Gate Charge (Qg)
It is the electric charge that has to be transferred to a MOSFET's gate to turn it fully on, given in nanocoulombs. The switching time is found from the relation t = Qg / I: a 50 nC gate charges in 50 ns with a 1 A driver current.
Gate charge is a more useful measure than capacitance, because while a MOSFET's gate capacitance changes noticeably with voltage, Qg gives the total charge required for the whole transition as a single number.
It forms the basis of two practical calculations. The first is the switching time: the larger the current the driver can supply, the shorter the transition. The second is the driver's own consumption: at every transition a charge of Qg is drawn from the supply voltage, which means a power of P = Qg × Vgs × f. For 50 nC, 12 V and 20 kHz this is approximately 12 mW; small per switch, but in multi-channel and high-frequency designs the total becomes measurable.
On the selection side there is a trade-off: low channel resistance usually means a large die area, and a large die area means high gate charge. That is, lowering the conduction loss tends to increase the switching loss. High-current, low-frequency designs favour the low RDS(on) side, and high-frequency designs the low Qg side.
The context in which this term is used: DC Motor Driver Selection Guide