The PMBF4391 from NXP Semiconductors is a Junction Field Effect Transistor (JFET) with Gate Source Voltage 40 V, Continous Drain Current 50 mA, Drain Source Resistance 30 Ohm, Gate Current 50 mA, Power Dissipation 0.25 W. Tags: Surface Mount. More details for PMBF4391 can be seen below.