DepletionMode

Depletion-mode refers to a class of field-effect transistors (FETs) that are normally conductive at zero gate-source voltage. In depletion-mode devices, the conducting channel exists without gate bias, and applying a gate voltage depletes carriers to reduce current or turn the device off. This behavior contrasts with enhancement-mode devices, which are normally off and require a gate voltage to create a conducting channel.

The most familiar example is the depletion-mode JFET, which is commonly an n-channel device. At Vgs = 0, current flows freely; applying a negative gate-source voltage reduces the channel width by expanding the depletion region, eventually pinching off the channel at a characteristic pinch-off voltage. Depletion-mode MOSFETs also exist, though they are less common in modern digital design. In depletion-mode MOSFETs, the channel is present at zero gate bias, and a gate voltage of the opposite polarity relative to the channel type depletes carriers to reduce conduction and can turn the device off.

Properties and behavior vary by device type and material system, but the general principle is consistent: gate-induced depletion controls conductance rather than requiring gate-induced channel formation. For N-channel depletion devices, turning off typically requires a negative gate voltage; for P-channel depletion devices, a positive gate voltage achieves depletion.

Applications for depletion-mode devices include simple current regulation, analog switching, and certain RF or biasing networks where a normally-on margin is advantageous. They are often used where a stable, wide-range current path is desirable without a continuous gate drive, though enhancement-mode transistors have become predominant in many modern digital circuits.

See also: field-effect transistor, JFET, MOSFET, enhancement-mode transistor, pinch-off voltage.