<< Chapter < Page Chapter >> Page >

Field effect transistors

Arguably the most important invention of modern times, the transistor was invented in 1947 at Bell Labs by John Bardeen, William Shockley, and Walter Brattain. The result of efforts to replace inefficient and bulky vacuum tubes in current regulation and switching functions. Further advances in transistor technology led to the field effect transistors (FETs), the bedrock of modern electronics. FETs operate by utilizing an electric field to control the flow of charge carriers along a channel, analogous to a water valve to control the flow of water in your kitchen sink. The FET consists of 3 terminals, a source (S), drain (D), and gate (G). The region between the source and drain is called the channel. The conduction in the channel depends on the availability of charge carriers controlled by the gate voltage. [link] depicts a typical schematic and [link] the associated cross-section of a FET with the source, drain, and gate terminals labeled. FETs come in a variety of flavors depending on their channel doping (leading to enhancement and depletion modes) and gate types, as seen in [link] . The two FET types are junction field effect transistors (JFETs) and metal oxide semiconductor field effect transistors (MOSFETs).

The n-channel enhancement mode MOSFET symbol.
A typical cross-section of a n-channel enhancement mode MOSFET.
Field effect transistor family tree. Adapted from P. Horowitz and W. Hill, in Art of Electronics , Cambridge University Press, New York, 2nd Edn., 1994.

Jfet fundamentals

Junction field effect transistors (JFETs) as their name implies utilize a PN-junction to control the flow of charge carriers. The PN-junction is formed when opposing doping schemes are broght together on both sides of the channel. The doping schemes can be made to be either n-type (electrons) or p-type (holes) by doping with boron/gallium or phosphorus/arsenic respectively. The n-channel JFETs consists of pnp junctions where the source and drain are n-doped and the gate is p-doped. [link] shows the cross section of a n-channel JFET in the “ON” state obtained by applying a positive drain-source voltage in the absence of a gate-source voltage. Alternatively the p-channel JFET consists of npn junctions where the source and drain are p-doped and the gate is n-doped. For p-channel a negative drain-source voltage is applied in the absence of a gate voltage to turn “ON” the npn device, as seen in [link] . Since JFETs are “ON” when no gate-source voltage is applied they are called depletion mode devices. Meaning that a depletion region is required to turn “OFF” the device. This is where the PN-junction comes into play. The PN-junction works by enabling a depletion region to form where electrons and holes combine leaving behind positive and negative ions which inhibit further charge transfer as well as depleting the availability of charge carriers at the interface. This depletion region is pushed further into the channel by applying a gate-source voltage. If the voltage is sufficient the depletion region on either side of the channel will “pinch off” the flow through the channel and the device will be “OFF”. This voltage is called the pinch off voltage, V P . The n-channel V P is obtained by increasing the gate-source voltage in the negative direction, while the p-channel V P is obtained by increasing the gate-source voltage in the positive direction.

Questions & Answers

Text editor project in c++
Ayesha Reply
hi daddy
Sreenu
hello sir
Ranjith
are unmanned flying machines that are built and equipped with various kinds of software systems that allow them to see, hear, and act. Discuss some of the societal challenges of building such kinds of systems.
johni Reply
You have developed a prototype of a software system and your manager is very impressed by it. She proposes that it should be put into use as a production sys tem, with new features added as required. This avoids the expense of system dev immediately useful. Write a short report for your manager expl
johni
You have developed a prototype of a software system and your manager is very impressed by it. She proposes that it should be put into use as a production sys tem, with new features added as required. This avoids the expense of system dev immediately useful.
johni
You have developed a prototype of a software system and your manager is very impressed by it. She proposes that it should be put into use as a production sys tem, with new features added as required. This avoids the expense of system dev immediately useful.
johni
You have developed a prototype of a software system and your manager is very impressed by it. She proposes that it should be put into use as a production sys tem, with new features added as required. This avoids the expense of system dev immediately useful. Write a short report for your manager expl
johni
You have developed a prototype of a software system and your manager is very impressed by it. She proposes that it should be put into use as a production sys tem, with new features added as required. This avoids the expense of system dev immediately useful.
johni
what is cocomo ?
Surabhi Reply
?
StReeT
give me coding of these projects
Aman Reply
feasibility study&fact gathering techniques
Nachi Reply
write about software engineering
Mandala Reply
8.10.0.
BARBU
define iterative model. example of iterative model. advantages and disadvantages of iterative model. when to use iterative model
Okello Reply
What is DFD in Software engineering
Hamid
i try out in netbeans this code:public class Profile { private Profile(int w) { // line 1 System.out.println(w); } public final Profile() { // line 5 System.out.println(10); } public static void main(String args[]) { Profile obj = new Profile(50); } } It is the question 5, and the answer is 50
Jacqueline Reply
how to join conversation
Bablu
i need help....Discuss the factors that influence the choice of the software development methodology to use when implementing software projects?(20)
tatenda Reply
The success rate of software development projects can be increased by using a methodology that is adequate for the specific characteristics of those projects.  then the focus on RAD,XP,RUP.
Hemant
hemant hie...it is a presentation i am working on...can you help me wth the introduction....please a powerful one hahahahha
tatenda
yes
Hemant
Define and show all Dipthongs and vowels used in english phonetics
Okendro Reply
what is the software engineering
Axmed Reply
please
Jennifer
hhahhaha
martin
software engineering is a organized process of activities for development a use full software . it may consist of : 1:spcecification 2:design 3: implementation 4: evolution:
hamayun
Please how can I create an application?
Eric Reply
please how can I create an application
Nchor
How do we create a software
Florence Reply
give a more direct question
Louis Reply
what is semantics?
amjad
what r the computer codes
Kawuba Reply
machine code, source code , object code, and byte code etc
Hemant
advantages of NAA
Sai Reply
how I can reaction of mercury?
Sham Reply

Get the best Physical methods in ch... course in your pocket!





Source:  OpenStax, Physical methods in chemistry and nano science. OpenStax CNX. May 05, 2015 Download for free at http://legacy.cnx.org/content/col10699/1.21
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Physical methods in chemistry and nano science' conversation and receive update notifications?

Ask