Question

What is current in BJT?

Answer 1

In a Bipolar Junction Transistor (BJT), **current** refers to the flow of electric charge carriers (electrons or holes) through the transistor's different regions. BJTs have three main parts: the **Emitter**, **Base**, and **Collector**. Each region of the BJT is involved in the flow of current:

1. **Emitter Current (I_E)**: This is the current that flows into the emitter. It consists of two parts:
   - **Base current (I_B)**: A small current that enters the base.
   - **Collector current (I_C)**: The majority of the emitter current flows through the collector.

2. **Base Current (I_B)**: This is the smallest current in the BJT. It controls the flow of the larger collector current. The base current is injected into the base and is necessary for the transistor to work as a switch or amplifier.

3. **Collector Current (I_C)**: This is the current that flows from the collector to the emitter. It is the main current that flows through the BJT, and its value is primarily determined by the base current (I_B) and the transistor's **current gain (β)**.

### Relationship between these currents:
In an ideal BJT:
- The **Emitter current (I_E)** is the sum of the **Base current (I_B)** and the **Collector current (I_C)**:
  \[
  I_E = I_B + I_C
  \]
- The **Collector current (I_C)** is roughly proportional to the **Base current (I_B)**, with the proportionality constant being the **current gain (β)** of the transistor:
  \[
  I_C = β \cdot I_B
  \]

The key idea is that a small base current controls a much larger collector current, allowing the transistor to act as an amplifier or switch.

In summary, in a BJT, current plays a crucial role in how the transistor functions, and the flow of current is governed by the interactions between the emitter, base, and collector regions.