While the threshold voltage of MOSFETs is frequently discussed, are you familiar with the subthreshold voltage? Simply put, when the gate voltage is below the threshold voltage and the semiconductor surface is only in a 'weak inversion' state, the corresponding drain current is called subthreshold current. Here are some points to note:

(Weak inversion refers to a state where the minority carrier concentration at the semiconductor surface is greater than or equal to the majority carrier concentration at the surface but much less than the majority carrier concentration inside the body.)

In the subthreshold region, carrier diffusion rather than drift plays a decisive role in the drain current. The drain current can be derived using the method of deriving the collector current of a bipolar transistor with uniformly doped base, which can be expressed as: ID~eV-Vm)/NT

This shows that Ip is exponentially related to VG-VTH.

When MOSFETs are used as low-voltage, low-power devices, such as in digital logic circuit switches or memory, the subthreshold region becomes particularly important. This is because it describes how the switch conducts and cuts off.

How Is the Threshold Voltage Generated in MOSFETs?

The generation of the subthreshold region is due to the fact that when the gate voltage is below the threshold voltage, electrons in the MOSFET cannot fully flow from the source to the drain. In the subthreshold region, there are still some electrons flowing from the source to the drain, but the current is small. This is because in the subthreshold region, the channel formation of the MOSFET is restricted and cannot be fully opened.

Role and Advantages of the Subthreshold Region in MOSFET Devices

1. Current and Power Consumption: In the subthreshold region, the drain current is exponentially related to the gate-source voltage, which gives the device high current amplification capability for small signal inputs. At the same time, it allows the device to have a certain conductivity even in a low-power state, reducing overall power consumption.

2. Noise Reduction: The existence of the subthreshold region makes the device have lower noise for small signal inputs, improving the noise performance of the device.

3. Reliability and Stability: The subthreshold region reduces the on-resistance of the device, improves the conduction capacity and reliability of the device. It can also reduce performance fluctuations caused by temperature changes, improving the stability of the device.

There Are Some Drawbacks

Slow Speed: MOSFETs in the subthreshold region have a slow switching speed due to the small current, which may lead to longer response times.

2. Limits Current Gain: MOSFETs in the subthreshold region have a low current gain, making them unsuitable for applications requiring high currents.

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