3c, and the amplification factor of the peak current (I C = 9.7469 μA and I TENG = 0.0974 μA) can be reached at 100 × . Moreover, measurements for 2000 cycles are carried out to validate the stability and repeatability of the device, as described in Fig. 3d. Even after 2000 test cycles, the changes in the peak collector current I C are <5%, showing its small hysteresis and excellent reproducibility. The above experimental results indicate that the collector current increases with the increasing frequency, which corresponds to good electrical characterization. Owing to the direct interaction mechanism between the external environment and electric device, the TBJT is likely to have excellent applications in sensors and HMI.

Application of the TBJT for mechanical frequency monitoring
First, the TBJT was applied to mechanical frequency monitoring (Fig. 4). Based on the electrical characterization of the TBJT, the I C can be tuned by the separation frequency of the FEP film. Based on this principle, we have designed a mechanical frequency monitoring sensor based on the TBJT. The structures of the TBJT with contact-separating and sliding single electrode modes, which are developed for vibrational and sliding frequency monitoring, are shown in Fig. 4a, b, respectively. By exploiting the excellent sensitivity, the TBJT as a mechanical frequency monitoring sensor shows promising prospects in HMI and intelligent sensing.

Application of the TBJT as a finger-triggered touch switch in smart home
In addition to being a mechanical signal sensor, another intriguing application of the TBJT is as a finger-triggered active tactile touch switch for smart home control systems. Traditional transistors cannot be touched by human fingers directly owing to the presence of electrostatic charges. However, on the basis of the strong advantages of the TBJT, we designed a finger-triggered active tactile switch with a high sensitivity that can be used in the fields of HMI and remote control. In this configuration, the human skin (finger) can also act as a mobile triboelectric layer, replacing the FEP film according to the triboelectric series. The equivalent electrical circuits of the TBJT and TENG used for controlling the light-emitting diode (LED) are demonstrated in Fig. 5a. The demo optical graph in the inset shows that an LED is controlled directly by the finger touching or releasing the mobile triboelectric layer (FEP film) of the TBJT. As depicted in Fig. 5b, the LED brightness can be tuned by the switch, and the driving energy of TENG is greatly improved. The LED is turned on/off when the finger presses or releases the switch, and the dynamic demonstrations are shown in Video S1. Moreover, as shown in Fig. 5c, the smart home control system includes a person; a touch switch based on the TBJT; a simple signal processing circuit; and household appliances, such as a table lamp, an electric fan, and a household security bell. The signal processing module is made up of a delayer and a relay based on a single-chip microcomputer, and the detailed circuit board is demonstrated in Figure S2. When a user touches the Cu pad of the TBJT, the signal is detected and converted into a switching signal by the signal processing circuit for the household appliances ( Fig. 3d and Video S2 to S4).

Fabrication of the tribotronic bipolar junction transistor
First, a 100 μm polyimide (PI) substrate (28 mm × 28 mm) was prepared and cleaned in an ultrasonic cleaner with deionized water, ethanol, and acetone, sequentially, and then it was blow-dried in a drying oven at 100°C for 1 h. Subsequently, three localized via holes were drilled on the substrate and 10 μm Cu used as electrical wires were deposited onto the selective area of back side of the substrate according to the designed circuit configuration. Next, layers of 3 μm Ni and 25 μm Cu were electroplated onto the substrate to form the pad (25 mm × 25 mm). Finally, a chip of the NPN-triode was adhered to the bottom of the PI substrate, in which the base was connected to the Cu pad through via the hole, and the collector and emitter could be electrically connected to the  external circuitry by the electrical wires. The mobile layer vertically controlled by the external force was fabricated based on a piece of FEP film, which was also used as a mechanical frequency monitoring sensor.

Surface modification of the FEP film
First, a 30 μm FEP film (25 mm × 25 mm) was prepared and cleaned. Before etching, a layer of Cu was RF sputtered onto the surface of the FEP surface, and then O 2 , Ar, and CF 4 gases were fed into the inductively coupled plasma (ICP) chamber to etch the surface with flow rates of 10.0, 15.0, and 30.0 standard cubic centimeter per minute. Then, a 400 W power source was used to generate the plasma, while another 100 W power source was used to accelerate the plasma ions moving to the FEP surface. After 5 min, the desired nanostructures were obtained on the Cu coated FEP surface.

Characterization and electrical measurements of the TBJT
The current and voltage was quantitatively determined by a linear motor (Akribis-DGL-SH417), a Low Noise Current Preamplifier (SRS-SR570) and a programmable electrometer (Keithley 6514) under ambient conditions at room temperature. A DC power supply (RIGOL-DP-832) was used to supply an electrically applied voltage to the TBJT and the external circuits.

Conclusion
In summary, we have demonstrated a tribotronic BJT (TBJT) based on a bipolar junction and a TENG in the single-electrode mode. With the sliding of the mobile triboelectric layer, a bias voltage is created, and the base current is amplified. The fabricated TBJT is used as a mechanical frequency monitoring sensor with a high sensitivity (1 μA Hz −1 ) and excellent stability. Moreover, the device can also be used as a finger-triggered active tactile switch. This work has extended emerging tribotronics to a device with simultaneously controlled voltage and current and has demonstrated that the new field may have innovative and promising potential applications in micro/nano-sensors, HMIs, wearable electronics, and other applications.