Overdrive voltage

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Overdrive voltage, usually abbreviated as V_OV, is typically referred to in the context of

V_OV is important as it directly affects the output drain terminal current (I_D) of the transistor, an important property of amplifier circuits. By increasing V_OV, I_D can be increased until saturation is reached.^[2]

Overdrive voltage is also important because of its relationship to V_DS, the drain voltage relative to the source, which can be used to determine the region of operation of the MOSFET. The table below shows how to use overdrive voltage to understand what region of operation the MOSFET is in:

A more physics-related explanation follows:

In an NMOS transistor, the channel region under zero bias has an abundance of holes (i.e., it is p-type silicon). By applying a negative gate bias (V_GS < 0) we attract more holes, and this is called accumulation. A positive gate voltage (V_GS > 0) will attract electrons and repel holes, and this is called depletion because we are depleting the number of holes. At a critical voltage called the threshold voltage (V_TH) the channel will actually be so depleted of holes and rich in electrons that it will INVERT to being n-type silicon, and this is called the inversion region.

As we increase this voltage, V_GS, beyond V_TH, we are said to be then overdriving the gate by creating a stronger channel, hence the overdrive (often called V_ov, V_od, or V_on) is defined as (V_GS − V_TH).

See also

• MOSFET
• Threshold voltage
• Electronic amplifier
• Short-channel effect
• Biasing

References