## 1 Introduction  
UV (ultraviolet) radiation, including the UV-A band (320-400 nm), UV-B band (280-320 nm), and UV-C band (100-280 nm), has extensive applications, such as in light sources, photo catalysts, sewage treatment and communications [1; 2]. As the third generation semiconductor material, III-Nitride materials have excellent optoelectronic performance. GaN with a 3.4 eV bandgap can be composed of MQWs (multiple quantum wells) with InN (1.9 eV) and AlN (6.2 eV). The corresponding direct bandgap of MQWs covers the UV range and deep ultraviolet (DUV) range, and optoelectronics devices based on III-Nitrides.  
MQWs have been a significant area of research in recent years [3; 4; 5]. Optical communication systems transmit information through modulated optical signals from ultraviolet to infrared. The motivation for developing ultraviolet optical communication contains unique channel characteristics, rapid evolution of III-Nitride devices, and emerging application demands in the fields of military and environmental protection and so on [6]. However, due to the limitation of speed and optoelectronic performance of key devices (such as transmitters) based on ultraviolet radiation, the performance of ultraviolet optical communication systems needs further improvement.  
For early ultraviolet optical communication systems, flashtubes and lamps with the disadvantages of being bulky, power hungry and with low bandwidth, were selected as light signal transmitters [6].  
Consequently, the transmission speed of early ultraviolet optical communication systems was limited around Kbps-Mbps [7,8]. Recently, III-Nitride LEDs with a high efficiency provided a potential application as light signal transmitter for ultraviolet optical communication systems [9, 10, 11]. [PERSON] et al. demonstrated that AlN/GaN/InGaN MQWs ultraviolet LEDs obtained high output power at 375/395 nm [12]. [PERSON] et al. reported a successful fabrication of room-temperature electrically injected InGaN/AlGaN MQWs near-ultraviolet laser diode at 390 nm, which was grown on Si by preparing an Al-composition step downgraded AlN/AlGaN multilayer buffer [13].  
Our group has studied III-Nitride integrated optoelectronics devices on silicon substrate as a light signal transmitter/receiver for visible and ultraviolet optical communication [14, 15, 16]. III-Nitride devices with MQWs have a stable carrier recombination lifetime, and the modulation bandwidth and transmission speed are significantly influenced by RC (resistance-capacitance) time [17]. Reducing the active area of III-Nitride devices with MQWs used in visible and ultraviolet optical communication systems, the modulation bandwidth and transmission speed could be improved by decreasing capacitance [18].  
A high-speed light signal transmitter based on ultraviolet radiation is realized on a commercial GaN-on-silicon platform in this paper. The light signal transmitter with ultra-small active area is obtained by a double-etching process. We characterize the optoelectronic properties of the light signal transmitter. The absolute value of the negative junction capacitance of the transmitter is as low as the pF scale in positive bias voltage. The emission peak remains stable at 380 nm in the ultraviolet range under different currents. A free space communication test presents that the light signal transmitter has a high transmission speed in the ultraviolet range. This study provides a potential approach for a light signal transmitter with high speed used in ultraviolet optical communication systems. The schematic of the high-speed light signal transmitter for optical communication based on ultraviolet radiation is shown in Figure 1.