Understanding mosfet leakage current is essential for anyone designing or troubleshooting power electronics and high‑gain analog circuits. This parameter, often specified in datasheets as IDS(Q) or IGS, represents the small current that flows even when the device is intended to be fully off. While a single unit might draw mere nanoamps, in large integrated circuits or high‑voltage systems these pathways can combine to cause significant static power loss, thermal stress, and output voltage drift.
What Is Mosfet Leakage Current
At its core, mosfet leakage current refers to the unintended conductive paths that exist in a metal‑oxide‑semiconductor field‑effect transistor. The most common contributors are subthreshold leakage, gate‑oxide tunneling, and leakage through the body or bulk region. These mechanisms are present in both N‑channel and P‑channel devices, although their magnitudes and temperature dependencies differ. In many applications, controlling this current is as important as managing the on‑state resistance, especially when the design targets battery life or high‑impedance nodes.
Key Mechanisms Behind Leakage
Subthreshold and Tunneling Paths
Subthreshold leakage occurs when the gate‑source voltage is below the nominal threshold, yet a weak inversion layer still allows carriers to traverse the channel. As process nodes shrink, the gate oxide becomes thinner, enabling direct tunneling through the dielectric. These two mechanisms often dominate the overall mosfet leakage current in modern low‑voltage CMOS technologies, where the supply voltage approaches the threshold voltage itself.
Body and Gate‑Oxide Contributions
The body terminal introduces additional paths, particularly in lateral structures where the source and body are closely spaced. Leakage here can be influenced by substrate bias and temperature, sometimes varying by an order of magnitude over industrial ranges. Gate‑oxide defects, such as oxide traps or thin spots, can also create leakage channels that bypass normal shielding, leading to higher than expected current in supposedly off states.
How to Measure and Specify Leakage
Datasheets typically quote mosfet leakage current under defined conditions, including a specific gate‑source voltage, often zero volts, and a specified drain‑source voltage. For bipolar transistors, the collector‑emitter leakage with the base open is an analogous specification. It is important to note that the measurement junction temperature, package type, and test time all influence the reported value. A device characterized at 25°C may show significantly higher leakage at the board‑level operating temperature due to the inherent temperature dependence of carrier mobility and intrinsic carrier concentration.
