1. Maximum Power Dissipation: DTC123JM3T5G: 260 mW DTC123JMT2L: 150 mW The onsemi device offers a 73% higher thermal design margin. Under identical ambient temperature and PCB layout, the DTC123JMT2L has a significantly lower allowable continuous operating current or maximum temperature rise. Direct replacement may introduce overheating risks in high-load or high-temperature applications. 2. Collector-Emitter Saturation Voltage Test Condition: DTC123JM3T5G: 250mV @ Ib=1mA, Ic=10mA DTC123JMT2L: 300mV @ Ib=250µA, Ic=5mA The test conditions are entirely different, making a direct numerical comparison invalid. The DTC123JMT2L's 300mV@5mA specification, tested with a lower base drive current, may translate to significantly worse performance in practical applications—especially when driving 10mA or higher currents—compared to the DTC123JM3T5G's 250mV@10mA. This could result in higher conduction loss and voltage drop in the switched state. 3. DC Current Gain Test Condition: DTC123JM3T5G: 80 @ Ic=5mA, Vce=10V DTC123JMT2L: 80 @ Ic=10mA, Vce=5V Although the specified minimum gain is identical (80), the test points differ. This suggests the gain characteristics over the operating region (collector current and voltage) may vary between the two parts, potentially affecting the precision of bias point design or small-signal behavior in the linear amplification region. 4. Transition Frequency: DTC123JM3T5G: Not specified DTC123JMT2L: 250 MHz The DTC123JMT2L explicitly specifies its high-frequency performance (250MHz), making it suitable for applications requiring fast switching or specific frequency response. The DTC123JM3T5G lacks this parameter, introducing uncertainty regarding its high-frequency capability. It is not recommended as a direct substitute in applications involving high-speed switching. Recommendation Summary: Substitution may be considered for low-frequency, small-signal switching or amplification applications where the operating current is well below 100mA and sufficient power dissipation margin exists. However, saturation voltage and actual temperature rise must be verified. For applications involving higher load currents, constrained thermal environments with limited heat sinking, or requiring specific switching speed, direct replacement is not advised. A circuit re-evaluation or selection of an alternative part with more closely matched parameters is recommended.