What Is Threshold Voltage?

Threshold voltage : Generally, the input voltage corresponding to the midpoint of the transition region where the output current in the transmission characteristic curve changes dramatically with the input voltage is called the threshold voltage. It has different parameters when describing different devices. When describing the characteristics of field emission, the voltage when the current reaches 10 mA is called the threshold voltage.

Such as MOS tube, when the device changes from depletion to inversion, it has to undergo a Si surface electron concentration equal to

The threshold voltage of a particular transistor is related to many factors, including the doping of the backgate, the thickness of the dielectric, the gate material and the excess charge in the dielectric.

Threshold voltage back gate doping

The doping of the backgate is the main factor determining the threshold voltage. If the back gate is doped

The heavier it is, the harder it is to reverse. A stronger electric field is required to reverse, and the threshold voltage rises. The back gate doping of the MOS tube can be adjusted by a slight implant under the surface of the dielectric layer. Such implants are called threshold-adjusted implants (or Vt-adjusted implants). Consider the impact of Vt adjustment implant on NMOS tubes. If the implant consists of acceptors, the silicon surface is more difficult to reverse and the threshold voltage is raised. If the implant is composed of donors, the silicon surface is more likely to reverse and the threshold voltage drops. If enough donors are implanted, the silicon surface is actually reverse-doped. In this way, there is a thin layer of N-type silicon at zero bias to form a permanent channel. As the gate bias voltage rises, the channel becomes more and more inverted. As the gate bias voltage drops, the channel becomes weaker and weaker, and eventually disappears. The threshold voltage of this NMOS tube is actually negative. Such a transistor is called a depletion mode NMOS, or simply a depletion mode NMOS. In contrast, an NMOS with a positive threshold voltage is called enhanced mode NMOS, or enhanced NMOS. Most of the commercially produced MOS tubes are enhanced devices, but there are some applications that require depleted devices. Depletion PMOS can also be produced. The threshold voltage of such a device is positive. Depleted devices should be clearly identified as much as possible. The sign of the threshold voltage cannot be judged because usually many engineers ignore the polarity of the threshold voltage. Therefore, it should be said that the "depletion PMOS with a threshold voltage of 0.7V" is not a PMOS with a threshold voltage of 0.7V. Many engineers will interpret the latter as an enhanced PMOS with a threshold voltage of -0.7V instead of a depleted PMOS with a threshold voltage of + 0.7V. It is clear that depletion devices can save a lot of misunderstandings.

Threshold voltage dielectric

The dielectric also plays an important role in determining the threshold voltage. Thick dielectrics weaken the electric field because they are relatively thick. So thick dielectrics increase the threshold voltage, while thin dielectrics decrease the threshold voltage. Theoretically, the dielectric composition also affects the electric field strength. In fact, almost all MOS transistors use pure silicon dioxide as the gate dielectric. This substance can grow into very thin films with extremely pure purity and uniformity; no other substance can be compared to it. Therefore, other dielectric materials have very few applications. (High dielectric constant materials such as silicon nitride are also used as gate dielectric devices. Some authors refer to all MOS transistors, including non-oxide dielectrics, as insulated-gate field effect transistor (IGFET))

Material composition of threshold voltage gate

The material composition of the gate also affects the threshold voltage. As mentioned above, when GATE and BACKGROUND are shorted, an electric field is applied to the gate oxide. This is mainly due to the difference in work function between GATE and BACKGATE substances. Most practical transistors are heavily doped

Threshold voltage (8 photos)

Polysilicon is used as the gate. Changing the doping level of polysilicon can control its work function.

Excess charge at the threshold voltage dielectric layer and gate interface

The excess charge at the interface between GATE OXIDE or the oxide and silicon surface may also affect the threshold voltage. These charges may have ionized impurity atoms, trapped carriers, or structural defects. The charge trapped on the dielectric or its surface can affect the electric field and further affect the threshold voltage. If the captured electrons change with time, temperature, or bias voltage, the threshold voltage also changes.