SUNY Poly Researchers Advance Bipolar Junction Transistors in Two-Dimensional WSe2 with Large Current and Photocurrent Gains

Most semiconductor devices rely on the p-n junction which is achieved by doping. Doping is a critical process that enables a semiconductor to be either electron-conducting (n-type) or hole-conducting (p-type). Doping, however, has been difficult to achieve in nanostructured semiconductors, dating back to the discovery of carbon nanotubes.

Researchers led by Professor Ji Ung Lee at SUNY Polytechnic Institute’s Colleges of Nanoscale Science and Engineering in New York published an article in the journal Nano Letters, which demonstrates bipolar junction transistors (BJT) in the exfoliated two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductor WSe₂ with both current and photocurrent gains.

The technique for doping created by the authors allows reconfigurability. Unlike the rigidly defined dopants in bulk devices that do not allow the conduction property to change, in the new approach, a single device can be used to create either an n-p-n or a p-n-p bipolar junction transistor device.  The authors believe that a reconfigurable device may one day allow a new type of computer architecture that consumes less power.

A bipolar junction transistor device consists of two back-to-back p-n diodes, a feature that has remained the same since their inception in 1947 as the first semiconductor transistor. In order to create the device, the authors used three gates labeled G1, G2 and G3 to define the three doped regions of a BJT.

The source S and drain D makes contact to the two ends of the device, while a narrow base B makes contact with the side of WSe₂ to prevent shunting the current in the base. The width of the base contact ranges from 50 to 70nm which is made narrower than width G2 which is 100nm.

Agnihotri et al. (2016) demonstrated ambipolar conduction between S-D contacts, which shows large current flow when all the gates are biased identically to allow electron conduction (V_{G 1, 2,3} >0) or hole conduction (V_{G 1,2,3} <0). In addition, the authors demonstrated the two diode characteristics that make-up the BJT device by biasing the gates to create p-p-n and n-p-p diodes. The authors confirmed the individual diode characteristics by measuring the current-voltage I-V characteristics of S-B and B-D electrodes.

The most important property of a BJT is current gain β. To demonstrate gain, one junction was forward-biased while the second junction was reverse-biased, which maximizes the collection of carriers that are injected into the base. With the base at ground, the corresponding bias configurations are V_S<0 and V_D>0 for an n-p-n bipolar junction transistor. Specifically, the device is configured as n⁺-p^—n bipolar junction transistor, which was achieved by using V_G1=2.5V, V_G2=-0.2V and V_G3=1.5V. In order to better assess the working principles of the device, V_D was swept while maintaining a forward bias of the first junction.At V_S=0, three currents I_S, I_D and I_B followed a diode-like behavior as a function V_D which is expected since the second junction determines the I-V characteristics as V_D is swept from reverse (V_D>0) to forward (V_D<0) bias. With V_S<0, the current I_B maintains the same sign throughout the bias range as the device enters a regime of transport with large current gain. The gain is tunable, unlike in bulk devices, because of the electrostatic doping technique. The authors demonstrate the utility of electrostatic doping by demonstrating record gain by reducing the voltage V_G2, which decreases the base doping and increases the lifetime of the injected carriers. The authors demonstrate β~1000, which is more than ten times larger than those from Si-based BJT devices.

To better understand the gain mechanism, Agnihotri et al. (2016) performed finite-element modeling and showed that the effective base width W_B scaled with V_G2.  In addition, the lifetime τ_n can depend inversely with doping (αI/|V_G2|). The combined effects result in βαI/|V_G2|³, which helps to explain the record gain in the transition metal dichalcogenide BJT device.

Using the similar principle that give rises to current gain, the bipolar junction transistor was operated as a phototransistor to demonstrate photocurrent gain. Here, light is used to generate the carriers in the base, which helps to forward-bias the junctions. A voltage on the drain favors one junction over the other, giving rise to a photocurrent gain. Further experiments based on calculated lifetime showed that phototransistor should be able to operate with a bandwidth in the sub-GHz range.