BJT Depletion Region Recombination

While studying bipolar transistors, we often simplify the picture by ignoring recombination in the depletion region. However, just like in a p-n diode, this phenomenon contributes an additional current component that deserves attention.

The unique voltage dependence of depletion region recombination allows us to identify its impact separate from other currents. Take the Gummel plot in Figure 1 as an example. This graph depicts the collector and base currents of a silicon transistor in forward active mode with a fixed collector-base voltage.

Here's the key point: between roughly 0.2 and 0.4 volts of base-emitter voltage, the base current exhibits a noticeable rise, distinct from the exponential trend with lower voltages. This "bump" arises solely from depletion region recombination, a process not contributing to the collector current as it doesn't directly involve electrons flowing through the base.

In conclusion, recognizing the influence of depletion region recombination is crucial for a complete understanding of transistor behavior. This hidden contributor, while sometimes omitted for simplicity, significantly impacts the base current characteristics, especially at low base-emitter voltages.

In bipolar junction transistors (BJTs), a hidden force subtly influences their behavior: recombination in the depletion region. While often overlooked, this phenomenon plays a significant role in shaping the current characteristics, particularly at low voltages. Let's delve into its effects and understand why it deserves attention.

The Depletion Region: A Stage for Carrier Disappearance

Imagine the BJT as two p-n junctions stacked back-to-back. When biased, depletion regions form around each junction, devoid of mobile charge carriers. This creates an electric field that guides injected minority carriers (electrons in a pnp transistor, holes in an npn) across the base. However, within the depletion region, something unexpected happens:

• Carrier Collision and Recombination: Injected minority carriers can collide with majority carriers (holes in a pnp transistor, electrons in an npn) residing in the depletion region. These collisions can lead to recombination, where both carriers "disappear," resulting in a loss of current.

Impact on BJT Currents:

This depletion region recombination has several consequences for BJT operation:

• Additional Base Current: The recombined carriers don't contribute to the collector current. However, the holes consumed in the process need to be replenished from the base, resulting in an additional base current. This current component exhibits a different voltage dependence compared to other base currents, often appearing as a "bump" in Gummel plots (graphs of collector and base currents vs. base-emitter voltage).

• Reduced Current Gain: Since some injected electrons are lost to recombination, fewer reach the collector. This translates to a lower current gain, the key parameter amplifying the base current into a larger collector current.

• Temperature Dependence: Recombination rates increase with temperature, leading to a decrease in current gain and an increase in base current at higher operating temperatures.

Visualization: Depicting the Drama

To visualize this hidden interaction, imagine the following scenario:

1. Electrons injected: Electrons are injected from the emitter into the p-type base of a pnp transistor.

2. Depletion region crossing: These electrons traverse the depletion region, guided by the electric field.

3. Collision and recombination: Some electrons collide with resident holes in the depletion region, leading to recombination and their disappearance.

4. Base current boost: To compensate for the lost holes, additional holes flow from the base, contributing to an increased base current.

5. Reduced collector current: Fewer electrons reach the collector due to recombination, resulting in a lower collector current and ultimately, a reduced current gain.