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Semiconductor Science

Aaron Snowberger

14주차:

MOSFETs

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MOSFETs (Metal Oxide FETs)

p. 382-428

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Learning Objectives

By the end of this chapter, you should be able to:

Discuss the functional differences between MOSFETs and JFETs.
Draw and explain a basic DC bias model for a MOSFET.
Graph the transconductance curves for both DE-MOS and E-MOS transistors, and describe their functional differences.
Perform DC bias analysis on various MOSFET circuits.
Explain necessary ESD precautions for MOS devices.

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12.1 Introduction

MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are similar to JFETs in having low gate current and functioning as voltage-controlled current sources. They come in N- and P-channel types and exist in two forms:

DE-MOSFET: Depletion and enhancement mode capable.
E-MOSFET: Enhancement-only.

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12.1 Introduction

DE-MOSFETs can use JFET biasing techniques; E-MOSFETs require new ones. Both types can be used in AC amplifiers like common source and drain configurations with similar input impedance and gain equations as JFETs.

E-MOSFETs are also used in power applications, offering advantages like higher speed and better thermal stability compared to BJTs.

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12.1 Introduction

A key concern is ESD (Electrostatic Discharge) sensitivity due to their insulated gate (no PN junction), which makes them also known as IGFETs. The insulation leads to very high input resistance but requires careful handling.

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12.2 The DE-MOSFET

A DE-MOSFET operates by modulating current through the drain-source channel using gate-source voltage, but unlike a JFET, it has a capacitor-like structure with an insulated gate.

When VGS = 0, current flows normally.
For small VDS, the channel behaves ohmically; for higher VDS, it saturates like a constant current source.
If VGS is negative, a depletion region forms, reducing current.
VGS(off) is the point at which the current is fully cut off.
IDSS is the current when VGS = 0.

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12.2 The DE-MOSFET

DE-MOSFETs are unique because they also operate in enhancement mode when VGS > 0, increasing current. This is unlike JFETs, which lose control in this region.

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12.2 The DE-MOSFET

The device operates in both depletion and enhancement modes, extending the VGS range and resulting in a modified current-voltage characteristic curve.

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12.2 The DE-MOSFET

The maximum VGS is limited (e.g., 20–30V), beyond which the insulation may break down. ESD can easily damage the device due to its high sensitivity, even from common static charges on the human body.

ESD Protection Measures:

Manufacturing with Zener diodes (though they may leak current).
Conductive packaging.
Grounded mats, wrist straps, and controlled environments to prevent static buildup.

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12.2 The DE-MOSFET

Since the operating equation is largely the same as for JFETs, most JFET analysis techniques (like transconductance equations) apply. However, gm0 (initial transconductance) may no longer represent the maximum due to the ability to enhance current beyond IDSS.

Schematic symbols show the insulated gate and usually tie the substrate back to the source, with arrows indicating N-type material flow.

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12.3 DE-MOSFET Biasing

Since DE-MOSFETs and JFETs share the same characteristic equations, they can use the same DC biasing methods—self bias, combination bias, and current source bias—all second-quadrant schemes with negative V_GS. However, DE-MOSFETs also support first-quadrant operation, enabling zero bias and voltage divider bias techniques.

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12.3 DE-MOSFET Biasing

Zero Bias

In zero bias, V_GS = 0 V.
No separate DC source or source resistor is needed—minimal component design.
Acts like a mix of self and constant voltage bias.
Since gate current is ideally zero, the gate voltage is 0 V.
The source is grounded, so V_GS = 0 V.
AC input causes operation to swing between depletion and enhancement modes.
Operating point is easy to find: I_D = I_DSS, g_m = g_m0.
Poor Q-point stability.
Not suitable for source follower or swamped amplifiers.

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12.3 DE-MOSFET Biasing

Zero Bias

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12.3 DE-MOSFET Biasing

Voltage Divider Bias

A constant voltage bias method that operates in enhancement mode.
Gate voltage is set using a resistor voltage divider (R1 and R2).
Source is grounded, so V_GS = V_G = V_R2 (positive).
Positive V_GS ensures enhancement-mode operation.
Drain current (I_D) is found via characteristic equation or graph.
High-value resistors are acceptable due to negligible gate current.

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12.3 DE-MOSFET Biasing

Voltage Divider Bias

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12.4 The E-MOSFET

E-MOSFETs come in low and high power forms and operate only in enhancement mode (first quadrant).

Structure: Similar to DE-MOSFET but with P material reaching the gate oxide, preventing conduction at zero or negative V_GS.
"Normally off" devices: No conduction unless V_GS exceeds a threshold voltage V_GS(th).
When V_GS exceeds the threshold, an N-type inversion layer forms, enabling current flow.
Increasing V_GS increases conduction.

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12.4 The E-MOSFET

E-MOSFETs come in low and high power forms and operate only in enhancement mode (first quadrant).

Characteristic Regions: Ohmic (triode), Saturation (constant current), Breakdown.
Equation: E-MOSFETs follow a square-law equation in saturation mode.
Transconductance (g_m): Derived from the characteristic equation.
Drain current curve is similar to a BJT's, rising after V_GS(th).

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12.4 The E-MOSFET

Power E-MOSFETs

Use a different structure, often with trench-style gates.
Feature low r_DS(on), fast switching, and low drive current needs.
Preferred in power electronics like DC-DC converters and switching regulators.
Vertical current flow (vs. lateral in low power devices) improves performance.
Drain is placed opposite the gate and source to reduce resistance and boost current capacity.
Characteristic curves resemble low power E-MOSFETs but are steeper.

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12.5 E-MOSFET Data Sheet Interpretation

The FDMS86180 is a high-power N-channel E-MOSFET with trench construction. It's RoHS compliant and housed in a Power 56 package.

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12.5 E-MOSFET Data Sheet Interpretation

rDS(on): Very low, just a few milliohms
Current: 151 A continuous, 775 A pulsed (at room temperature)
Breakdown Voltage: 100 V
IDSS: 1 μA (leakage current for normally-off devices)
VGS(th): 2.0–4.0 V (typical 3.2 V)
Transconductance (gFS): 144 S at 67 A
Switching Speed: Turn-on/off in tens of nanoseconds

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12.5 E-MOSFET Data Sheet Interpretation

Performance graphs show:

Ohmic region up to VDS = 5 V
rDS(on) increases with temperature (about 3x across range)
gm decreases with temperature, indicating a negative temp coefficient

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12.6 E-MOSFET Biasing

E-MOSFETs work only in the first quadrant, so JFET-style biasing won't apply. For switching, only sufficient gate drive is needed. For linear amplifier use, constant voltage bias methods are employed.

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12.6 E-MOSFET Biasing

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12.6 E-MOSFET Biasing

Voltage Divider Bias

Similar to BJT biasing:

Gate must be above source voltage
Input gate current is negligible
VGS is not fixed; it depends on the rest of the circuit

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12.6 E-MOSFET Biasing

Voltage Divider Bias

In this bias setup:

Resistors R1 and R2 create a voltage divider
With source grounded, VGS = VG
VG must exceed VGS(th)
Use the characteristic curve or a normalized plot to determine drain current once the device constant k is found (from ID(on), VGS(on))

Decoupling can be used to allow for more flexible resistor values.

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12.6 E-MOSFET Biasing

Voltage Divider Bias

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12.6 E-MOSFET Biasing

Drain Feedback Bias

Uses the device's characteristic curve:

Sets drain current by choosing suitable drain resistor and supply voltage
Gate current is negligible, simplifying KVL equations
Simple, effective circuit with minimal components

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12.6 E-MOSFET Biasing

Drain Feedback Bias

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Summary

MOSFET Types: DE-MOSFET (depletion-enhancement) and E-MOSFET (enhancement-only)
Both use insulated gates and exhibit square-law characteristics
Gate currents are extremely small; devices are sensitive to electrostatic discharge (ESD)
DE-MOSFETs: Operate in both quadrants, can use JFET biasing methods
E-MOSFETs: Operate only in enhancement mode (first quadrant), IDSS is leakage current
High-power E-MOSFETs have vertical current paths → higher current capacity and lower rDS(on)

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Review Questions

What are the differences between JFETs and MOSFETs?
What are the differences between DE-MOSFETs and E-MOSFETs?
Why are MOSFETs sometimes referred to as “Insulated Gate” or IGFETs?
How does the DC bias model of the MOSFET compare to that of the BJT?
What biasing circuits are available for use with the DE-MOSFET?
What biasing circuits are available for use with the E-MOSFET?
What is “trench” construction and where is it used?
Explain typical precautions taken when handling MOSFETs and why they are necessary.

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MOSFETs Small Signal Amplifiers

p. 348-444

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Learning Objectives

By the end of this chapter, you should be able to:

Draw and explain a basic AC model for a MOSFET.
Analyze MOSFET amplifiers (common source) for voltage gain, input impedance, and output impedance.
Analyze MOSFET voltage followers (common drain) for the same parameters.

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13.1 Introduction

MOSFETs are used in common source amplifiers (voltage amplifiers) and common drain amplifiers (voltage followers). They offer high input impedance because of very low gate current. However, at higher frequencies, input capacitance reduces this impedance. Not all bias types work well with all AC circuits—e.g., zero bias for DE-MOSFETs isn't ideal for followers or swamped amplifiers.

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13.1 Introduction

MOSFET amplifiers are known for:

Good high-frequency performance
Low noise
Low distortion with small signals

However, they typically offer lower voltage gain than BJTs. Gain depends on transconductance (gm), which varies widely among MOSFET types.

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