When a P-N junction diode is forward biased, it means that the positive terminal of the battery is connected to the P-side of the diode and the negative terminal is connected to the N-side. This creates a potential difference across the diode, causing the P-N junction to conduct current. The current in the circuit is controlled by the resistance.

Resistance:
The resistance in the circuit determines the flow of current. In a forward-biased P-N junction diode, the resistance offered by the diode is relatively low. This allows a significant amount of current to flow through the diode. The resistance of the diode is determined by the doping levels and the physical characteristics of the material.

When the diode is forward biased and a voltage is applied across it, the P-N junction allows the majority carriers (holes in the P-side and electrons in the N-side) to move across the junction. This movement of carriers constitutes the flow of current through the diode. The resistance of the diode determines the ease with which the carriers can move through the junction. A lower resistance allows for a higher current flow.

External voltage, capacitance, and internal voltage:
While the external voltage (applied across the diode), capacitance (related to the storage of charge), and internal voltage (voltage drop across the diode) are important factors in the behavior of a diode, they do not directly control the current in the circuit.

The external voltage simply provides the potential difference required to forward bias the diode. The capacitance of the diode plays a role in its transient response but does not directly control the current flow. The internal voltage refers to the voltage drop across the diode, which is typically around 0.7V for a silicon diode. This voltage drop is a result of the forward biasing and the resistance of the diode.

In summary, when a P-N junction diode is forward biased, the current in the circuit is primarily controlled by the resistance offered by the diode. The lower the resistance, the higher the current flow. The external voltage, capacitance, and internal voltage play secondary roles in the behavior of the diode but do not directly control the current.