1. Bias Resistor R2 (Emitter-Base Resistor): The DDTC114WCA-7-F integrates a 4.7 kΩ resistor, while the BCR135E6327HTSA1 features a 47 kΩ resistor. Under the same operating voltage, the BCR135 exhibits significantly higher base input impedance, requiring less drive current. However, it may be more susceptible to noise, and switching speed may be slower due to the increased discharge time constant associated with R2. 2. DC Current Gain (hFE): The minimum hFE of the BCR135 (70 @ 5mA, 5V) is substantially higher than that of the DDTC114 (24 @ 10mA, 5V). This indicates that the BCR135 can deliver higher collector current for the same base current, offering stronger drive capability. When substituting, the original base current must be re-evaluated to avoid unintended changes in saturation depth. 3. Transition Frequency (fT): The BCR135 has an fT of 150 MHz, which is lower than the 250 MHz of the DDTC114. Consequently, the BCR135 offers relatively slower high-frequency amplification and switching response, potentially making it unsuitable for frequency-sensitive applications such as high-speed switching or RF front-end circuits. 4. Collector Cutoff Current (ICBO): The ICBO of the BCR135 is 100 nA, lower than the 500 nA of the DDTC114. This indicates that the BCR135 has superior leakage characteristics under high temperature or high voltage conditions, contributing to reduced static power dissipation and improved thermal stability.