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JFET

 
Computer Desktop Encyclopedia: FET
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(Field Effect Transistor) One of two major categories of transistor; the other is bipolar. FETs use a gate element that, when charged, creates an electromagnetic field that changes the conductivity of a silicon channel and turns the transistor on or off. FETs are fabricated as individually packaged discrete components as well as by the hundreds of millions on a single chip.

FETs Vs. Bipolar

FET-based silcon chips are easier to construct than their bipolar counterparts. FETs switch a little slower than bipolar transistors, but use less power. Once the gate terminal on an FET has been charged, no more current is needed to keep that transistor on (closed) for the duration of time required. By comparison, a bipolar transistor requires a small amount of current flowing to keep the transistor on. While the current for one transistor may be negligible, it adds up when millions are switching simultaneously. The heat dissipated on bipolar limits the total number of transistors that can be built on the chip, which is why CMOS logic (based on FETs) is used to build chips with millions of transistors.

MOSFETs

The most widely used and widely known FETs are MOSFETs (metal oxide semiconductor FETs), which come in NMOS (n-channel) and PMOS (p-channel) varieties. On a chip, NMOS and PMOS transistors are wired together in a complementary fashion to create CMOS logic, which is the most predominant and used in almost every electronic device today. See MOSFET and n-type silicon.

There Are Many Kinds of FETs

Similar to MOSFETs are JFETs (junction FETs), which use a PN junction gate rather than a poly-crystalline gate. Used for microwave communications, MESFETs (metal semiconductor FETs) are similar to JFETs, but use a Schottky metal gate and are made from gallium arsenide or indium phosphide, not silicon. Evolving from MESFETs for higher-frequency applications are HEMTs and PHEMTs (high electron mobility transistors and pseudomorphic high electron mobility transistors). HEMTs are also called MODFETs, TEGFETs and SDHTs (modulation doped FETs, two-dimensional electron gas FETs and selectively doped heterojunction transistors).

Another high-frequency FET is the gallium arsenide-based CHFET (complementary heterostructure FET), which uses a complementary architecture similar to CMOS.

FETs Vs. Bipolar
After the gate is charged in an FET, no more current flows, but the transistor remains closed (turned on) during the required time period. Bipolar transistors (BJTs) require current the entire time the transistor must be closed.

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Wikipedia: JFET
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Electric current from source to drain in a p-channel JFET is restricted when a voltage is applied to the gate.

The junction gate field-effect transistor (JFET or JUGFET) is the simplest type of field effect transistor. It can be used as an electronically-controlled switch or as a voltage-controlled resistance. Electric charge flows through a semiconducting channel between "source" and "drain" terminals. By applying a bias voltage to a "gate" terminal, the channel is "pinched", so that the electric current is impeded or switched off completely.

Contents

Structure

Circuit symbol for an n-Channel JFET
Circuit symbol for a p-Channel JFET

The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers (p-type), or of negative carriers (n-type). Contacts at each end form the source and drain. The gate (control) terminal has doping opposite to that of the channel, which it surrounds, so that there is a P-N junction at the interface. Terminals to connect with the outside are usually made ohmic.

Function

JFET operation is like that of a garden hose. The flow of water through a hose can be controlled by squeezing it to reduce the cross section; the flow of electric charge through a JFET is controlled by constricting the current-carrying channel. The current depends also on the electric field between source and drain.

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 diode, the arrow points from P to N, the direction of conventional current when forward-biased. An English mnemonic is that the arrow of an N-channel device "points in".

To pinch off the channel needs a certain reverse bias (VGS) of the junction. This "pinch-off voltage" varies considerably, even among devices of the same type. For example, VGS(off) for the Temic J201 device varies from -0.8V to -4V.[1] Typical values vary from 0.3V to 10V.

To switch off an n-channel device requires a negative gate-source voltage (VGS). Conversely, to switch off a p-channel device requires VGS positive.

Comparison with other transistors

I–V characteristics and output plot of a JFET n-channel transistor.

JFET gate current (the reverse leakage of the gate-to-channel junction) is comparable to that of a MOSFET (which has insulating oxide between gate and channel), but much less than the base current of a bipolar junction transistor. The JFET has higher transconductance than the MOSFET and is therefore used in some low-noise, high input-impedance op-amps.

The JFET was predicted by Julius Lilienfeld 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 Bardeen, Walter Houser Brattain, and William Shockley were trying to make a JFET when they discovered the point-contact transistor. The first practical JFETs were made many years later, in spite of them having been conceived long before the junction transistor.

Mathematical model

The current in N-JFET due to a small voltage VDS is given by:

I_{DSS} = (2a) \frac{W}{L} q N_d \mu_n V_{DS}

where

  • 2a = channel thickness
  • W = width
  • L = length
  • q = electronic charge = 1.6 x 10-19 C
  • μn = electron mobility
  • Nd = n type doping concentration

In the saturation region:

I_{DS} = I_{DSS}\left[1 - \frac{V_{GS}}{V_P}\right]^2

In the linear region

I_D = (2a) \frac{W}{L} q N_d {{\mu}_n} \left[1 - \sqrt{\frac{V_{GS}}{V_P}}\right]V_{DS}

or (in terms of IDSS):

I_D = \frac{2I_{DSS}}{V_P^2} (V_{GS} - V_P - \frac{V_{DS}}{2})V_{DS}
v • d • e
Transistor amplifiers
JFET N-dep symbol.svg BJT NPN symbol (case).svg

References

  1. ^ J201 data sheet

External links


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