Unit 3: Field effect transistors

Introduction

The Field Effect Transistor (FET) is a fundamental semiconductor device widely used in robotics for switching applications, motor control, power management, and sensor signal amplification. Unlike Bipolar Junction Transistors (BJTs), which are current-controlled, FETs are voltage-controlled devices. They rely on an electric field to control the shape and electrical conductivity of a "channel" in a semiconductor material.

Advantages of FET over BJT

Understanding why FETs are preferred over standard transistors (BJTs) is crucial in robotics, especially for designing energy-efficient and highly integrated circuits.

• Voltage-Controlled Operation: A FET is a voltage-controlled device ( controls ), whereas a BJT is a current-controlled device ( controls ). This means FETs draw virtually zero input current, significantly reducing power consumption in control circuitry.
• Extremely High Input Impedance: Because the input (Gate) is reverse-biased (JFET) or insulated (MOSFET), the input impedance is incredibly high (typically to ). This is ideal for interfacing with delicate robotic sensors without loading the sensor output.
• Unipolar Device: FETs operate using only one type of charge carrier (either electrons in N-channel or holes in P-channel). BJTs use both (bipolar). This unipolar nature leads to faster switching times in modern MOSFETs.
• High Thermal Stability: FETs have a negative temperature coefficient at high current levels. As temperature increases, carrier mobility decreases, increasing channel resistance and reducing drain current. This prevents the "thermal runaway" that can easily destroy BJTs in heavy-duty robotic motor drivers.
• Lower Noise Generation: Since charge carriers do not cross a P-N junction in the same way they do in a BJT, FETs generate less inherent electronic noise, making them superior for amplifying low-level signals from analog sensors.
• Smaller Size and High Packing Density: FETs (specifically MOSFETs) are smaller and easier to fabricate on a silicon wafer. This allows for the creation of densely packed microcontrollers and ICs that act as the "brain" of a robot.

Junction Field Effect Transistor (JFET)

The JFET is the simplest type of FET. It utilizes a reverse-biased P-N junction to control the width of the conducting channel.

Structure of JFET

A JFET consists of a single piece of semiconductor material (either N-type or P-type) forming a channel, with two heavily doped regions of the opposite type embedded on the sides.

• N-Channel JFET: The main body is N-type silicon. At the two ends of this bar, ohmic contacts are made to form the Source (S) and Drain (D) terminals. On the sides of the N-type bar, two P-type regions are diffused. These P-type regions are internally connected to form a single Gate (G) terminal.
• P-Channel JFET: The structure is exactly the inverse; the channel is P-type material, and the Gate is made of heavily doped N-type regions.

Working Principle of JFET (N-Channel)

1. Zero Gate Voltage (), Applied Drain Voltage ():
   When a positive voltage is applied to the Drain relative to the Source, electrons flow from Source to Drain through the N-channel. The Drain current () is limited only by the natural resistance of the channel.
2. Applying Reverse Bias to the Gate ():
   When a negative voltage is applied to the Gate relative to the Source, the P-N junctions between the Gate and the Channel become reverse-biased.
3. Depletion Region Modulation:
   Reverse biasing creates a depletion region (an area devoid of charge carriers) that extends into the N-channel. As becomes more negative, the depletion regions grow wider, physically narrowing the conductive channel and increasing its resistance. This reduces the Drain current ().
4. Pinch-Off:
   If the Gate voltage is made negative enough (reaching a specific value called the Pinch-off Voltage, ), the depletion regions from both sides touch. The channel is essentially closed, and Drain current drops to approximately zero.

Note: In normal operation, the Gate-Source junction of a JFET is ALWAYS reverse-biased to prevent Gate current from flowing.

Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET)

The MOSFET (also known as Insulated Gate FET or IGFET) improves upon the JFET by placing an insulating layer between the Gate and the channel. This allows for even higher input impedance and different modes of operation.

Types of MOSFET

1. Depletion-type (D-MOSFET): Contains a physically implanted channel between the Source and Drain. It can operate in both "depletion" mode (reducing current) and "enhancement" mode (increasing current).
2. Enhancement-type (E-MOSFET): Does not have a physical channel initially. A channel must be electrically induced to allow current to flow. This is the most common transistor used in modern digital logic and robotic motor drivers.

Structure of MOSFET

• Substrate (Body): A lightly doped semiconductor base (e.g., P-type for an N-channel MOSFET). Often internally connected to the Source.
• Source and Drain: Two heavily doped regions of the opposite type to the substrate (e.g., N+ regions in a P-type substrate) diffused into the substrate.
• Insulating Layer: A thin layer of Silicon Dioxide () is grown over the surface between the Source and Drain.
• Gate: A metal (or polycrystalline silicon) electrode is deposited on top of the layer. The Gate is completely electrically isolated from the semiconductor material.

Working Principle of MOSFET

1. Enhancement MOSFET (E-MOSFET - N-channel)

• Off State (): With no Gate voltage, there is no channel between the N-type Source and N-type Drain. They are separated by the P-type substrate (forming back-to-back P-N junctions). No Drain current () flows, regardless of the applied .
• Turn-On / Channel Creation (): When a positive voltage is applied to the Gate, it acts like the positive plate of a capacitor. The electric field penetrates the layer and repels holes (majority carriers) deeper into the P-substrate. Simultaneously, it attracts minority electrons from the substrate to the surface directly beneath the oxide layer.
• Inversion Layer: Once the Gate voltage exceeds a specific Threshold Voltage (), enough electrons accumulate under the Gate to form an N-type "inversion layer." This forms a continuous N-channel connecting the Source and Drain.
• Conduction: Applying a positive now causes current to flow. Increasing further widens the channel, allowing more current to flow.

2. Depletion MOSFET (D-MOSFET - N-channel)

• Physical Channel: A narrow N-channel already exists between the Source and Drain during manufacturing.
• Depletion Mode (): Applying a negative Gate voltage repels electrons out of the existing channel, depleting it of charge carriers and reducing (similar to a JFET).
• Enhancement Mode (): Applying a positive Gate voltage attracts more electrons into the existing channel, enhancing its conductivity and increasing .

Output and Transfer Characteristics of JFET/MOSFET

Characteristic curves graphically represent the electrical behavior of FETs. They are divided into Output (Drain) characteristics and Transfer characteristics.

1. JFET Characteristics

Output (Drain) Characteristics

This is a plot of Drain Current () versus Drain-Source Voltage () for various constant values of Gate-Source Voltage (). It consists of three main regions:

1. Ohmic (Linear) Region: At low values of , the JFET acts like a voltage-controlled resistor. The current increases linearly with .
2. Saturation (Active) Region: As increases, the channel narrows near the drain end due to increased reverse bias at that specific point. Once reaches the pinch-off point, the current saturates and becomes almost constant ( - Drain to Source Saturation Current), regardless of further increases in . The JFET operates as a constant current source here, and this region is used for amplification.
3. Breakdown Region: If is increased excessively, avalanche breakdown occurs across the reverse-biased Gate-Drain junction, causing to spike uncontrollably, potentially destroying the device.

Transfer Characteristics

This is a plot of Drain Current () versus Gate-Source Voltage () for a constant (usually taken in the saturation region).

• For an N-channel JFET, the curve exists entirely in the negative quadrant.
• Maximum current () flows when .
• Current is completely cut off when (Pinch-off voltage).
• The relationship is non-linear and governed by Shockley's Equation:

2. MOSFET Characteristics

A. D-MOSFET Characteristics

• Output Characteristics: Visually similar to the JFET output curve, but with a critical difference: the curves extend into positive values. The region below is the depletion mode, and the region above is the enhancement mode.
• Transfer Characteristics: The curve follows Shockley's equation but crosses the vertical axis (y-axis). It spans from negative (where it hits zero at or ) through (where ), and continues upward into positive values.

B. E-MOSFET Characteristics

Because E-MOSFETs are the most prominent in robotics (used as switches in H-bridges and logic circuits), their characteristics are vital.

• Output Characteristics:
  • No current flows until (Threshold Voltage).
  • Once , the output curves look similar to other FETs, featuring Ohmic and Saturation regions.
  • In digital robotics, E-MOSFETs are rapidly switched between the "Cut-off" region (, acting as an open switch) and the "Ohmic/Linear" region (High , low , acting as a closed switch with very low ).
• Transfer Characteristics:
  • The curve is entirely in the positive quadrant (for N-channel).
  • remains zero until reaches .
  • Above , increases non-linearly. The relationship is given by:

(Where is a constant depending on the physical construction of the MOSFET).