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

MOSFET upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...

transistors A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...

. The overdrive voltage is defined as the voltage between transistor gate and source (V_GS) in excess of the

threshold voltage The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important s ...

(V_TH) where V_TH is defined as the minimum voltage required between gate and source to turn the transistor on (allow it to conduct electricity). Due to this definition, overdrive voltage is also known as "excess gate voltage" or "effective voltage." Overdrive voltage can be found using the simple equation: V_OV = V_GS − V_TH.

Technology

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.Lecture Note of Prof Liu, UC Berkeley
/ref> 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: {, class="wikitable" , - ! Conditions ! Region of Operation ! Description , - , V_DS > V_OV; V_GS > V_TH , Saturation (CCR) , The MOSFET is delivering a high amount of current, and changing V_DS won't do much. , - , V_DS < V_OV; V_GS > V_TH , Triode (Linear) , The MOSFET is delivering current in a linear relationship to the voltage (V_DS). , - , V_GS < V_TH , Cutoff , The MOSFET is turned off, and should not be delivering any current. 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).

References

Electrical parameters Semiconductors MOSFETs

Technology

See also

References