Cgd

Cgd, or gate-to-drain capacitance, is the capacitance between the gate and drain terminals of a field-effect transistor, most commonly a MOSFET. It is one of the intrinsic parasitic capacitances of the device, alongside Cgs (gate-to-source) and Cgb (gate-to-body). Cgd arises from the gate electrode overlapping the drain region and from fringing electric fields. Its magnitude depends on device geometry, oxide thickness, and operating bias, and it can vary with drain-to-source voltage due to depletion effects and changes in fringe fields.

In AC and switching operation, Cgd couples drain voltage fluctuations back to the gate, a phenomenon known as the Miller effect. The effective input capacitance is increased by the Miller contribution, roughly C_in ≈ C_gs + C_gd(1 − A_v) for inverting configurations, where A_v is the small-signal gain from gate to drain. This can slow down switching and alter bandwidth, especially in high-gain or high-speed circuits.

Modeling and measurement: In circuit models such as SPICE, Cgd is represented as a mutual capacitance between the gate and drain (often denoted CGD). Its value can be treated as bias-dependent in more advanced models, and it is typically characterized experimentally or provided in device datasheets. Values are usually in the femtofarad to picofarad range, scaling with transistor width and process technology.

Importance: Cgd is a key parameter in RF design and high-speed electronics, influencing input capacitance, feedback through the Miller effect, and overall frequency response. It is considered together with Cgs and Cgb when analyzing MOSFET capacitances and transient behavior.