1. Field Effect Transistor Uses
2. Field Effect Transistor Tutorial
3. Npn Transistor
4. Field Effect Transistor Circuits

In 1945, Shockley had an idea for making a solid state device out of semiconductors. He reasoned that a strong electrical field could cause the flow of electricity within a nearby semiconductor. He tried to build one, then had Walter Brattain try to build it, but it didn't work.

Three years later, Brattain and Bardeen built the first working transistor, the germanium point-contact transistor, which was manufactured as the 'A' series. Shockley then designed the junction (sandwich) transistor, which was manufactured for several years afterwards. But in 1960 Bell scientist John Atalla developed a new design based on Shockley's original field-effect theories. By the late 1960s, manufacturers converted from junction type integrated circuits to field effect devices. Today, most transistors are field-effect transistors. You are using millions of them now.

Field Effect Transistor Uses

The field effect transistor, FET is a key electronic component using within many areas of the electronics industry. The FET used in many circuits constructed from discrete electronic components in areas from RF technology to power control and electronic switching to general amplification. Field oxide Source metal Poly gate Drain metal n+ n+ (b) ˙' ˇ˘ ˚ ˙ˇ ˚ ˘ ˘ ˘˘ ˘˘˚˘ ˚ ˙ '# ˘˙ ˘ ˘ ˘˘ '# ˘ˇ ˆ ˙ $˙ ˘˙ ˜ ˘ ˇ p-type Gate (G) Substrate or body (B) Source (S) Drain (D) n+ n + L S D p Electron inversion layer G SD ––––––– (a) (b) (c) ˙' ˇ˘(% ˘ ˘ ˘˘ '# ˚ ˘ ˘ ˘. 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. Lab 1: Field Effect Transistor; The J-FET OBJECTIVES. Familiarity with basic characteristics and parameters of the J-FET. Applications of J-FET as a current source and a variable resistor. Draw a circuit for measurements of characteristics of a depletion mode, n-channel JFET, described in part 1 of the Laboratory (below).

What is Field Effect Transistor: Field effect transistor is a unipolar device. Meaning is current conduction takes place only due to one type of carrier electron or holes. This is the major difference between the bipolar junction transistor and field effect transistor. BJT takes the conduction in electron and holes.

MOS-FETs

Most of today's transistors are 'MOS-FETs', or Metal Oxide Semiconductor Field Effect Transistors. They were developed mainly by Bell Labs, Fairchild Semiconductor, and hundreds of Silicon Valley, Japanese and other electronics companies.

Field-effect transistors are so named because a weak electrical signal coming in through one electrode creates an electrical field through the rest of the transistor. This field flips from positive to negative when the incoming signal does, and controls a second current traveling through the rest of the transistor. The field modulates the second current to mimic the first one -- but it can be substantially larger.

How it works

On the bottom of the transistor is a U-shaped section (though it's flatter than a true 'U') of N-type semiconductor with an excess of electrons. In the center of the U is a section known as the 'base' made of P-type (positively charged) semiconductor with too few electrons. (Actually, the N- and P-types can be reversed and the device will work in exactly the same way, except that holes, not electrons, would cause the current.)

Three electrodes are attached to the top of this semiconductor crystal: one to the middle positive section and one to each arm of the U. By applying a voltage to the electrodes on the U, current will flow through it. The side where the electrons come in is known as the source, and the side where the electrons come out is called the drain.

If nothing else happens, current will flow from one side to the other. Due to the way electrons behave at the junction between N- and P-type semiconductors, however, the current won't flow particularly close to the base. It travels only through a thin channel down the middle of the U.

There's also an electrode attached to the base, a wedge of P-type semiconductor in the middle, separated from the rest of the transistor by a thin layer of metal-oxide such as silicon dioxide (which plays the role of an insulator). This electrode is called the 'gate.' The weak electrical signal we'd like to amplify is fed through the gate. If the charge coming through the gate is negative, it adds more electrons to the base. Since electrons repel each other, the electrons in the U move as far away from the base as possible. This creates a depletion zone around the base – a whole area where electrons cannot travel. The channel down the middle of the U through which current can flow becomes even thinner. Add enough negative charge to the base and the channel will pinch off completely, stopping all current. It's like stepping on a garden hose to stop the flow of water. (Earlier transistors controlled this depletion zone by making use of how electrons move when two semiconductor slabs are put next to each other, creating what is known as a P-N junction. In a MOS-FET, the P-N junction is replaced with metal-oxide, which turned out to be easier to mass produce in microchips.)

Now imagine if the charge coming through the gate is positive. The positive base attracts many electrons – suddenly the area around the base which used to be a no-man's-land opens up. The channel for current through the U becomes larger than it was originally and much more electricity can flow through.

Alternating charge on the base, therefore, changes how much current goes through the U. The incoming current can be used as a faucet to turn current on or off as it moves through the rest of the transistor.

On the other hand, the transistor can be used in a more complex manner as well -- as an amplifier. Current traveling through the U gets larger or smaller in perfect synch with the charge coming into the base, meaning it has the identical pattern as that original weak signal. And, since the second current is connected to a different voltage supply, it can be made to be larger. The current coming through the U is a perfect replica of the original, only amplified. The transistor is used this way for stereo amplification in speakers and microphones, as well as to boost telephone signals as they travel around the world.

Footnote on Shockley

Shockley watched as Silicon Valley grew but could not seem to enter The Promised Land he had envisioned. He never was able to make field effect transistors, while other companies designed, grew, and prospered. Fred Seitz called Shockley 'The Moses of Silicon Valley.'

Other transistor types:
-- Point-Contact Transistor
-- Junction ('Sandwich') Transistor

Resources:
-- The Way Things Work by David Macaulay
-- Van Nostrand's Scientific Encyclopedia
-- The Field Effect Transistor
-- Interview, Walter Brown, May 3, 1999

Copyright 1999, ScienCentral, Inc, and The American Institute of Physics.No portion of this web site may be reproduced without written permission. All Rights Reserved.

OBJECTIVES

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

Field Effect Transistor Tutorial

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

Npn Transistor

PRELAB

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.

Field Effect Transistor Circuits

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.

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

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.