1. DC Current Gain (hFE) and Linearity: The minimum hFE of the 2SCR513 (180 @ 50mA) is significantly higher than that of the BCX54 (100 @ 150mA), and its test condition current is lower. At the same operating point, the 2SCR513 typically offers higher gain, requiring less base current to drive the same collector current (Ic). Note that hFE varies with Ic; since the original circuit was designed around a lower gain device, substitution may affect feedback loop stability or bias point. 2. Saturation Voltage (Vce(sat)): At the same 500mA Ic, the maximum Vce(sat) of the 2SCR513 (350mV) is lower than that of the BCX54 (500mV). In switching or linear power applications, the 2SCR513 exhibits lower conduction loss, reduced heating, and higher efficiency. 3. Cutoff Characteristics and Leakage Current: The maximum collector cutoff current (ICBO) of the 2SCR513 (1µA) is an order of magnitude higher than that of the BCX54 (100nA). Under high-temperature or high Vce operating conditions, the 2SCR513 may have higher off-state leakage power dissipation, which is a consideration for low-power or high-precision cutoff circuits. 4. Power Handling and Thermal Performance: The maximum power dissipation of the 2SCR513 (2W) is higher than that of the BCX54 (1.25W), despite both using the same TO-243AA package. This indicates that the 2SCR513 has a lower chip-to-package thermal resistance (RthJA) or allows higher power dissipation at elevated junction temperatures, offering greater thermal design margin. 5. Frequency Characteristics: The transition frequency (360MHz) of the 2SCR513 is twice that of the BCX54 (180MHz), giving it superior high-frequency small-signal performance. It is better suited for higher-frequency amplification or switching applications, with less gain roll-off at elevated frequencies.