MOSFETs: Structure & Operation Modes

Hello Alex,

Welcome back. In our last lesson, we built the final piece of our BJT analysis toolkit: the hybrid-pi small-signal model. You learned how to linearize the transistor's behavior around its DC Q-point, allowing us to analyze its performance as an AC amplifier.

Today, we pivot to the other giant of the transistor world: the MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). While BJTs are foundational, MOSFETs are the undisputed backbone of modern digital electronics and are also vital in power and RF applications. This lesson will cover their fundamental structure and the different ways they are designed to operate.

Learning Outcome: MOSFET Structure and Modes of Operation (Enhancement vs. Depletion).

Approximate time to complete: 50 minutes.

1. From Current to Voltage Control: The MOSFET Paradigm

Before diving into the physics, it's crucial to understand the fundamental conceptual shift from BJTs to MOSFETs. A BJT is a current-controlled device: a small base current ( $I_{B}$ ) controls a large collector current ( $I_{C}$ ). A MOSFET, by contrast, is a voltage-controlled device: the voltage applied to its gate terminal controls the current flowing between its drain and source terminals.

This difference has profound implications. The MOSFET's gate is electrically isolated from the main current-carrying channel by a thin layer of silicon dioxide ( $S i O_{2}$ )—an excellent insulator. This means that, in a DC state, almost no current flows into the gate, giving it a near-infinite input impedance. This is a major advantage over the BJT, which requires a continuous base current to stay on.

The video below uses a fantastic water-flow analogy to illustrate this core principle and highlight some key applications.

Watch: MOSFET Basic Function and Comparison to BJT (00:35 - 03:41). Focus on the analogy of using pressure (voltage) versus a small flow (current) to control the main flow. Note the points on efficiency and high current handling.

This voltage-controlled nature makes MOSFETs highly efficient switches and simplifies the design of the circuits that drive them.

2. The Two Main Flavors: Enhancement vs. Depletion

All MOSFETs fall into one of two primary operational modes, which determines their "default" state (i.e., their state when no voltage is on the gate).

Enhancement-Mode (E-MOSFET): These are normally OFF. There is no conductive channel between the drain and source by default. Applying a specific gate voltage creates or enhances a channel, turning the device ON. This is the most common type of MOSFET, especially in digital logic.
Depletion-Mode (D-MOSFET): These are normally ON. A physical channel is fabricated into the device, allowing current to flow by default. Applying a gate voltage depletes this channel of charge carriers, reducing the current and potentially turning the device OFF.

The following clip provides a clear distinction using the same water-flow analogy and introduces the corresponding circuit symbols.

Watch: Enhancement vs. Depletion Type MOSFETs (07:00 - 08:06). Pay close attention to the circuit symbols: the broken line for enhancement mode signifies a normally-off channel, while the solid line for depletion mode signifies a normally-on channel.

These two modes, combined with two possible channel types (N-channel and P-channel), give us four basic MOSFET varieties.

3. Inside the MOSFET: Structure and Operation

To understand how MOSFETs work, we need to look at their internal structure. As with BJTs, it all starts with doped silicon. Let's quickly review the basics of creating N-type and P-type semiconductors.

Watch: Internal Structure and Semiconductor Doping (09:33 - 11:42). This is a quick refresher on how doping silicon with elements like phosphorus (N-type) or boron (P-type) creates free electrons or holes, respectively.

Now, let's see how these materials are assembled to create the four types of MOSFETs.

A. N-Channel Enhancement-Type MOSFET

This is the most common and important type to understand thoroughly. Its operation is based on creating a channel where one doesn't normally exist.

Watch: N-Channel Enhancement Type MOSFET Structure and Operation (11:42 - 15:53). This is the core segment of the lesson. Follow the explanation carefully.

Structure:

A base of lightly doped P-type substrate.
Two heavily doped N-type regions diffused into the substrate, which act as the Source and Drain.
A very thin insulating layer of Silicon Dioxide ( $S i O_{2}$ ) grown over the substrate between the source and drain.
A metal contact on top of the oxide layer, forming the Gate.

Operation:

Cutoff Region ( $V_{GS} = 0$ ): With no voltage on the gate, we have two back-to-back P-N junctions between the drain and source. No current ( $I_{D}$ ) can flow. The device is OFF.
Channel Formation ( $V_{GS} > V_{T}$ ): When a sufficiently positive voltage is applied between the gate and source ( $V_{GS}$ ), the resulting electric field pushes away the majority carriers (holes) in the P-substrate beneath the gate. Simultaneously, it attracts the minority carriers (electrons).
Inversion Layer: Once $V_{GS}$ exceeds a specific Threshold Voltage ( $V_{T}$ ), enough electrons accumulate under the gate to form a continuous N-type channel connecting the source and drain. This is called an inversion layer.
Current Flow: With the channel formed, a voltage applied across the drain and source ( $V_{D S}$ ) will cause a current of electrons to flow from source to drain (conventional current $I_{D}$ flows from drain to source). The conductivity of this channel is controlled by the magnitude of $V_{GS}$ .

The video also introduces two modes of current flow, which we will analyze mathematically in the next lesson:

Ohmic (or Triode) Region: At low $V_{D S}$ , the channel acts like a voltage-controlled resistor.
Saturation Region: At higher $V_{D S}$ , the channel "pinches off" near the drain, and the current becomes relatively constant, independent of further increases in $V_{D S}$ .

B. N-Channel Depletion-Type MOSFET

The depletion-mode device is structurally different because it's designed to be "normally on."

Watch: N-Channel Depletion Type MOSFET Operation (15:53 - 16:57).

Structure and Operation:

The key difference is a physical N-type channel is fabricated directly between the source and drain.
Depletion Mode ( $V_{GS} < 0$ ): Applying a negative gate voltage creates an electric field that repels electrons from this channel, "depleting" it of charge carriers. This increases the channel's resistance and reduces $I_{D}$ . A sufficiently negative $V_{GS}$ will "pinch off" the channel entirely, stopping the current.
Enhancement Mode ( $V_{GS} > 0$ ): Applying a positive gate voltage attracts more electrons into the channel, enhancing its conductivity and increasing $I_{D}$ .

C. and D. P-Channel MOSFETs

P-channel devices are the complement of N-channel devices. The roles of P- and N-type silicon are swapped, charge carriers are holes instead of electrons, and all voltage polarities are reversed.

Watch: P-Channel Enhancement and Depletion Operation (16:57 - 18:36).
P-Channel Enhancement: Built on an N-type substrate with P-type source/drain regions. It's normally OFF. A negative $V_{GS}$ (below a negative $V_{T}$ ) is required to form a channel of holes.
P-Channel Depletion: Built on an N-type substrate with a physical P-type channel. It's normally ON. A positive $V_{GS}$ is used to deplete the channel of holes and turn the device OFF.

4. Summary Table of MOSFET Types

This table summarizes the four basic types and their operating characteristics.

MOSFET Type	Channel Type	Default State (V_GS = 0)	V_GS to Turn ON / Enhance Current	V_GS to Turn OFF / Deplete Current
N-Channel Enhancement	N (electrons)	OFF	Positive ( $> V_{T}$ )	Zero or Negative
N-Channel Depletion	N (electrons)	ON	Positive	Negative (to pinch-off)
P-Channel Enhancement	P (holes)	OFF	Negative ( $< V_{T}$ )	Zero or Positive
P-Channel Depletion	P (holes)	ON	Negative	Positive (to pinch-off)

Conclusion

In this lesson, you've transitioned from the current-controlled world of BJTs to the voltage-controlled paradigm of MOSFETs. We have explored the physical structures and fundamental operating principles that distinguish the four main types of MOSFETs.

Key Takeaways:

MOSFETs are voltage-controlled devices with extremely high input impedance due to their insulated gate.
Enhancement-mode MOSFETs are normally OFF, requiring a gate voltage to create a conductive channel.
Depletion-mode MOSFETs are normally ON, featuring a built-in channel that can be "pinched off" with a gate voltage.
The channel type can be N-channel (electron flow, controlled by positive $V_{GS}$ for enhancement) or P-channel (hole flow, controlled by negative $V_{GS}$ for enhancement).
We've conceptually defined the operating states: cutoff, ohmic (triode), and saturation.

This physical understanding is the foundation for analyzing MOSFET circuits. In our next lesson, we will dive into the MOSFET I-V Characteristics, where we will put numbers and graphs to these concepts. We'll explore the equations that precisely describe the drain current $I_{D}$ as a function of gate-source voltage $V_{GS}$ and drain-source voltage $V_{D S}$ in each region of operation.