Resonant tunneling diodes (RTDs) have received a lot of attention due to their structural simplicity, ultra-high frequency capability, low power consumption, and their ability to operate at room temperature^[1−6]. They are widely applied in micro/nanoelectronics and communications (oscillators, logic circuits, transmitter, wireless link), optoelectronics (phototransistor, photodetector, light emitting diode, optical modulator, single photon detector), and recently in neuromorphic applications^[7−21]. RTDs typically consist of a quantum well sandwiched in between two barriers and work on the principle of quantum tunneling effect. Negative differential resistance (NDR), which is a unique characteristic of this semiconductor device, allows them to be applicable in a wide range of areas (as mentioned above). Research on RTDs commenced with the start of the theoretical studies on tunneling phenomenon in a superlattice by Leo Tsu and Raphael Esaki in the 1970s and along with Chang in 1974^[22−24]. These RTDs were experimentally demonstrated for the first time in 1974 at liquid nitrogen temperature, where the NDR region was observed in the current voltage (I−V) characteristics; however, the first room temperature experiment was only conducted in 1985^{[24, 25]}.

The conventional fabrication process for RTDs requires six lithography steps and mainly employs the polyimide process^{[26, 27]} for passivation. The first step here is the deposition of top metal contact, followed by the top mesa step (where etching is conducted until the emitter layer). The bottom mesa step, which etches till the substrate, is followed by the formation of a bottom metal contact. A polyimide is used in the passivation step. The use of a polyimide passivation layer is essential to protect the device from external contaminants and to prevent unwanted surface recombination effects that can degrade the device performance at the cost introducing parasitic capacitance, which is crucial for high frequency applications. Finally, via-opening and metal bond pads deposition completes the fabrication process. Opening of the top contact (i.e., via-opening) after passivation is a critical step in enabling an electrical connection between top contact and bond pad, and is a challenging step, especially while fabricating the nanoscale dimension RTDs. The air-bridge approach overcomes the critical top-contact connection step of the conventional processes, where a suspended air-bridge structure is made that connects the top metal and the bond pad. Although similar fabrication approaches have been reported in the literature^[28−32], they all require at least four lithography steps.

In this work, we report details of the process and compare the DC performance of a 4 × 4 μm² DBQW RTD using the proposed air-bridge technique with that from similarly sized devices made from the same epitaxial wafers but fabricated using the conventional polyimide process. DC characterization shows that both these processes have nearly the same performance.

This paper is organized as follows: Section 2 describes the RTD device epi-structure and fabrication details. Section 3 deals with the device characterization, while concluding remarks and future perspectives are discussed in Section 4.

The epi-structure of the RTD device used in this work was grown by MBE (molecular beam epitaxy) on a semi-insulating (SI) indium phosphide (InP) substrate. This structure, shown in Fig. 1, consists of a 4.5 nm indium gallium arsenide (In_0.53Ga_0.47As) quantum well that is sandwiched in between 1.4 nm AlAs barriers; the barriers being surrounded by undoped and lightly doped In_0.53Ga_0.47As spacer layers (In_0.53Ga_0.47As) on both sides. The collector and emitter layers are made of heavily doped In_0.53Ga_0.47As material doped with silicon. The devices were fabricated by both photolithography and electron beam lithography. Only two lithography steps were required. Fig. 2 shows the cross section of the device. The emitter and collector contact pads were patterned by using electron beam lithography. The metal pattern was then used as a mask for an orthophosphoric based chemical wet etch to the emitter (bottom) contact layer. Next, the collector pad and RTD were protected with photoresist by using photolithography, followed by a final chemical wet etching step to the substrate. Fig. 3 illustrates the process steps, while Fig. 4 shows the scanning electron microscopic view of the fabricated RTD device showing the air-bridge, which is 5 µm wide and 55 µm long.

Performance analysis for the fabricated RTD device is done by DC characteristics measurement. The setup is shown in Fig. 5. A semiconductor device parameter analyzer (Keysight B1500A) was used for DC characterization of the device. The bond pads (60 × 100 µm²) are connected to the terminals on the parameter analyzer through two DC probes. The measured I−V characteristic of the 4 × 4 µm² RTD at room temperature is shown in Fig. 6(a). The device exhibits a peak-to-valley current ratio of 3 and peak current density of 143 kA/cm². The detailed results, including the comparison with conventionally fabricated devices are shown in Table 1 and Fig. 6. It can be seen that both processes have nearly the same performance. Thus, the proposed simple RTD fabrication process is robust and has comparable performance with the conventional polyimide process.

The performance of 4 × 4 μm² RTD device (fabricated by our proposed air-bridge process) is also compared with the available literature (based on air-bridge work) and the results are given in Table 2. It should be noted that the comparison here is made with the available work where the clear fabrication steps of RTD device and results are described.

Double barrier quantum well resonant tunneling diode of active device area 4 × 4 μm² based on the InGaAs/AlAs system is developed using a novel air-bridge process. Current−voltage characteristics at room temperature for this device results in a peak-to-valley current ratio of 3.02 and a peak current density of 143.46 kA/cm². The measured results nearly match with the polyimide process with similar device dimension. This novel fabrication process may be extended to other material systems.