A p-n heterojunction made of MgO and Ga4Se3S single crystal has been successfully produced. The current-voltage curve analysis has shown that the current conduction mechanism is mostly governed by the Richardson-Schottky mechanism. The width of the effective interface region of the p-n junction was found to be 3.72x10(-5)cm. The work function and the electron affinity of the Ga4Se3S crystals were also determined as 4.32 and 3.96 eV, respectively. On the other hand, the capacitance-voltage curve analysis, which was carried out in the power range that extends from Bluetooth to WLAN power outputs, reflected a built-in voltage of 0.48 eV and density of noncompensated carriers of 8.2 x 10(16)cm(-3). The device is observed to exhibit a wide range of negative resistance associated with the tunneling of charged particles at reverse biasing down to similar to 1.28 V. At that voltage, when exposed to a He-Ne laser beam of similar to 3 mW, the device reflected a responsivity of similar to 80. The charge storability increased and the I-V characteristics are significantly shifted. These properties are promising because it indicates the applicability of these tunneling devices in optoelectronics.