Junction field effect transistor

The JFET (. SFET, engl junction FET , JFET or non-insulated-gate FET , NIGFET) is the easiest constructed unipolar from the group of field effect transistors ; a distinction is made between n-channel and p-channel JFETs .

Circuit symbols of JFETs
n-channel	p-channel

history

The development of the junction field effect transistor goes back to Julius Edgar Lilienfeld , who first described the functionality in 1925. At that time, however, the doping of the semiconductor material was not so far advanced in terms of production technology that JFETs could be produced reproducibly. The first practically implemented JFET with a pn junction (positive-negative) and a so-called gate as a control electrode go to Herbert F. Mataré and Heinrich Welker , and independently and parallel to this, William B. Shockley and Walter H. Brattain , from 1945 back.

construction

n-channel JFET: change in size of the junction with the gate-source voltage U _GS

The following explanations apply to n-channel JFETs. In the p-channel JFET, the n and p zones are swapped and the signs of all voltages and currents are reversed.

An n-channel JFET consists of an n- doped area, which is surrounded by a p-zone (see also pn junction ). The drain (D; drain = sink, drain) and source (S; source = source, inflow) connections are alloyed into the n-layer . The drain-source path is called the n-channel.

The p-zone is the so-called gate connection (G; gate = gate) . This connection is used to control the JFET. A space charge zone is formed between the p-zone and the n-channel, the extent and shape of which depend on the voltages between source and drain and between gate and source. The JFET is thus similar to the MESFET ( metal-semiconductor FET ) in which a rectifying metal-semiconductor junction ( Schottky junction ) is used instead of the pn junction . The functionality of JFET and MESFET does not differ .

function

Example of an output characteristic field of an n-channel JFET

When the gate connection is open or connected to the source connection, the n-channel behaves similarly to an ohmic resistor . Without triggering at the gate, the JFET is conductive.

If the gate is connected to the source and the drain-source voltage U _DS on the n-channel is increased (plus pole on the drain connection), the space charge zone between the gate and the n-channel in the vicinity of the drain expands as the drain increases -Source voltage continues to increase and narrows the remaining current channel more and more. When the drain-source voltage rises, the current through the n-channel (= drain current) rises until a maximum constriction of the channel has been reached. The corresponding voltage is referred to as pinch-off or pinch-off voltage U _p (see output characteristic curve); it corresponds to the threshold voltage U _th for MOSFETs . With a further increase in U _DS , the drain current I _D remains almost constant. The constriction has stabilized and expanded horizontally (in the example image), i. In other words, the “additional” channel voltage is now absorbed by the “pinch off” in the channel. This is the normal operating range of this transistor and the corresponding drain current I _DSS (of English drain source horted to gate called). In this state, the transistor can (quasi) be used as a constant _current source with I _DSS . A disadvantage compared to "real" constant current sources is an increased temperature dependency. The size of the pinch-off voltage depends on the doping N _{D or A} and half the width a of the channel, as well as on the voltage drop U _D ( diffusion voltage ) across the space charge zones:

{\ displaystyle U _ {\ mathrm {p}} = U_ {D} -U _ {\ mathrm {p0}}}

with the internal pinch-off voltage:

{\ displaystyle U _ {\ mathrm {p0}} = {\ frac {eN _ {\ mathrm {D}} a ^ {2}} {2 \ varepsilon _ {\ mathrm {r}} \ varepsilon _ {0}}} }

Origin of the pinch-off and control effect in the JFET

Here is the elementary charge . The pinch-off voltage is counted positively in the case of an n-channel in the gate-source direction and in the case of a p-channel in the source-gate direction. ${\ displaystyle e}$

By applying a negative bias voltage between gate and source, the space charge zone of the gate-source diode is enlarged (n-channel JFET). The channel is also modulated in width and length (pinch-off region; see picture). This enables a control effect with a high output resistance at the drain (similar to that of a bipolar transistor or a pentode ). In the output characteristic field it can be seen that the current of the horizontal branches of the characteristic curve shifts to smaller values in the active area. In this case too, an increase in the drain-source voltage causes only a very small change in the drain current.

Operating point setting with R _S and temperature compensation

The desired operating point setting for operation is very simple and takes place, analogously to an electron tube , either with a source resistor or with a negative gate-source bias voltage. As with an electron tube, the slope of the JFET is very low and for a high voltage gain z. B. relatively large work resistances required. ${\ displaystyle S = {\ frac {\ mathrm {d} I _ {\ mathrm {D}}} {\ mathrm {d} U _ {\ mathrm {GS}}}}}$

As with electron tubes or MOSFETs, the almost powerless control of the JFET for stationary operation is advantageous. Since the gate-source path used to control the drain current is always operated in the reverse direction, in steady-state operation, the gate never flows more than the reverse current of a few picoamps. At higher frequencies, significantly larger capacitive currents occur.

If the JFET is operated below the pinch-off voltage in the linear range (ohmic region; see picture), it can e.g. B. can be used for an automatic gain control (AGC) as part of a voltage divider. There it behaves like a triode .

The control characteristic ( I _D as a function of U _GS ) is a complicated function and can be approximated by a quadratic function . The following formula describes the simple model of the transistor in the pinch-off area. As described above, I _DSS and U _p are manufacturing-dependent parameters and are specified in the data sheet .

{\ displaystyle I _ {\ mathrm {DS}} = I _ {\ mathrm {DSS}} \ left [1 - {\ frac {U _ {\ mathrm {GS}}} {U _ {\ mathrm {p}}}} \ right] ^ {2}}

Small signal equivalent circuit diagram

Small-signal equivalent circuit diagram of the JFET

In the saturation area or in the pinch-off region, the behavior in small-signal operation can be described by a simple small-signal equivalent circuit diagram . Are in it

{\ displaystyle s = {\ frac {\ partial I _ {\ mathrm {D}}} {\ partial U _ {\ mathrm {GS}}}} \ quad {\ text {and}} \ quad r _ {\ mathrm {DS }} = r _ {\ mathrm {i}} = {\ frac {\ partial U _ {\ mathrm {DS}}} {\ partial I _ {\ mathrm {D}}}}}

To be precise, the power source behaves in the model as an energy sink . So that it can develop, the transistor must be operated in a suitable circuit that is fed by an actually existing energy source. ${\ displaystyle i _ {\ mathrm {D}}}$

Channel length modulation

Similar to a MOSFET, the channel constriction at the drain contact in the pinch-off area results in a shorter effective line length in the transistor, depending on the drain-source voltage. The reason for this is the decrease in mobility due to the high field strengths at the end of the channel and the associated finite saturation speed of the charge carriers. This leads to an increase in the saturation current in the output characteristic field. In a very simplified way, this can be taken into account by introducing an early voltage U _A :

{\ displaystyle I _ {\ mathrm {D}} \ left (U _ {\ mathrm {GS}}, U _ {\ mathrm {DS}} \ right) = I _ {\ mathrm {DS}} \ left (U _ {\ mathrm {GS}} \ right) \ cdot \ left (1 + {\ frac {U _ {\ mathrm {DS}}} {U _ {\ mathrm {A}}}} \ right)}

Areas of application

Compared to the bipolar transistor, the JFET generates a significantly lower noise power at frequencies below approx. 1 kHz; at higher frequencies it makes sense if the source resistance is greater than approx. 100 kΩ ... 1 MΩ (typical for capacitor microphones , piezoelectric sensors , high-quality photodetectors or active antennas with low height, often used in measurement technology).

Ready- made types with graduated values are available for applications as a current source, the so-called current control diode , or an adjustable resistor.

It is also used to switch signal voltages in the low and high frequency range (LF and HF range), as a switching mixer with a particularly high dynamic range and low intermodulation in shortwave receivers and in auto-zero amplifiers and chopper amplifiers, and as a signal diode with low reverse current .

Furthermore, a negative differential resistance can be implemented with two JFETs similar to tunnel diodes , which can be used in oscillators to generate vibrations. This JFET circuit is known as a lambda diode .

Well-known types of small signals

BF245A / B / C: n-channel JFET; typical parameters: U _DS ≤ 30 V; P _max = 0.30 W; I _DSS = 2… 6.5 mA (A type) / 6… 15 mA (B type) / 12… 25 mA (C type); U _p = -0.5… −8 V; Type TO-92 (discontinued)
J310: n-channel JFET; typical parameters: U _DS ≤ 25 V; P _max = 0.35 W; I _DSS = 24 ... 60 mA; Type TO-92 (discontinued)
MMBF4416: n-channel JFET; typical parameters: U _DS ≤ 30 V; P _max = 0.225 W; I _DSS = 5… 15 mA; Design SOT-23

Web links

Commons : JFET - collection of images, videos and audio files

Stefan Goßner: Basics of electronics. January 1, 2016, accessed March 1, 2016 .

Individual evidence

↑ Reinhold Paul: field effect transistors - physical principles and properties. Verlag Berliner Union, Stuttgart 1972, ISBN 3-408-53050-5 .
↑ Bo Lojek: The MOS Transistor . In: History of Semiconductor Engineering . Springer, Berlin 2007, ISBN 978-3-540-34257-1 , pp. 317 ff .
^ The Semiconductor Data Book, Motorola Inc. 1969 AN-47
↑ Data & Design Manual, Teledyne Semiconductors 1981, Junction FETs in Theory and Application
↑ Low Power Discretes Data Book, Siliconix incorporated 1989, Application Note LPD-1
^ Hans Heinrich Meinke , Friedrich-Wilhelm Gundlach : Pocket book of high frequency technology - Volume 1: Basics . Springer-Verlag, Berlin 1992, ISBN 3-540-54714-2 . See G20
↑ Erwin Böhmer: Elements of Applied Electronics , Vieweg + Teubner, 2009

[1] Reinhold Paul: field effect transistors - physical principles and properties. Verlag Berliner Union, Stuttgart 1972, ISBN 3-408-53050-5 .

[2] Bo Lojek: The MOS Transistor . In: History of Semiconductor Engineering . Springer, Berlin 2007, ISBN 978-3-540-34257-1 , pp. 317 ff .

[3] The Semiconductor Data Book, Motorola Inc. 1969 AN-47

[4] Data & Design Manual, Teledyne Semiconductors 1981, Junction FETs in Theory and Application

[5] Low Power Discretes Data Book, Siliconix incorporated 1989, Application Note LPD-1

[6] Hans Heinrich Meinke , Friedrich-Wilhelm Gundlach : Pocket book of high frequency technology - Volume 1: Basics . Springer-Verlag, Berlin 1992, ISBN 3-540-54714-2 . See G20

[7] Erwin Böhmer: Elements of Applied Electronics , Vieweg + Teubner, 2009