Field Effect Transistor

• Field Effect Transistor Circuits
• Field Effect Transistor Diagram
• Field Effect Transistor Images
• Field Effect Transistor Symbol
• Field Effect Transistor Biosensor
• Field Effect Transistor Pdf

Field Effect Transistors Module 4.1 Junction Field Effect Transistors Field Effect Transistors Although there are lots of confusing names for field effect transistors (FETs) there are basically two main types: 1. The reverse biased PN junction types, the JFET or Junction FET, (also called the JUGFET or Junction Unipolar Gate FET). Field-effect transistors, also known as unipolar transistors, use either electrons or holes for the transport of electricity. The transistor is the basic component in semiconductor manufacturing, in modern microchips there are found several millions to billions of transistors. Field-Effect Transistors (FETs) have much higher input impedance than do bipolar junction transistors (BJTs) and would therefore seem to be ideal devices for op amp input stages. However, they cannot be manufactured on all bipolar IC processes, and when a process does allow their manufacture, they often have their own problems. Field Effect Transistors in Theory and Practice INTRODUCTION There are two types of field-effect transistors, theJunction Field-Effect Transistor (JFET) and the “Metal-Oxide Semiconductor” Field-Effect Transistor (MOSFET), or Insulated-Gate Field-Effect Transistor (IGFET). The principles on which these devices operate (current controlled. Field effect transistor (FET) width and length are key variables available to the circuit designer to optimize circuit performance. Fully depleted silicon on insulator (FDSOI) is a planer technology, so width can be continuously varied, unlike finFET where the width comes in.

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A Field Effect Transistor (FET) is a three-terminal semiconductor device. Its operation is based on a controlled input voltage. By appearance JFET and bipolar transistors are very similar. However, BJT is a current controlled device and JFET is controlled by input voltage. Most commonly two types of FETs are available.

• Junction Field Effect Transistor (JFET)
• Metal Oxide Semiconductor FET (IGFET)

Junction Field Effect Transistor

The functioning of Junction Field Effect Transistor depends upon the flow of majority carriers (electrons or holes) only. Basically, JFETs consist of an N type or P type silicon bar containing PN junctions at the sides. Following are some important points to remember about FET −

• Gate − By using diffusion or alloying technique, both sides of N type bar are heavily doped to create PN junction. These doped regions are called gate (G).

• Source − It is the entry point for majority carriers through which they enter into the semiconductor bar.

• Drain − It is the exit point for majority carriers through which they leave the semiconductor bar.

• Channel − It is the area of N type material through which majority carriers pass from the source to drain.

There are two types of JFETs commonly used in the field semiconductor devices: N-Channel JFET and P-Channel JFET.

N-Channel JFET

It has a thin layer of N type material formed on P type substrate. Following figure shows the crystal structure and schematic symbol of an N-channel JFET. Then the gate is formed on top of the N channel with P type material. At the end of the channel and the gate, lead wires are attached and the substrate has no connection.

When a DC voltage source is connected to the source and the drain leads of a JFET, maximum current will flow through the channel. The same amount of current will flow from the source and the drain terminals. The amount of channel current flow will be determined by the value of V_DD and the internal resistance of the channel.

A typical value of source-drain resistance of a JFET is quite a few hundred ohms. It is clear that even when the gate is open full current conduction will take place in the channel. Essentially, the amount of bias voltage applied at ID, controls the flow of current carriers passing through the channel of a JFET. With a small change in gate voltage, JFET can be controlled anywhere between full conduction and cutoff state.

P-Channel JFETs

It has a thin layer of P type material formed on N type substrate. The following figure shows the crystal structure and schematic symbol of an N-channel JFET. The gate is formed on top of the P channel with N type material. At the end of the channel and the gate, lead wires are attached. Rest of the construction details are similar to that of N- channel JFET.

Normally for general operation, the gate terminal is made positive with respect to the source terminal. The size of the P-N junction depletion layer depends upon fluctuations in the values of reverse biased gate voltage. With a small change in gate voltage, JFET can be controlled anywhere between full conduction and cutoff state.

Output Characteristics of JFET

The output characteristics of JFET are drawn between drain current (I_D) and drain source voltage (V_DS) at constant gate source voltage (V_GS) as shown in the following figure. Download netgear driver.

Field Effect Transistor Circuits

Initially, the drain current (I_D) rises rapidly with drain source voltage (V_DS) however suddenly becomes constant at a voltage known as pinch-off voltage (V_P). Above pinch-off voltage, the channel width becomes so narrow that it allows very small drain current to pass through it. Therefore, drain current (I_D) remains constant above pinch-off voltage.

Parameters of JFET

The main parameters of JFET are − Resmed port devices driver download for windows.

• AC drain resistance (Rd)
• Transconductance
• Amplification factor

AC drain resistance (R_d) − It is the ratio of change in the drain source voltage (ΔV_DS) to the change in drain current (ΔI_D) at constant gate-source voltage. It can be expressed as,

R_d = (ΔV_DS)/(ΔI_D) at Constant V_GS

Transconductance (g_fs) − It is the ratio of change in drain current (ΔI_D) to the change in gate source voltage (ΔV_GS) at constant drain-source voltage. It can be expressed as,

g_fs = (ΔI_D)/(ΔV_GS) at constant V_DS

Amplification Factor (u) − It is the ratio of change in drain-source voltage (ΔV_DS) to the change in gate source voltage (ΔV_GS) constant drain current (ΔI_D). It can be expressed as,

u = (ΔV_DS)/(ΔV_GS) at constant I_D

Field Effect Transistor Diagram

OBJECTIVES

Familiarity with basic characteristics and parameters of the J-FET.

Applications of J-FET as a current source and a variable resistor.

PRELAB

Field Effect Transistor Images

Draw a circuit for measurements of characteristics of a depletion mode, n-channel JFET, described in part 1 of the Laboratory (below). Sketch basic characteristics of a n-channel J-FET (I_D vs. V_DS and I_D vs. V_GS) and explain why it may be used as a constant current source and a voltage controlled resistor. Indicate the parts of the characteristics where these functions can be realized.

LABORATORY

Equipment needed from the stockroom: ECE 392 parts kit, analog universal meter, resistance substitution box, leads.

1. JFET CHARACTERISTICS; V_P AND I_DSS.

1. 1. Insert a JFET into the protoboard, connect the source to ground and the drain to a 15 V power supply through an ammeter, which will measure the drain current (I_D). Measure this current for different voltage values between the gate and the source (V_GS). Use only negative voltage on the gate. Determine the pinch-off voltage (V_P), i.e. the gate voltage at which the drain current is (practically) zero. Get a few measurements at low current, with V_GS close to V_P so that you have enough points on the logI_D vs. V_GS graph to determine V_P. (see description of the report, below). Measure also I_DSS, the drain current with V_GS = 0. This current flows through the transistor when the gate connected to the source. Repeat measurements of V_P and I_DSS values for another transistor of the same type in your kit and see if there is significant difference between the two transistors. If so, make sure that you can identify these transistors when you use them in other measurements.

1. 2. Next, measure I_D(V_DS) characteristics of one of the transistors for V_GS = 0 and two different negative values. Note the linear part of the characteristics, where I_D is proportional to V_DS (behaves like a resistor) and the saturation part, where current is (almost) independent of the voltage.

You will explore saturation range of the JFET transistor characteristic in part 2 and the linear range in part 3, below.

2. FET AS A CURRENT SOURCE.

The flat parts of the I_D vs. V_DS characteristics of the FET allow to use this device as a simple constant current sources because the current is (almost) independent of the voltage across it. Test this idea with two transistors. Measure the current with different values of the load resistor R_L (100 Ω - 100 kΩ)chosen from the resistance substitution box.

How good is this current source? Determine the range of the load resistor values which allows the current to stay constant within a given interval (say 2 % or 5%). What is the range of voltage across the transistor operating as a current source.

You can buy JFETs with the gate connected to the source, so called current regulator diodes. These two terminal devices, calibrated for different current values, are current equivalents of Zener diodes which provide a constant voltage.

A variation of JFET current source, with self-biasing, is shown on the next schematic. One of its advantages is that you can obtain different current values by adjusting the resistor R (a few k). Try this simple circuit and again determine the range of load resistor R_L which allows you to keep the current constant.

Is this a better current source than the one without a resistor? How does it work? Do you see feedback in this circuit? What does the voltmeter here show?

3. JFET AS A VARIABLE RESISTOR.

In the linear part of the JFET I_D vs. V_DS characteristics, the current through the transistor is (roughly) proportional to the voltage across it, like in a resistor. Moreover, the slope of these characteristics depends on V_GS so that changing the latter changes the value of the 'resistance'. This effect can be used in many 'voltage controlled circuits'.

Experiment with the JFET as a variable resistor by using it instead of a regular resistor in a two resistor voltage divider.
Chose R = 10 k.

Apply a small sinewave signal (about 0.2 V) to the input and observe variation of the output amplitude while changing V_GS (negative voltage must be used!). To see if the transistor really behaves as a resistor, switch the waveform generator to a triangular wave. Nonlinear dependence of voltage on current will show as a distortion of the straight lines of the waveform. A resistor has a linear I-V characteristic and will not distort a triangular wave.

From observation of the output waveform with a triangular wave at the input estimate in what range of input voltage the transistor behaves approximately as a resistor? Explain your observation.

The circuit shown below is an improved version of a two resistor voltage divider, with R a regular resistor and the transistor being an adjustable resistor. The divider ratio can be adjusted by the control voltage V_C. A compensation circuit (between the output and the transistor gate) greatly improves the circuit linearity as a part of output voltage (what fraction?) is added to V_GS. Check that this circuit behaves much better as a voltage controlled resistive divider.

Compare the range of V_in with undistorted triangular waveform with the previous case of the uncompensated circuit. Explain.

REPORT

Describe briefly the measurements. Include all schematics. Show all results with proper units. Do not forget to include the frequency used in ac measurements. For part 1, make a graph of I_D vs. V_GS characteristic and indicate the values of I_DSS and V_P on the graph. V_P is best determined from a plot of logI_D vs. V_GS. If you have data for two transistors, plot them on the same graph. For part 2, you may plot I_D vs. log R_L to cover a wide range of resistance. In the discussion, comment whether the parameters I_DSS and V_P are the same for a given transistor type. Address the topics and answer the questions printed in bold letters in the manual. Add any observations or conclusions you wish to make.

Field Effect Transistor Symbol

Field Effect Transistor Biosensor

A PROJECT IDEA (OPTIONAL): ONE TRANSISTOR AM TRANSMITTER.

Field Effect Transistor Pdf

You could use the last circuit for amplitude modulation of a high frequency carrier signal, just as it is done in AM radio transmission. Supply the input with a high frequency sinewave (about 1 MHz) and modulate its amplitude by feeding a low frequency signal (in the kilohertz range) through a capacitor (~ 1 µF) to the slider of the potentiometer. The low frequency signal may be picked up by an AM radio tuned to the appropriate frequency (in this case about 1 MHz). If you supply an amplified signal from a microphone you may hear your voice 'on the air'. A piece of wire attached to the drain may serve as a transmitter antenna, extending the reception distance.