1. Opposite Polarity: This is the fundamental difference. The MMBT3904T is an NPN type, with current flowing from collector to emitter and turning on with a high base voltage. The MMBT3906T is a PNP type, with current flowing from emitter to collector and turning on with a low base voltage. Power supply polarity, bias resistor design, and drive logic must be completely reconfigured. 2. Saturation Voltage (Vce(sat)): The MMBT3906T has a higher Vce(sat) (400mV) compared to the MMBT3904T (300mV) under identical test conditions. This means in hard-saturation switching applications, the PNP transistor will exhibit slightly higher conduction loss and self-heating than its NPN counterpart, potentially impacting efficiency and temperature rise. 3. DC Gain (hFE) Test Conditions: While both specify a minimum hFE of 100, the test condition for the MMBT3906T is Vce=1V, whereas for the MMBT3904T it is Vce=10V. This indicates the PNP device can maintain its gain at lower operating voltages (close to 1V). However, it also suggests its characteristics may differ from the NPN transistor at higher Vce levels, requiring attention to gain matching at the actual operating point. 4. Power Dissipation Capability: The MMBT3906T is rated for a higher maximum power (Pmax=250mW) than the MMBT3904T (150mW). Given their similar package sizes (SOT-523 vs. SC-89-3 with comparable thermal resistance), this difference is more likely attributable to variations in manufacturer testing standards or labeling conservatism rather than a genuine improvement in thermal performance. For design purposes, it is advisable to use the lower value as a general constraint. 5. Frequency Response: The MMBT3906T has a lower transition frequency (fT=250MHz) than the MMBT3904T (300MHz), indicating that the PNP transistor's high-frequency response or switching speed is slightly inferior. In circuits with stringent requirements for switching speed or bandwidth, this discrepancy can lead to signal asymmetry or performance degradation.