The
junction gate field-effect transistor (
JFET or
JUGFET) is the simplest type of
field-effect transistorThe field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
. It can be used as an
electronicallyElectronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
-controlled
switchIn electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
or as a voltage-controlled
resistanceThe electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
.
Electric chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
flows through a semiconducting channel between "source" and "drain" terminals. By applying a bias
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
to a "gate" terminal, the channel is "pinched", so that the
electric currentElectric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
is impeded or switched off completely.
Structure
The JFET is a long channel of
semiconductorA semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
material,
dopedIn semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
to contain an abundance of positive
chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
carriers (
p-type), or of negative carriers (
n-type). Contacts at each end form the source(S) and drain(D). The gate(G) (control)
terminalA terminal is the point at which a conductor from an electrical component, device or network comes to an end and provides a point of connection to external circuits. A terminal may simply be the end of a wire or it may be fitted with a connector or fastener...
has doping opposite to that of the channel, which surrounds it, so that there is a
P-N junctionA p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
at the interface. Terminals to connect with the outside are usually made
ohmicAn ohmic contact is a region on a semiconductor device that has been prepared so that the current-voltage curve of the device is linear and symmetric. If the I-V characteristic is non-linear and asymmetric, the contact is not ohmic, but is a blocking or Schottky contact...
.
Function
JFET operation is like that of a
garden hoseA garden hose is a flexible tube used to carry water. There are a number of common attachments available for the end of the hose, such as sprayers and sprinklers...
. The flow of water through a hose can be controlled by squeezing it to reduce the
cross sectionIn geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
; the flow of
electric chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
through a JFET is controlled by constricting the current-carrying channel. The current also depends on the electric field between source and drain (analogous to the difference in pressure on either end of the hose).
Schematic symbols
The JFET gate is sometimes drawn in the middle of the channel (instead of at the drain or source electrode as in these examples). This symmetry suggests that "drain" and "source" are interchangeable, so the symbol should be used only for those JFETs where they are indeed interchangeable (which is not true of all JFETs).
Officially, the style of the symbol should show the component inside a circle (representing the envelope of a discrete device). This is true in both the US and Europe. The symbol is usually drawn without the circle when drawing schematics of integrated circuits. More recently, the symbol is often drawn without its circle even for discrete devices.
In every case the arrow head shows the polarity of the P-N junction formed between the channel and gate. As with an ordinary
diodeIn electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
, the arrow points from P to N, the direction of conventional current when forward-biased. An English
mnemonicA mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...
is that the arrow of an N-channel device "points i
n".
To pinch off the channel, it needs a certain reverse bias (V
GS) of the junction. This "pinch-off voltage"(V
p) varies considerably, even among devices of the same type. For example, V
GS(off) for the Temic J201 device varies from -0.8V to -4V. Typical values vary from -0.3V to -10V.
To switch off an
n-channel device requires a
negative gate-source voltage (V
GS). Conversely, to switch off a
p-channel device requires V
GS positive.
In normal operation, the electric field developed by the gate must block conduction between the source and the drain.
Comparison with other transistors
JFET gate current (the reverse leakage of the gate-to-channel
junctionA p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
) is comparable to that of a
MOSFETThe metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
(which has insulating oxide between gate and channel), but much less than the base current of a
bipolar junction transistor|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...
. The JFET has higher
transconductanceTransconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm...
than the MOSFET and is therefore used in some low-noise, high input-impedance
op-ampsAn operational amplifier is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output...
.
History of the JFET
The JFET was predicted by
Julius LilienfeldJulius Edgar Lilienfeld was an Austro-Hungarian physicist. He was born in Lemberg in Austria-Hungary , moved to the United States in the early 1920s, and became American citizen in 1934...
in 1925 and by the mid-1930s its theory of operation was sufficiently well known to justify a patent. However, it was not possible for many years to make doped crystals with enough precision to show the effect. In 1947, researchers
John BardeenJohn Bardeen was an American physicist and electrical engineer, the only person to have won the Nobel Prize in Physics twice: first in 1956 with William Shockley and Walter Brattain for the invention of the transistor; and again in 1972 with Leon Neil Cooper and John Robert Schrieffer for a...
,
Walter Houser BrattainWalter Houser Brattain was an American physicist at Bell Labs who, along with John Bardeen and William Shockley, invented the transistor. They shared the 1956 Nobel Prize in Physics for their invention. He devoted much of his life to research on surface states.- Early life and education :He was...
, and
William ShockleyWilliam Bradford Shockley Jr. was an American physicist and inventor. Along with John Bardeen and Walter Houser Brattain, Shockley co-invented the transistor, for which all three were awarded the 1956 Nobel Prize in Physics.Shockley's attempts to commercialize a new transistor design in the 1950s...
were trying to make a JFET when they discovered the
point-contact transistorA point-contact transistor was the first type of solid-state electronic transistor ever constructed. It was made by researchers John Bardeen and Walter Houser Brattain at Bell Laboratories in December 1947. They worked in a group led by physicist William Bradford Shockley...
. The first practical JFETs were made many years later, in spite of their having been conceived long before the junction transistor. To some extent it can be treated as a hybrid of a MOSFET and a BJT though an IGBT resembles more of the hybrid features.
Mathematical model
The current in N-JFET due to a small voltage V
DS is given by:
-
where
- 2a = channel thickness
- W = width
- L = length
- q = electronic charge = 1.6 x 10-19 C
- μn = electron mobility
In solid-state physics, the electron mobility characterizes how quickly an electron can move through a metal or semiconductor, when pulled by an electric field. In semiconductors, there is an analogous quantity for holes, called hole mobility...
- Nd = n type doping concentration
In the saturation region:
-
In the linear region
-
or (in terms of
):
-
External links