Theory

Introduction:
The Gummel plot is a fundamental tool in semiconductor device characterization, providing insights into the behavior and parameters of BJTs essential for practical application in electronic circuits. A typical Gummel Plot shown in Figure 1:

Fig. 1. Circuit diagram

Operating Regions of BJT

A BJT has three main operating regions:

• Cut-off Region: Both I_B and I_C are very small.
• Active Region: The transistor operates as an amplifier.
• Saturation Region: The transistor is fully turned on.

Key Equations

Collector Current (I_C):

$$I_C = I_S (e^{V_{BE}/V_T} - 1)$$

Where:

• I_S is the saturation current.
• V_BE is the base-emitter voltage.
• V_T is the thermal voltage.

Base Current (I_B):

$$I_B = I_C / β$$

Where β is the current gain of the transistor.

Gummel Plot Analysis

The Gummel plot involves plotting:

1. I_B vs V_BE: Shows exponential relationship.
2. I_C vs V_BE: Also shows exponential relationship.

Importance of Gummel Plot

• Parameter Extraction: Enables extraction of key parameters such as I_S, β and the ideality factor of the transistor.
• Device Characterization: Provides understanding of the transistor's behavior.
• Circuit Design: Essential for designing circuits using BJTs.