Construction & Working of Enhancement-Type MOSFET (Part 2)

Summary

This lecture focuses on the working of an N-channel enhancement-type MOSFET. It explains how a channel is formed between the source and drain by increasing the gate voltage, making the gate more positive. The lecture contrasts enhancement-type MOSFETs with depletion-type MOSFETs, explains the role of threshold voltage in enabling current flow, and introduces key concepts like depletion region and pinch-off condition. Additionally, the impact of gate-to-source voltage (Vgs) and drain current (ID) is discussed, along with various scenarios for voltage behavior in the circuit.

Takeaways

• 🔍 The lecture discusses the construction and working principles of n-channel and Hansmann type MOSFETs.
• 📚 MOSFETs are classified into depletion type and enhancement type, with key differences in channel formation.
• ⚡ In enhancement type MOSFETs, no channel exists initially between the source and drain; it forms by applying a positive gate voltage.
• 🔬 N-channel enhancement MOSFETs have a P-type substrate and N-type wells, with a thin silicon dioxide layer separating the gate and the body.
• ⚖️ Channel formation occurs when electrons accumulate near the gate terminal, converting the surface region from P-type to N-type.
• 📈 Increasing the gate-to-source voltage (VGS) widens the channel, allowing significant current flow once the threshold voltage (VT) is surpassed.
• 🔑 Threshold voltage (VT) is the key parameter that determines when a sufficient channel forms to allow current flow between source and drain.
• ⚙️ VGS is used to represent the voltage between the gate and source, simplifying the circuit where the source and body are connected internally.
• 📏 The channel width depends on the excess voltage (VGS - VT), with greater VGS leading to a wider channel and higher current flow.
• 🛑 When the drain voltage (VDS) reaches a certain level (VGS - VT), the channel near the drain pinches off, limiting the current flow, a state called pinch-off.

Q & A

What are the key differences between depletion-type and enhancement-type MOSFETs?

The key difference is that in depletion-type MOSFETs, the channel is already present between the source and drain, whereas in enhancement-type MOSFETs, the channel is not initially present and must be formed by applying a positive gate voltage.

How is the channel formed in an enhancement-type MOSFET?

The channel in an enhancement-type MOSFET is formed by applying a positive gate voltage. This causes positive charges to accumulate at the gate terminal, which attracts negative charge carriers (electrons) in the P-type substrate, creating an N-type channel between the source and drain.

What is the significance of the thin layer of silicon dioxide in the structure of a MOSFET?

The thin layer of silicon dioxide acts as an insulator between the gate terminal and the body (substrate) of the MOSFET. This ensures that there is no direct electrical contact between the gate and the body, allowing the gate voltage to control the formation of the channel without current flowing through the gate.

What is the threshold voltage (Vt) in a MOSFET, and why is it important?

The threshold voltage (Vt) is the minimum gate-to-source voltage (Vgs) required to form a conductive channel between the source and drain in an enhancement-type MOSFET. It is important because the channel only forms and significant current flows when the gate voltage exceeds this threshold.

Why are the source and substrate terminals connected in most MOSFETs?

The source and substrate terminals are often connected internally to simplify biasing and reduce the need for multiple voltage sources. This ensures that the potential difference between the source and substrate is zero.

What happens to the depletion region when the gate-to-drain voltage (Vgd) is reduced?

When the gate-to-drain voltage (Vgd) is reduced, the depletion region near the drain widens, as there is less attraction for negative charges in this area. This causes the channel to become narrower near the drain, leading to an increase in the width of the depletion region.

What is meant by the term 'pinch-off' in the context of a MOSFET?

Pinch-off refers to the condition where the channel near the drain becomes extremely narrow due to a reduction in Vgd (gate-to-drain voltage) as Vds (drain-to-source voltage) approaches Vgs - Vt. Although the channel narrows, current continues to flow, but its increase is limited.

What is VD saturation, and why is it important?

VD saturation is the condition where the drain-to-source voltage (Vds) reaches a point where increasing Vds further does not increase the drain current (Id). This voltage is known as the saturation voltage (Vdsat), and it marks the transition to the saturation region of MOSFET operation, where Id becomes relatively constant.

How does the width of the channel depend on the gate-to-source voltage (Vgs)?

The width of the channel depends on the excess voltage, which is the difference between Vgs and the threshold voltage (Vt). As Vgs increases beyond Vt, the channel width increases, allowing more current to flow. A higher Vgs results in a wider channel.

What happens to the drain current (Id) when Vgs exceeds the threshold voltage?

When Vgs exceeds the threshold voltage, a conductive channel forms between the source and drain, allowing significant drain current (Id) to flow. The amount of current is proportional to the width of the channel, which increases with the excess voltage (Vgs - Vt).