The FDTA series of digital transistors is designed to replace a single device and its external resistor bias network. The bias resistor transistor contains a single transistor with a monolithic bias network consisting of two resistors; a series base resistor and a base− emitter resistor. The FDTA eliminates these individual components by integrating them into a single device. The use of a FDTA can reduce both system cost and board space.

Bias resistor built-in transistors (BRTs), also known as digital transistors, are designed to function as switches. The BRT is a bipolar transistor containing a series base resistor (R1) and a base-emitter resistor (R2) to simplify on-board circuit design.

Typical bipolar transistors are used as amplifiers, buffers, and switches. Amplifiers and buffers are used in the linear active region of the V_CE-I_C curve. source region (called saturation region and cutoff region respectively). In the saturation region, the collector-emitter voltage drop (V_CE) becomes minimum, resulting in maximum collector current. In the cutoff region, the transistor is completely cut off. Under cutoff conditions, only a tiny collector cutoff current (I_CEO) can flow from the collector to the emitter.
Due to the built-in bias resistor, the BRT operates in the saturation region when it is turned on and in the cut-off region when it is turned off.
The bias resistor configuration is shown in the figure. R1 converts the voltage applied to the B terminal into current to stabilize BRT operation, while R2 acts as a pull-down resistor, pulling the base voltage to the GND level when the BRT is turned off.

Without R2, leakage current from the input or collector-off current (I_CBO) in the "off" state could cause BRT failure due to charge accumulation in the base. R2 helps prevent faults by passing leakage current to GND.