pp 110-120
4.5 transverse field contact transistor (JFET) 4.5.1 Structure
The transverse field transistor contact (Junction Field Effect Transistor, FET), known as the JFET, is a unipolar transistor. This is because it bases its operation on a single type of institution, ie electrons <vents, in contrast with the public and called transistors and bipolar operation is based on both types of operators.
Surge (D)
Source (S)
Gate
(A)
(C)
Figure 4.5.1 Principle (a) constructing a JFET, with (b) two gates or (c) with a gate
N
The principle construction of a JFET-based selection of a semiconductor unit, eg N-type, which are added on both sides of the P-type regions (see sch.4.5.1). The concentration of dopant in the P-type regions is much higher than that of the channel. Each of the P-type regions called gate (gate). The two ends of the Press-N are called respectively source (source) and surge (drain), while the share of semiconductor-type N, which lies between the popular type-P, called the channel or channel (channel). Depending on the type of semiconductor channel is defined and the type of JFET. So there JFET n-channel or p-channel (n-channel or p-channel). A JFET can have only one gate, if you combine the two P-type regions. Otherwise, the JFET will have two gates (such electronic devices used in special circuits mutation frequency receivers). Because it is the most widespread devices with a gate, they will focus on the analysis that follows.
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A parallel between the JFET and bipolar transistors. Because this ratio, which describe many types of circuits JFET from the respective bipolar transistors, simply adapted for the case of the JFET. The correspondence of names and symbols of the terminals shown in pin.4.5.1. A simple example of this correlation is the symbolism of the DC emitter O base IB and IC of the collector of bipolar transistor with that of the DC power source IS, IG the gate and drain of the ID of the JFET.
Bipolar Transistors
JFET
Bipolar Transistors
JFET
Emitter
Source
E
S
Basis
Gate
B
G
Collector
Abductor
C
D
Table 4.5.1 Correlation names and symbols pin bipolar transistor and JFET
The D
GO
(A) 0 S (b) The S
Figure 4.5.2 Circuit Symbols JFET (a) n-channel and (b) p-channel
The circuit symbol for a JFET is shown in Fig 4.5.2. For mnemonic device must be considered that the thin vertical line corresponds to the channel. The source and the surge associated with this line. Furthermore, the arrow shows the gate to the semiconductor-type N as the common pathway. Thus, if the arrow points to the channel JFET is the n-channel (sch.4.5.2a) whereas if the opposite direction, are p-channel (sch.4.5.2v).
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4.5.2 Principle of operation
The operation is based on a JFET gate-channel contact, hereinafter called contact or diode gate for simplicity. The cargo area of the contact area has resulted in a reduction of the cross section of the channel and therefore the change in resistance, see Figure 4.5.3. So by changing the polarity of the diode gate, changing the width of the space charge region and hence the contact resistance of the channel so check the current which flows through the JFET. O term effect of field associated with the cargo area space is created in the gate contact.
(A)
(C)
Figure 4.5.3 (a) Areas of space charge, (b) normal bias JFET
and (c) characteristic curve in the current situation shorted gate
Typical bias circuit a JFET N-channel type shown in Figure 4.5.3v. In the circuit uses two voltage sources. The first is connected between the drain and source VDD voltage and provides the abductor VDS. The second is connected between gate and source of the VGG and provides the gate voltage VGS. The VGG reverse biases the diode gate so the gate current of IG to be too small and therefore the input impedance too high. Furthermore, the shape of the gate voltage, as mentioned above, the resistance of the channel. Figure (4.5.4) shows a graph of drain currents ID characteristics versus drain voltage VDS for various values of gate voltage. The similarity of the characteristic curve of the bipolar transistor collector is
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remarkable. For small values of the voltage drain current prohibition
Wu increases sharply and becomes almost horizontal in the active region. Between VP and VDS (max), the drain current is almost constant.
When the drain voltage becomes too large and exceed the price BN ^, which is the tendency to collapse with the grounded source, JFET, as shown in Figure (4.5.3) and (4.5.4), collapses. Like the bipolar transistor active area extends along the (almost) horizontal part of the curve. In this region the JFET operates as a power source. In each curve drain current versus drain voltage discern the following:
VGS = 0
VcS (off)
Vp
VD
Figure 4.5.4 Characteristic curves abductor
Gate shorted condition: When the gate voltage of zero, then the gate and source shorted. This condition is called gate and shorted condition corresponds to the "current drain gate shorted to the situation."
Drain current of the gate shorted condition: In
Figure 4.5.3g IDSS ratios obtained from the expression "from the hood (Drain) to the source (Source) with gate shorted (Shorted-gate)". The leaflets give manufacturers the IDSS in the active region. It is fundamental that the reader remembers this: Because the active region of the curve is almost flat, very close to satisfactory IDSS drain current at which
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any point in the active region in the state shorted gate. Moreover, because the IDSS refers to the situation shorted gate is the maximum current drain can have a JFET in normal mode. All other values of the gate voltage is negative and less drain currents.
Compressive stress: The compressive stress (pinch-off voltage) VP is the drain voltage above which the direct current drain is approximately constant in the state shorted gate. When done drain voltage equal to VP, the conductive channel is extremely narrow (compressed), and areas of cargo space almost touching. If the drain voltage increased further, the drain current will increase slightly because the width of the conductive channel is very small and does not change very little. The compressive stress separates a typical JFET in two regions: the active site, which applies when the voltage
drain voltage is higher compression, and shoulder area, which applies when the drain voltage is lower than the compressive stress. The latter is also called ohmic because in this current of
cupboard is almost proportional to the drain voltage, ie apply Ohm's law, and practically does not depend on the gate voltage. Moreover, the ohmic region corresponds to the saturation region of bipolar transistors. The ohmic resistance area between the source terminal and drain very easily calculated by the equation
VP
Rds = c ^ 4.5.1
1DSS
Voltage cutoff gate - source: The typical abductor resembles the typical collector of bipolar transistors. For example sch.4.5.4 given the characteristic of a typical JFET. The higher characteristic given for VGS = 0 V, for gate shorted condition. The compressive stress is about 4 V and the voltage breakdown around 30 V. As shown in the figure the IDSS is 10 mA. When VGS = VGS (off), where VGS (off) is the cutoff voltage gate-source (gate cutoff voltage), the cargo area adjacent areas and severing the direct current drain. Since the VP is the voltage kidnapping, which limits the current in the shorted state of the gate must be valid
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Transconductance characteristic: The typical transconductance (transconductance curve) JFET is a graph of drain current versus gate voltage (see sch.4.5.5). In general, the transconductance characteristic of any JFET has the same form, ie is part parable. This is due to start operation of the JFET. The typical transconductance is described by the equation:
4.5.3
4.5.2
Some manufacturers' brochures do not mention the VP, but
V
GS (off)
. These two trends are numerically equal in absolute value.
VGS (off)
Figure 4.5.5 Typical transconductance
which can be used to approximate each JFET. With the help of Eq. 4.5.2 it is possible to calculate the drain current of each gate voltage value is known when the drain current of a state shielded gate, ie the maximum drain current, and the cutoff voltage of the gate-source. Because of the parabolic shape of the curve is called the JFET square law provisions.
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4.5.3 Reading the technical characteristics
The leaflets of the technical characteristics of the JFET resemble those of bipolar transistors. So in each table divided by the maximum principle estimates (maximum ratings), the static and dynamic characteristics, ie, dc and ac, the characteristic curves and mechanical characteristics such as type JFET shell under consideration. The Annex presents a copy of the technical part of the booklet JFET MPF102, which is manufactured by many companies.
In the data sheet indicating the code number and type of JFET. Following are some key applications for which provision is made, such as high-frequency amplifiers, oscillators and converters. Here are the absolute maximum estimates, which are usually referred to at 25AC unless noted. Are an impediment for applications in which a designer or conservator intends to use its JFET. It then shows the type of housing with the terminals, followed by continuous features (static) and AC (dynamic), which distinguished the voltage cutoff VcS (off), the current shorted gate IDSS, the breakdown voltage of the diode gate - source BVcSS etc. . Features AC excels diagogimotita gfs, parasitic capacities, etc. Finally, each data sheet, as already mentioned, contains the characteristic curves of DC and AC.
Example 4.5.1
The technical characteristics of the JFET MPF102, which is n - channel we find that the reverse current of the gate are:
IcSS = -2 nA for VcS = -15 V and VDS = 0 V
What is the gate-source resistance, ie the input device
on the continuum?
Solution
The input impedance of the continuum are:
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7500MO
cs 2nA '
RBE = 700KO
This result can be compared to the corresponding input impedance for the continuum, a bipolar transistor the base of which passes current 1mA:
0,7 V
1mA
The comparison shows the superiority, in terms of input impedance of JFET in bipolar transistors.
Example 4.5.2
O characteristics of a JFET table gives: IDSS = 25 mA and VP = 4
V. What is the maximum current drain what is the cutoff voltage
the gate-source?
Solution
For each value of gate voltage, the drain current must lie between the limits:
0 <ID <25 mA
When the gate voltage is zero, the drain current becomes the maximum value
ID = 25 mA
The cutoff voltage gate-source is equal to the vapor compression
but with opposite sign. Since the compressive stress is equal to 4V
we
VcS (off) = -4 V
Example 4.5.3
Using the data in Example 4.5.2 to compute the impedance of the JFET in the ohmic region.
Solution
The resistance for the continuous ohmic region is equal to the ratio of voltage to squeeze the maximum current drain. So,
Rdc = 160O DC 25TIA
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SUMMARY 4.5
• A JFET comprises a semiconductor unit-type N or type-P
called the channel. The two ends of the channel is called
source and kidnapping. On both sides of the channel is P-type contacts if the channel is N-type or type-N if the channel is
P-type, called gate <gates, they operate independently from each other.
• There is a correlation between the source electrode, gate JFET and evacuation of the electrodes emissions, base and collector of bipolar transistors.
• the gate area with cargo space limits the cross-section of the channel. The resistance of the channel or the current that leaks can be controlled "if the contact is inversely polarized gate source.
• In the cluster of characteristics-voltage drain current versus gate voltage of a JFET, divided as follows:
A. The current drain on the state shorted gate, which is the maximum current in the active region.
B. The compressive stress (VP), which is the drain voltage on the drain current of which is practically constant in the state shorted gate.
C. The gate voltage cutoff - source (VGS (off)), for which cut off the current of kidnapping and is numerically equal to the compressive stress, but with opposite sign. D. The ohmic region where the drain current of practically does not depend on the gate voltage.
• The typical transconductance allows calculation of the drain current for each value of gate voltage when the direct current is known to drain a state shielded gate and the gate source voltage cutoff.
