1. Maximum Power Dissipation (Power - Max): The NSBC123JF3T5G is rated for 254mW, while the RN1105MFV,L3F is rated for 150mW. Under identical operating conditions, the RN1105MFV,L3F will exhibit a higher junction temperature rise and reduced thermal margin. If the original circuit design operates near 254mW, a direct substitution will cause device overheating, leading to degraded long-term reliability or failure. A re-evaluation of the actual power dissipation at the operating current and the thermal environment is mandatory. 2. Package / Thermal Characteristics: The NSBC123JF3T5G uses an SOT-1123 package, whereas the RN1105MFV,L3F uses an SOT-723 (VESM) package. They differ in physical dimensions, pin geometry, and most critically, thermal resistance (RθJA). The SOT-723 is typically smaller and may have weaker heat dissipation capability. A direct replacement requires confirming PCB land pattern compatibility and re-evaluating the thermal design, as the package change affects the efficiency of heat transfer to the PCB. 3. DC Current Gain (hFE) Test Condition: Both specify a minimum hFE of 80, but the test points differ. The NSBC123JF3T5G is tested at Ic=5mA, Vce=10V, while the RN1105MFV,L3F is tested at Ic=10mA, Vce=5V. Manufacturers guarantee the specification in different regions of the device's characteristic curve. Although the minimum values are identical, the actual hFE curves across the operating range may vary slightly. Circuits with stringent gain accuracy requirements, such as those for biasing, should take note. 4. Saturation Voltage (Vce(sat)) Test Condition: The test current and base drive conditions differ between the two parts. This means their specified saturation voltage values (250mV vs. 300mV) cannot be compared directly on a numerical basis. This discrepancy reflects different manufacturer definitions of "typical" saturation characteristics. When used in low-voltage, high-current switching applications, a comparison of their saturation characteristic curves at the actual application current is necessary to evaluate potential differences in switching losses.