1. DC Current Gain (hFE) and Test Conditions: DTC123EM3T5G (onsemi): Minimum 8 (@ 5mA, 10V). DTC123EMT2L (Rohm): Minimum 20 (@ 20mA, 5V). The test conditions for collector current (Ic) and collector-emitter voltage (Vce) are completely different, rendering the hFE values not directly comparable. The Rohm part exhibits higher gain under its specified conditions, while the onsemi part is tested at a lower Ic and higher Vce. At the same actual operating point, their amplification capabilities will differ, potentially affecting drive levels, switching speed, or the linearity of analog amplification circuits. 2. Collector-Emitter Saturation Voltage (Vce(sat)) and Test Conditions: DTC123EM3T5G: Maximum 250mV (@ 5mA Ib, 10mA Ic). DTC123EMT2L: Maximum 300mV (@ 500µA Ib, 10mA Ic). The Rohm part achieves a similar saturation voltage with a much smaller base drive current (only 1/10th). This indicates superior saturation characteristics for the DTC123EMT2L, making it easier to drive into deep saturation. This is advantageous for low-power drive applications or where ensuring a low voltage drop in switching is critical. The onsemi part requires a larger base current to achieve a similar saturation effect. 3. Transition Frequency (fT) and Maximum Power Dissipation (Pd): DTC123EM3T5G: fT not specified, Pd = 260mW. DTC123EMT2L: fT = 250MHz, Pd = 150mW. The Rohm part specifies its high-frequency switching capability (250MHz), making it suitable for applications requiring a certain switching speed. The onsemi part's high-frequency performance is uncertain as it is not specified. However, the onsemi part has a significantly higher maximum power dissipation (+73%), suggesting better thermal performance or package heat dissipation capability. It can withstand the same power dissipation under higher ambient temperatures or more demanding conditions. 4. Package and Price: Package: Although the physical dimensions are both SOT-723, the supplier package names differ (SOT-723 vs. VMT3). There may be subtle differences in pad layout or internal connections; it is necessary to verify the package drawings in the respective datasheets. Price: The Rohm part price ($0.43) is significantly higher than the onsemi part ($0.24). Direct substitution would result in an approximate 79% cost increase. Summary: These are not parametrically equivalent, pin-compatible substitutes. The choice depends on the application's priorities: If low drive current and specified high-frequency performance are emphasized, the Rohm part can be selected, but thermal design must be verified. If higher power dissipation capability and lower cost are critical, with no specific requirement for high-frequency characteristics, the original onsemi part should be retained. Before any substitution, the matching of gain and saturation characteristics must be evaluated at the actual circuit operating points.