7.6.2. Reverse Bias Condition

In normal operation for a BJT, the base-collector junction (junction-J2) is reverse biased, meaning the n-region has a higher potential than the p-region, thus creating a reverse bias condition.

The reverse bias condition in a BJT occurs when the collector to base junction is exposed to a higher voltage on the collector's n-region compared to the p-region of the base. This prevents current from flowing easily through the junction; the minority carriers present in the junction face a barrier due to the reverse voltage. As the voltage increases, the depletion region widens, reducing the likelihood that electrons from the collector can recombine with holes in the base. This is critical for maintaining the transistor's cutoff or active states in many applications, allowing devices to switch or amplify signals effectively.

Think of the reverse bias condition like a dam holding back water. In this scenario, the dam is the reverse voltage that prevents water (current) from flowing from the collector (upstream) to the base (downstream). Just as a dam maintains water levels and controls flow, the reverse bias maintains the conditions that prevent current flow until a specific voltage threshold is reached.

Under reverse bias, the minority carrier concentrations in both the n-type and p-type regions drop significantly, and the current mainly comprises the reverse saturation current.

When the BJT is reverse biased, the minority carriers (electrons in the p-type base and holes in the n-type collector) diminish due to the widening of the depletion layer and the higher energy barrier posed by the reverse voltage. This causes the junction to have a very small current flowing, which can still be measured, referred to as the reverse saturation current. The current is primarily dependent on how many minority carriers can overcome the barrier imposed by the reverse bias. Thus, the overall effect minimizes the current flow but does not entirely stop it.

You can think of minority carriers under reverse bias like fish that can swim through a narrow passage under water to reach another side. While there may be some fish that can successfully make it through (those representing the minority carriers), the majority of fish (representing majority carriers) are kept back by the pressure and narrowness of the passage (the reverse voltage), making it a rare occurrence.

The total reverse current through the junction under reverse bias is approximated using expressions that resemble those used in the forward bias but consider the sign of the voltage appropriately.

In reverse bias, the total current flowing through the junctions of the BJT can be formulated similarly to forward bias conditions, but recognizes that the current flow is opposite. The equations typically yield very small currents given the nature of the junction being reverse biased; thus, they are represented by factors of the saturation current and the respective carrier concentrations. Here, the presence of both electrons and holes contributes to these small currents, but each will be affected inversely by the applied reverse voltage.

Imagine a crowded, narrow hallway (representing the junction under reverse bias). If people (electrons and holes, respectively) are pushed in from one side of the hallway, only a few might trickle through while the rest remain stuck due to the narrow passage (the reverse voltage imposing an energy barrier). The total movement through the hallway can be likened to the total current, which is minimal yet present due to those few who manage to sneak through.

--