JFET
The junction field effect transistor ( SFET, Eng junction - fet, JFET or non -insulated -gate fet, NIGFET. ) Is the most easily constructed from the group of unipolar field-effect transistors; a distinction is made between n -channel and p-channel JFETs.
History
The development of the junction field- effect transistor goes back to Julius Edgar Lilienfeld, who in 1925 first described the operation. However, the doping of the semiconductor material was then manufacturing technology not progressed so far in order to produce reproducible JFETs can. The first practically realized JFET with a pn junction (positive - negative) and a so-called gate go as a control electrode to Herbert F. Mataré, Heinrich Welker and parallel to William B. Shockley and Walter Brattain H. back from the year 1945.
Construction
The following explanations refer to the n-channel JFETs. The p- channel JFET, the n and p regions are reversed and the signs of all voltages and currents are reversed.
An n- channel JFET composed of an n - doped region of a p-type region ( depletion layer ) is enclosed (see also the pn junction ). At the n- layer, the drain terminals (D, = drain sink drain ) and source (S; Source = source, inflow ) alloyed. The drain -source path is called n-channel.
The p-type region is called the gate (G; gate = gate). This port is used to control the JFET. It forms with the N- channel, a p-n diode. The JFET is thus similar to the MESFET ( metal -semiconductor FET Sheet ) in which a rectifying metal -semiconductor junction ( Schottky junction ) is used in place of the pn junction. In the operation of JFET and MESFET is no different.
Function
With an open gate, the n-channel behaves similarly to an ohmic resistor. Without control at the gate of the JFET is therefore conductive. The gate is connected to the source, and further increases the drain -source voltage UDS across the n-channel, the current increases until a maximum constriction of the channel has been reached. This voltage is referred to as a pinch or pinch-off voltage U (see output characteristic), it corresponds to the threshold voltage Uth in MOSFETs. A further increase of the drain current ID UDS remains almost constant. The constriction has stabilized and extended horizontally (in the example picture), that is, the "additional" channel voltage is then absorbed by the "pinch off" away, in the channel. This is the normal operating range of the transistor and the corresponding drain current is called IDSS (of English. Drain source horted to gate ). The transistor may be used in this state ( virtually ) as a constant current source with IDSS disadvantageous compared with the "correct" the constant current sources is a high temperature dependence. The size of the pinch -off voltage is dependent on the doping ND or A and the half-width A of the channel, as well as the voltage drop UD ( diffusion potential ) of the space charge zones:
With the inner pinch -off voltage:
Here is the elementary charge. The pinch -off voltage in the case of an n- channel in the gate -to-source direction and in the case of a p- channel counted positive in the direction of the source-gate.
By applying a negative bias between gate and source of the space-charge zone of the gate-source diode is increased (N- channel JFET ). The channel is in the width and length additionally modulated ( pinch-off region, see figure). Thus, a control action is at high output resistance at the drain possible (similar to the bipolar transistor or a pentode ). The output characteristic can be seen that the current of the horizontal branches characteristic shifts in the active region to smaller values . Also in this case causes an increase in the drain-source voltage is only a very small change in the drain current.
The desired operating point setting for the operation is very simple and is done in analogy to an electron tube, either with a source resistor or by a negative gate - source bias. As in an electron tube, the slope of the JFET is very low and for high voltage amplification, for example, relatively large load resistors are required. Is preferable as the electron tube or MOSFETs virtually power-free control of the JFETs ( for steady-state operation ). Since the gate -source path is used for controlling the drain current is always operated in the reverse direction, is never more than the reverse current of a few picoamperes is flowing through the gate. At higher frequencies occur capacitive currents.
Is the JFET below the pinch-off voltage in the linear region of operation (resistive region, see Figure ), it can, for example, for automatic gain control (English Automatic Gain Control, AGC ) can be used as part of a voltage divider. It behaves there like a triode.
The control curve ( as a function ID from UGS) 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. IDSS and up are as described above from manufacturing -dependent parameters and are specified in the data sheet.
Small-signal equivalent circuit
In the saturation region or in the pinch-off region can describe the behavior in the small signal operation by a simple small-signal equivalent circuit. In it are
Channel length modulation
Similar to a MOSFET, and depending on the channel pinch- drain contact at the pinch-off region in a shorter effective length of the line transistor of the drain -source voltage. The reason for this is the loss of motion due to the high field strengths at the end of the channel and the associated saturation finite velocity of the charge carriers. This leads to an increase of the saturation current in the output characteristic. Simplistically, one can take this into account by introducing a Earlyspannung UA:
Areas of application
The JFET generates compared to the bipolar transistor at frequencies below about 1 kHz a significantly smaller noise power at higher frequencies, the use is useful when the source impedance ... 1 M is greater than about 100 k (Typical for condenser microphones, piezoelectric sensors, quality photodetectors or active antennas with low height, often used in metrology ).
For use as a power source, called the current regulating diode, or adjustable resistor prefabricated types are available with differentiated values.
He also is used to switch from signal voltages in low - and high-frequency range ( LF and HF range ) as a switching mixer with very high dynamic range and low intermodulation in shortwave receivers and auto-zero amplifiers and chopper amplifiers, as well as a signal diode with low reverse current.
Furthermore, it can be realized, which can be used in oscillators for generating oscillations of two similar JFETs tunnel diode a negative differential resistance. This circuit is referred to as JFET lambda diode.