Let, JE(emitter junction) and JC(collectors junction)
JE - forward, JC- forward , transistor operates in saturation
JE - forward, JC- reverse, transistor operates in active
JE - reverse, JC- reverse, transistor operates in cut-off

Explanation:
When a BJT (Bipolar Junction Transistor) is biased in such a way that one junction is forward biased and the other junction is open, it operates in saturation mode.

Saturation mode:
In saturation mode, both the base-emitter junction and the base-collector junction are forward biased. This means that a high current flows through the transistor, and it can be considered as a closed switch. The transistor is in its most conductive state in saturation mode.

Forward biasing:
Forward biasing means that the positive terminal of the battery is connected to the P-type region of the transistor, and the negative terminal is connected to the N-type region.

Emitter open:
When the emitter is open, it means that the emitter terminal is not connected to any circuit. In this case, the collector-base junction is forward biased, and the collector current is controlled by the base current.

Collector open:
When the collector is open, it means that the collector terminal is not connected to any circuit. In this case, the emitter-base junction is forward biased, and the emitter current is controlled by the base current.

Operating in saturation:
When one junction is forward biased and the other junction is open, the BJT operates in saturation mode. In saturation mode, the transistor is fully turned on and acts like a closed switch. The collector current (for emitter open) or the emitter current (for collector open) is at its maximum value.

Importance of saturation mode:
Saturation mode is important in applications where the transistor needs to act as a switch. In saturation mode, the transistor can pass a high amount of current, allowing it to drive loads such as relays, motors, or LED arrays.

Conclusion:
When a BJT has one junction forward biased and either the emitter or collector is open, it operates in saturation mode. In saturation mode, the transistor acts like a closed switch, allowing a high current to flow through it.