Overdrive voltage

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Overdrive voltage, usually abbreviated as VOV, is typically referred to in the context of MOSFET transistors. The overdrive voltage is defined as the voltage between transistor gate and source (VGS) in excess of the threshold voltage (VTH) where VTH 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." [1] Overdrive voltage can be found using the simple equation: VOV = VGS − VTH.

MOSFET Transistor used for amplifying or switching electronic signals.

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of field-effect transistor that is fabricated by the controlled oxidation of a semiconductor, typically silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The MOSFET is the basic building block of modern electronics. Since its invention by Mohamed M. Atalla and Dawon Kahng at Bell Labs in November 1959, the MOSFET has become the most widely manufactured device in history, with an estimated total of 13 sextillion (1.3 × 1022) MOS transistors manufactured between 1960 and 2018.

Threshold voltage Minimum source-to-gate voltage for a field effect transistor to be conducting from source to drain

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

Contents

Technology

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

The saturation current, more accurately, the reverse saturation current, is that part of the reverse current in a semiconductor diode caused by diffusion of minority carriers from the neutral regions to the depletion region. This current is almost independent of the reverse voltage.

Overdrive voltage is also important because of its relationship to VDS, 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:

ConditionsRegion of OperationDescription
VDS > VOV; VGS > VTHSaturation (CCR)The MOSFET is delivering a high amount of current, and changing VDS won't do much.
VDS < VOV; VGS > VTHTriode (Linear)The MOSFET is delivering current in a linear relationship to the voltage (VDS).
VGS < VTHCutoffThe 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 (VGS < 0) we attract more holes, and this is called accumulation. A positive gate voltage (VGS > 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 (VTH) 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, VGS, beyond VTH, we are said to be then overdriving the gate by creating a stronger channel, hence the overdrive (often called Vov, Vod, or Von) is defined as (VGS − VTH).

See also

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References

  1. ↑ Sedra and Smith, Microelectronic Circuits, Fifth Edition, (2004) Chapter 4, ISBN   978-0-19-533883-6
  2. ↑ Lecture Note of Prof Liu, UC Berkeley
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