I. INTRODUCTION

Micro-light emitting diodes (micro-LEDs) are rapidly emerging as a next-generation display technology, particularly suited for advanced applications in augmented reality, extended reality, and wearable devices ^[1,2]. Due to their exceptional brightness, energy efficiency, and long lifespan, micro-LEDs are expected to potentially replace traditional LCD and OLED technologies. Despite these advantages, the widespread adoption of micro-LEDs has faced challenges related to fabrication and high production costs. Extensive research is underway to overcome these barriers and drive commercialization.

A key challenge in micro-LED fabrication is the efficient integration of thousands of micro-LED devices with the driving circuits that control individual pixels ^[3]. Techniques such as pick-and-place, wafer bonding, and monolithic integration, where both micro-LEDs and driving circuits are fabricated on a single substrate, are being investigated ^[4,5]. Among these, monolithic integration offers significant advantages, including simplified fabrication processes and reduced parasitic effects ^[6].

Gallium nitride (GaN) is known for its excellent properties, such as a wide bandgap, high electron mobility, and high breakdown voltage, making it suitable for high-efficiency switching, high-power devices, and LED applications ^[7]. GaN-based LEDs are particularly suited for micro-LED applications due to their broad emission spectrum, which spans from ultraviolet to blue and green wavelengths ^[8]. As the number of micro-LED pixels and panel sizes increases, the interconnection between driving ICs and micro-LED pixels becomes a significant issue, as conventional driving ICs are typically fabricated on a Si CMOS platform, which is different from the micro-LED pixels. To address this challenge, there is a growing demand for the monolithic integration of driving ICs into micro-LED pixels. GaN transistors are also widely used in various applications, including high-speed amplifiers and high-efficiency switching devices ^[9,10]. Given that GaN transistors and micro-LEDs share the same material platform, it is fundamentally possible to fabricate both micro-LEDs and GaN-based driving ICs, provided that the epitaxial structure is designed to accommodate both optical and electrical components. In this study, we propose the use of a p-GaN/AlGaN/GaN heterojunction to simultaneously fabricate both LEDs and transistors.

II. CONCEPT AND APPROACH

The epitaxial wafer structure used in this study is shown in Fig. 1. It consists of a 70 nm p-GaN cap layer, a 15 nm Al$_{0.2}$Ga$_{0.8}$N barrier layer, a 300 nm i-GaN layer, and a 4 $\mu$m GaN buffer layer grown on a silicon (111) substrate (see Fig. 1(a)). This is a conventional epitaxial structure capable of fabricating an enhancement-mode (E-mode) field-effect transistor (FET) ^[11,12]. The upper p-GaN layer in the epitaxial structure can serve as the anode of an LED, enabling the monolithic fabrication of both FETs and LEDs on the same wafer without a regrowth process.

Fig. 1. Epitaxial structure for monolithic integration between micro-LED and its driver IC.

Previous studies have demonstrated the integration of LEDs with GaN FETs ^{[13,14,15,16]}. Depending on the epitaxial structure used for monolithic fabrication, GaN FETs can be implemented as p-channel FETs, depletion-mode (D-mode) FETs, or E-mode FETs. However, p-channel FETs exhibit significantly lower channel mobility, leading to low current density and D-mode FETs are not preferred for ICs.

In this study, utilizing the epitaxial structure shown in Fig. 1, we successfully implemented both micro-LED and E-mode FET structures on a single wafer, demonstrating the monolithic integration of micro-LEDs with an E-mode p-GaN/AlGaN/GaN FET-based 2T1C pixel-driving IC. The monolithically integrated 2T1C circuit consisted of two E-mode p-GaN/AlGaN/GaN FETs and a metal-insulator-metal (MIM) capacitor.

II. MONOLITHIC FABRICATION

The monolithic fabrication process is illustrated in Fig. 2. Fabrication began with solvent cleaning. Afterward, surface organic contaminants and native oxides were removed using a sulfuric peroxide mixture and buffered oxide etchant. Selective etching of the p-GaN layers was then performed using BCl$_{3}$/SF$_{6}$ gas to form the gate of E-mode FET and the anode of LED, followed by recovery annealing at 700$^\circ$C for 150 s ^[17]. Mesa isolation between devices was achieved using BCl$_{3}$/Cl$_{2}$-based inductively coupled plasma reactive ion etching (ICP-RIE). Ti/Al/Ni/Au ($= 20$/$120$/$25$/$50$ nm) was evaporated for ohmic contacts, followed by rapid thermal annealing at 830$^\circ$C for 30 s. A 120 nm SiO$_{2}$ layer was then deposited as a passivation layer after dipping the sample in a diluted HCl (3:1) solution for 30 s. The anode contact area of the LED was opened by RIE etching with SF$_{6}$ gas before forming the p-type ohmic contact. A thin Ni/Au ($=5$/$5$ nm) film was evaporated as the p-type ohmic contact and subsequently annealed at 450$^\circ$C for 10 min. The contact areas were defined by etching the SiO$_{2}$ film, after which a Ni/Au ($=50$/$150$ nm) film was evaporated to serve as the gate and electrodes of the GaN FETs, the anode of the LED, and the bottom electrode of the MIM capacitor. A second passivation layer of SiO$_2$ was then deposited, which also functioned as the dielectric layer of the MIM capacitor. Finally, contact openings for interconnections were created by etching the second passivation layer, followed by the evaporation of a Ni/Au film to form the metal interconnections.

During monolithic fabrication, individual FETs and LED-embedded FETs were also fabricated for device characterization.

Fig. 2. Fabrication process for the monolithic integration of a micro-LED and circuit components. (a) p-GaN etch, (b) mesa isolation, (c) ohmic contact, (d) 1st passivation, (e) p-GaN ohmic contact, (f) gate and electrode, (g) 2nd passivation, and (h) interconnection.

III. RESULTS AND DISCUSSION

1. Device Characteristics

Two device structures were investigated for comparison: one was an E-mode p-GaN/AlGaN/GaN FET, and the other was a FET with an embedded LED. The detailed FET dimensions and structures are shown in Fig. 3. Both FETs have a p-GaN gate length of 4 $\mu$m, a gate metal contact length of 2 $\mu$m, and gate-to-source and gate-to-drain lengths of 2 $\mu$m each. As indicated by the device symbols, the drain current flows directly into the embedded LED in the device shown in Fig. 3(b).

Fig. 3. Cross-sectional schematic of (a) E-mode p-GaN/AlGaN/GaN FET and (b) FET-integrated micro-LED.

Fig. 4. (a) Transfer and (b) output characteristics of the E-mode p-GaN/AlGaN/GaN FET and the FET-integrated micro-LED.

Figs. 4(a) and 4(b) present the transfer and output characteristics of the E-mode GaN FET and the FET with an embedded LED, respectively. Both devices demonstrated a threshold voltage of 2 V, confirming their E-mode operation. The identical threshold voltages are due to the consistent thickness of the p-GaN layer used in both devices. The E-mode GaN FET exhibited an ON-resistance of 13.9 $\Omega$·mm and an on/off ratio of $\sim 10^8$. The peak transconductance was 68 mS/mm. It should be noted that the FET with an embedded LED shows higher current density than the E-mode FET without an embedded LED. This was attributed to the conductivity modulation by the p-GaN drain structure ^[18]. Unlike the conventional p-GaN/AlGaN/GaN FET, the FET with an embedded LED exhibited a drain turn-on voltage of 2.4 V, as shown in Fig. 4(b). This behavior is attributed to the turn-on characteristics of the p-GaN/AlGaN/GaN heterojunction of LED ^[19].

Fig. 5 shows microscopic emission images of the FET-integrated micro-LED operated at different applied voltages. A gate voltage of 5 V was used with two different drain voltages, 8 V and 10 V. Although the emission intensity could not be measured in our system, it was confirmed that the emitted light intensity increased with higher voltages. A previous study reported that the peak emission wavelength of approximately $\sim365$ nm was measured using a similar epitaxial structure ^[14].

Fig. 5. Microscopic emission images of the FET-integrated micro-LED with a gate voltage of 5 V and a drain voltage of (a) 8 V and (b) 10 V.

2. 2T1C Driver IC Characteristics

Fig. 6(a) shows a circuit diagram of the fabricated monolithically integrated micro-LED pixel with a 2T1C driving IC. Figs. 6(b) and 6(c) display the 3D schematic of the pixel and a microscopic image of the fabricated 2T1C integrated LED pixel, respectively. In this configuration, T1 is an E-mode p-GaN/AlGaN/GaN FET, while T2, surrounding the circular LED, represents the FET-integrated micro-LED. The circular LED has a diameter of 100 $\mu$m. T1 has a channel width of 30 $\mu$m, while T2 has a channel width of 330 $\mu$m, corresponding to the perimeter of the circular LED. The storage capacitor (C$_{\rm st}$), with dimensions of $100 \times 30$ $\mu$m$^2$, is implemented as a 1 pF MIM capacitor.

Fig. 6. (a) 2T1C pixel circuit diagram, (b) 3D CAD design, and (c) microscopic image of the fabricated 2T1C pixel circuit.

It should be noted that the capacitance of C$_{\rm st}$ must be carefully determined, taking into account the charging and discharging behaviors during switching operation. C$_{\rm st}$ must charge quickly when the signal is applied but should not discharge when T1 is off. An important consideration is the leakage current when T1 is off, as it can accelerate the discharging process. The discharging behavior of C$_{\rm st}$ can be expressed as

$V(t) = V_{\rm initial}e^{-\frac{t}{R_{\rm leakage}\cdot {\rm C}_{\rm st}}},$

where V$_{\rm initial}$ is the charged voltage of C$_{\rm st}$, and R$_{\rm leakage}$ represents the leakage resistance. Therefore, the leakage current of T1 must be carefully considered when determining the appropriate capacitance for C$_{\rm st}$. The capacitance of C$_{\rm st}$ and the channel width of T1 were determined based on SPICE simulations, considering the leakage behavior of T1 while also taking pixel space constraints into account.

The fabricated 2T1C driver IC was tested at an operating frequency of 500 kHz. Fig. 7 shows the pulse sequence used for the 2T1C pixel circuit. The scan signal was applied with a pulse width of 0.5 $\mu$s and a period of 1 $\mu$s at an applied voltage of 5 V, while the data signal had a pulse width of 1 $\mu$s and a period of 2 $\mu$s, also at an applied voltage of 5 V. The V$_{\rm DD}$ was set to 10 V to drive the micro-LED. The measured waveform demonstrates the feasibility of the circuit's capability to scan and drive the micro-LED. No discharging phenomenon was observed during switching operation, verifying that the capacitance of C$_{\rm st}$ and the channel width of T1 were properly designed. The measured LED driving current was 19 mA, with rise and fall times of 20 ns and 50 ns, respectively, indicating a very fast response. These fast response characteristics are also attributed to the high electron mobility of AlGaN/GaN FETs, enabling high-frequency operation.

Fig. 7. Voltage pulse sequence and measured response current waveform of the fabricated 2T1C pixel circuit.

V. CONCLUSIONS

This study demonstrated the successful design and fabrication of a monolithically integrated 2T1C driving IC for a micro-LED, utilizing a p-GaN/AlGaN/GaN heterojunction platform. The p-GaN layer on top of the AlGaN/GaN heterojunction serves both as the gate for the E-mode FET switching device and as the anode of the micro-LED. The experimental results confirmed the effective driving capability of the monolithically integrated IC for the micro-LED. This approach offers a promising solution for high-performance micro-LED displays.