Understanding the Sources of Leakage Current in Transistors

Leakage current in transistors is an issue that significantly impacts the performance and power consumption of integrated circuits, especially in low-power and high-density applications. This article explores the various sources of leakage current in transistors, including reverse bias leakage, subthreshold conduction, gate leakage, junction leakage, and thermal generation.

Reverse Bias Leakage in Bipolar Junction Transistors (BJTs)

In bipolar junction transistors (BJTs), reverse bias leakage occurs when the base-emitter junction is reverse-biased. A small amount of minority carrier injection occurs, leading to a small current known as reverse saturation current. This leakage current can be particularly significant, especially in applications where low leakage is critical.

Subthreshold Conduction in Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs)

While MOSFETs are designed to switch between on and off states, a current can still flow even when the gate voltage is below the threshold voltage. This phenomenon, known as subthreshold conduction, occurs due to the diffusion of carriers in the channel. The leakage current in this case is a significant concern for low-power applications and can be more pronounced as the transistor size continues to shrink.

Gate Leakage in MOSFETs

In MOSFETs, particularly in advanced technologies, gate leakage can become a major issue as the technology scales down. Tunneling of carriers through the thin gate oxide layer contributes to gate leakage current. This leakage can significantly affect the performance and reliability of the transistor, especially in low-voltage and low-power applications.

Junction Leakage in Transistors

In transistors, junction leakage occurs at the junctions due to thermal generation of carriers. This leakage is particularly significant at elevated temperatures. The thermal energy at higher temperatures can generate electron-hole pairs, contributing to leakage current. This effect can be exacerbated by the increased thermal energy and the limited control over the channel under certain conditions.

Thermal Generation and Hot Carrier Injection

At higher temperatures, the increased thermal energy can generate electron-hole pairs, leading to a significant increase in leakage current. Hot carrier injection is another phenomenon that can contribute to additional leakage. In some conditions, high-energy carriers can be injected into the gate oxide, leading to further leakage current.

Conclusion: The Main Source of Leakage Current

Thermally generated minority charge carriers are identified as the primary source of leakage current in transistors. The insufficient isolation of the channel source and the drain also contribute to these leaks. Engineers employ various methods to reduce leakage currents, including creating diodes or capacitors at these locations. Advanced transistor structures, such as FinFETs and Gate All Around (GAA) FETs, increase the surface area of the gate, mitigating the leakage problem.

Typically, planar MOSFETs are used in modern microprocessors above 20nm. More advanced chips employ FinFETs or GAA FETs to address the leakage issue effectively. The integration of these advanced structures ensures better control and lower leakage currents, making transistors more reliable and power-efficient.