12.1 JFET and MOSFET structures and operation

JFET Structure and Operation

A field-effect transistor (FET) controls current through a semiconductor channel using an electric field applied at its gate terminal. Unlike bipolar junction transistors (BJTs), which are current-controlled, FETs are voltage-controlled devices with extremely high input impedance. The two main types, JFETs and MOSFETs, differ in how the gate controls the channel, but both are foundational to analog and digital circuit design.

JFET Components and Function

A Junction Field-Effect Transistor (JFET) uses a reverse-biased PN junction to control current flow through a semiconductor channel. Here are the key parts:

• Channel: A region of doped semiconductor (N-type or P-type silicon) connecting the source and drain terminals. This is where current actually flows.
• Gate: A terminal that surrounds or sits alongside the channel, forming a PN junction with it. Voltage applied here controls how much current the channel can carry.
• Source: The terminal where majority carriers enter the channel (electrons in an N-channel JFET, holes in a P-channel JFET).
• Drain: The terminal where majority carriers exit the channel.

The gate and channel are made of opposite doping types, so they naturally form a PN junction. That junction is the key to how a JFET works.

JFET Operation and Characteristics

JFETs operate in depletion mode by default. That means the channel is fully conductive when no gate voltage is applied ($V_{GS} = 0$). You turn the device off by applying voltage, rather than turning it on.

Here's how current control works, step by step:

1. With $V_{GS} = 0$, the channel is wide open and maximum drain current ($I_{DSS}$) flows.
2. Applying a reverse-bias voltage to the gate ($V_{GS} < 0$ for N-channel, $V_{GS} > 0$ for P-channel) widens the depletion region around the PN junction.
3. As the depletion region grows, it squeezes the conductive channel, reducing its cross-sectional area and increasing resistance.
4. At the pinch-off voltage ($V_P$), the depletion regions from both sides of the channel meet. The channel is "pinched off," and drain current levels off to a nearly constant value. This is the saturation region.

JFETs are valued in analog circuits as voltage-controlled resistors, constant current sources, and low-noise amplifiers because of their high input impedance (the reverse-biased gate draws almost no current).

MOSFET Structure and Operation

MOSFET Components and Function

A Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) also uses an electric field to control channel current, but its gate is physically insulated from the channel by a thin layer of silicon dioxide ($SiO_2$). This insulating oxide layer is what distinguishes MOSFETs from JFETs and gives them an even higher input impedance.

MOSFETs have four terminals:

• Gate: Sits on top of the oxide layer. Voltage here creates an electric field that reaches through the oxide to control the channel.
• Source: Where carriers enter the channel.
• Drain: Where carriers exit the channel.
• Body (substrate): The bulk semiconductor material. Often connected to the source in discrete devices, but it can be biased separately in integrated circuits.

Because the gate is insulated, essentially zero DC current flows into it. This is a major practical advantage over BJTs.

MOSFET Types: Enhancement vs. Depletion

MOSFETs come in two varieties, and the distinction matters:

• Enhancement-mode MOSFETs (the most common type) have no conductive channel when $V_{GS} = 0$. You must apply a gate voltage to create the channel. Think of it as "normally off."
• Depletion-mode MOSFETs have a built-in channel and conduct at $V_{GS} = 0$, similar to a JFET. Applying gate voltage can either enhance or deplete the channel.

Most digital and analog circuits use enhancement-mode MOSFETs, so that's where you should focus your attention.

MOSFET Operation and Characteristics

For an N-channel enhancement MOSFET, here's the turn-on process:

1. With $V_{GS} = 0$, no channel exists between source and drain. The device is off.
2. As you increase $V_{GS}$, the electric field through the oxide repels holes in the P-type substrate and attracts electrons toward the surface.
3. At the threshold voltage ($V_{TH}$), enough electrons accumulate to form a thin conductive layer (called an inversion layer) connecting source to drain. The device turns on.
4. Increasing $V_{GS}$ beyond $V_{TH}$ strengthens the channel and allows more drain current to flow.

For a P-channel enhancement MOSFET, the process is reversed: you apply $V_{GS} < V_{TH}$ (a negative voltage) to attract holes and form the channel.

Transconductance ($g_m$) measures how effectively the gate voltage controls drain current. It's defined as:

$g_m = \frac{\Delta I_D}{\Delta V_{GS}}$

A higher $g_m$ means a small change in gate voltage produces a large change in drain current, which is exactly what you want for amplification.

MOSFETs are the backbone of CMOS (Complementary MOS) technology, which pairs N-channel and P-channel devices to build logic gates with very low static power consumption. They're also widely used in analog amplifiers, power switches, and voltage-controlled current sources.