# Base-collector current in BJT Saturation

• Helena Wells
In summary, the conversation discusses the search for a formula for the current flowing from the base to the collector in saturation mode of a BJT. The conversation mentions references to the emitter current in forward active mode, but none for the base-collector current in saturation mode. The conversation also mentions the use of a base current that is approximately 1/10 of the collector current as a good indication for a forward biased B-C junction. It is noted that the forward biased diode equation is used in saturation mode and the device is completely flooded with carriers. A link to further equations is provided.
Helena Wells
TL;DR Summary
This thread is about the base-collector current in BJT saturation
I can't find the formula for the current flowing from the base to the collector in saturation mode of BJT.Any help?There are many references to the emitter current in the forward active mode but none for the base-collector current in saturation mode.Help appreciated.

You are looking for a formula?
The other way round: A pretty large base current is an indication for forward operation of the base-collector junction. It is common practice to say (for example) that a base current which is app. 1/10 of the collector current is a good and safe indication for a forward biased B_C junction. But this is a "rulre of thumb" only - a factor of 0.05 is also good enough.

yeah just write there is a formula for the relationship between collector , emitter current and base current in forward active mode i was wondering if there is for saturation.

Helena Wells said:
yeah just write there is a formula for the relationship between collector , emitter current and base current in forward active mode i was wondering if there is for saturation.
It is just the forward biased diode equation. The BC junction is forward biased in saturation and there can't be any "transistor" action because the voltage polarities aren't right. The device is completely flooded with carriers flowing through both the BE and BC junctions.

You can find some of the equations here: http://web.mit.edu/6.012/www/SP07-L18.pdf

edit: Removed some of the words, which aren't correct. The device is still a transistor, βf & βr still matter. But in normal application circuits it isn't working like a transistor, the collector current is normally determined by eternal things (resistors and voltages).

Last edited:
Ah ok thanks Dave!

## 1. What is BJT saturation?

BJT saturation is a state in which the transistor is fully turned on and the collector-emitter voltage is at its minimum level. In this state, the transistor behaves like a closed switch, allowing a large amount of current to flow from the collector to the emitter.

## 2. How is the base-collector current affected in BJT saturation?

In BJT saturation, the base-collector current is at its maximum value. This is because the transistor is fully turned on, allowing a large amount of current to flow from the collector to the emitter. The amount of base current required to achieve saturation is called the saturation current, and it is typically a few percent of the collector current.

## 3. What factors affect the base-collector current in BJT saturation?

The base-collector current in BJT saturation is affected by the base current, the collector-emitter voltage, and the characteristics of the transistor itself. The base current must be sufficient to turn on the transistor, and the collector-emitter voltage must be low enough to keep the transistor in saturation. The transistor's characteristics, such as its gain and saturation current, also play a role in determining the base-collector current in saturation.

## 4. How does BJT saturation differ from BJT active mode?

BJT saturation and active mode are two different operating states of a transistor. In active mode, the transistor is partially turned on and acts as an amplifier, while in saturation, the transistor is fully turned on and behaves like a switch. In active mode, the collector current is controlled by the base current, while in saturation, the collector current is determined by the characteristics of the transistor.

## 5. What is the significance of base-collector current in BJT saturation?

The base-collector current in BJT saturation is an important parameter in transistor circuits, as it determines the amount of current that can flow through the transistor. It is also used in the design and analysis of transistor circuits, as it helps in determining the operating point and the stability of the circuit. In addition, the base-collector current in saturation is used to calculate the transistor's power dissipation and efficiency.

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