Structure of the MOSFET
pp 120-130
4.6 MOSFET
The MOSFET (Metal-Oxide Semiconductor FET) is also a provision that has source, gate and hood. But unlike the JFET, the gate is electrically insulated on the channel leading to the gate current is too small, no matter the time of the voltage applied between gate and source. As an insulator, used a very thin layer of silicon dioxide, which is interposed between the metal gate and semiconductor. The order of succession of layers determines the name of the device.
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Figure 4.6.1 Structure MOXGET site dilution
The MOSFET is also called IGFET (Insulated Gate FET), because the gate is insulated. In one type of MOSFET channel is determined by the type of cargoes, electrons or holes, which carry the current from source to outlet. Moreover, the MOSFET is divided into two categories: the dilution type MOSFET (Depletion mode) and the MOSFET type condensation (Enhancement mode). depending on how they work. O MOSFET manufacturing method mentioned elsewhere (Technology Electronic Devices) and requires special laboratories, Laboratories Microelectronics.
4.6.1 MOSFET type dilution
The MOSFET extender consists of a semiconductor unit type N. The end of this section are the source and drain terminals (Fig. 4.6.1). Applying a voltage between source and drain, where the surge is connected to the positive terminal of the source, forcing electrons to flow from source to outlet. The MOSFET, unlike the JFET, has a single region R, called the substrate (substrate). This area restricts the channel cross section thus remains a very thin layer that allows electrons to move freely from source to outlet. A multi-coated silica-valuation contrast to the opposite side of the channel (sch.4.6.1) and is placed on the metal gate.
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(A)
Figure 4.6.2 MOSFET type extender (a) negative voltage to the gate and (b) positive gate voltage at
Figure (4.6.2) shows a MOSFET type dilution in which a negative voltage applied to the gate. The source voltage VDD requires the electrons to move from source to employ a producer through the thin channel, which is the area between P and the gate. The negative voltage applied to the gate repels the electrons in the channel, resulting in reduced density and thus reducing the power drain. This function is called thinning operation (depletion mode). When the gate voltage becomes more negative decreases the drain current when they become quite negative in the current is cut off. Because the gate is electrically insulated on the channel, it is possible to apply a positive voltage (sch.4.6.2v).
A positive voltage at the gate is likely to attract electrons towards the gate and thereby increasing the density in the channel. This function is called condensation function (enhancement mode). Thus, the channel, although practically does not change size, it becomes more conductive thereby increasing the drain current of. The possibility of further increasing the current drain by applying positive gate voltages to adjust the dilution of the MOSFET JFET.
The current of the gate is negligible in both modes. Thus, in both cases, the input impedance of the gate is very large, from 1010 to 1014 Ohm. An additional element MOSFET N-channel is the MOSFET P-channel.
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(A) Lpagogos (b) Lpagogos
Q Q
Source Source
Figure 4.6.3 Circuit somvola MOXEETtopou kanalioo dilution of N-(a) with a probe substrate, and (b) without substrate terminal
Figure (4.6.3) is given the circuit symbol for a MOSFET type dilution. The gate has the form opiismou a capacitor. Right from the gate there is a thin line, which represents the channel. The arrow points toward the substrate N-type semiconductor, so that this provision is an N-channel MOSFET. In most cases the manufacturer internally connects the substrate to the source, so we have a three terminal device as shown in Figure 4.6.3v.
O! kidnapping of a typical N-channel MOSFET are shown in Figure (4.6.4). You should notice that the curves are the upper features correspond to positive values of gate voltage, where we operate
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thickening (VGS> 0V). The lower kampyii for which the drain current of almost 0 corresponds to a voltage equal to the gate cutoff voltage (VGS = VGS (off)). The kampyies for which the gate voltage is between the VGS (off) and 0 corresponding to a dilution function (VGS <0 V).
Particular attention should be given to the fact that in the dilution type MOSFET IDSS is no longer the maximum current. As shown in sch.4.6.4, there are characteristic corresponding to positive values of VGS. Mode thicken the drain current exceeds the IDSS. The MOSFET type dilution used easily, why do not necessarily require a voltage bias. From a typical MOSFET transconductance type of dilution, which is given to technical brochures of the manufacturers can easily choose the operating point Q of the device. Such a case may correspond to the values and VGS = 0V ID = IDSS, thus simplifying the circuit polarization.
Bearing all this in mind, we can say that each MOSFET that can operate either by dilution or condensation-type MOSFET is called dilution.
Dilution MOSFET Applications
Dilution da MOSFET type used primarily in analog electronics. The amplifiers with MOSFET-type extender is similar to the JFET amplifiers and for this reason can follow the AC analysis, which is used in JFET. Moreover, they are at the entrance of the insulated gate, achieves very high input impedance. Delos because the resistance of the channel varies with the voltage of the gate, find application in automatic gain control circuits (AGC). Because this is non-zero current of kidnappings for zero gate voltage MOSFET called normal mode ON (normally on MOSFET).
Example 4.6.1
In the adjacent circuit given that IDSS = 10mA and VGS (off) = -4 V. What
is the voltage between drain and source when the gate voltage is
0?
Solution
The MOSFET operates as a power source so that this determines the current flowing through the circuit. Furthermore, as the gate voltage is zero, the current drain is equal to IDSS, ie 10 mA. Thus, the voltage between drain and source will be equal to
VDS = 16 V - (10 mA) · (1 KO) = 16 V - 10 V = 6 V
It should be given special attention. To apply the above calculation should be the MOSFET operates in the active region, ie the voltage between drain and source (VDS) should be larger trend of compression (VP).
Because
Vp = - VcS <off) = 4 V <VDS = 6 V
actually apply the above calculation.
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Example 4.6.2
In the previous example circuit alter the resistance of 1 to 4.7 KO KO. If the gate voltage is zero what is the voltage between drain and source?
Solution
Assume that the MOSFET operates as a power source. Because VCS = 0 V the drain current of is 10 mA, implies that the voltage between drain and source will be:
VDS = 16 V - (10 mA) · (4,7 KO) = 16 V - 47 V = -31 V
But this is impossible, because the drain voltage can not be negative. For this reason you should follow a different path. O above calculation clearly shows that the MOSFET
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does not work in the active region but in ohmic region. So
calculate the resistance of the MOSFET for DC:
VP 4V
VDS = 16 V =-400ch 16V = 0,0784 x16 V = 1,255 V
DS 4700 Z 5100 Z + 400
Rds = 400O DS IDss 10mA
In the ohmic region the MOSFET acts as resistance, which
connected in series with the resistance of 4.7 KO and creates a
voltage divider. Using the known type of divider
measure the voltage between drain voltage and source.
4.6.2 MOSFET-type densification
Another type of MOSFET, which applies primarily to digital circuits, is the thickening type MOSFET (enhancement mode MOSFET). This MOSFET is operated exclusively to thicken and allows the simultaneous completion of the same chip, N-channel devices and P-channel thus creating additional information systems and therefore the circuits CMOS (Complementary MOS). It should be noted that the development of microelectronics in digital circuits, especially microprocessors, which contain more than one million tiles in each MOSFET, due to MOSFET-type densification. In addition, MOSFET-type densification used in power applications with various names such as V-MOSFET (Siliconix, Temic), T-MOSFET (Motorola), HEXFET (IRF), etc., depending on the optimized structure, which have achieved several manufacturers them.
The structure of the MOSFET-type densification differs from that of the MOSFET type dilution. These substrate extends until the oxide layer (see Figure 4.6.5) with the result that
a channel between source and drain, but only a structure which looks like two diodes connected between them instead.
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(I) The Lpagogos
Gate
Figure 4.6.5 Structure MOSFET-type densification (a) structure and (b) normal polarization
To understand the operation of a MOSFET-type densification should in principle be applied polarizations normal polarity of the gate and drain (see Figure 4.6.5.v). Original theory that the gate voltage is zero, ie that we have shorted gate situation. In this case the direct current can not pass
from source to outlet because both the P-type substrate has too few free electrons and the other substrate diode - kidnapping is reverse biased. So the situation shorted gate drain current is negligible.
VGS3> 0
VGS (th)
VGS (on) VG
Figure 4.6.6 (a) Characteristic curves of abduction and
(B) transconductance of a MOSFET-type densification
In order to drain current of electrons must occur in the substrate, the substrate that is converted to - even locally - to N-type so as not to substrate diodes - source and substrate - kidnapping. This is possible thanks to the fact that the gate forms a capacitor with the substrate. So if you applied a fairly "high" positive voltage to the gate then the reinforcement of the gate, the gate capacitor - substrate positively charged. This has led to accumulated negative charge, ie electrons in the substrate. The load accumulates in the substrate, which is located just below the thin oxide layer covering the gate all the way from source to the hood. This leads, locally, a very thin layer where the concentration of electrons is larger than the holes. This material behaves as a type-N and the presence of good conductivity and therefore the electrical current between source and drain contacts. If you then increase the positive gate voltage will increase and pyknotita of electrons in the substrate. This will result in the increase of drain current (see Figure 4.6.6). Because the channel is N-type, the device will be thickening MOSFET N-channel (n-channel enhancement mode MOSFET).
The substrate is P-type and the presence of the strong elec-
Field trick him a thin layer, which abuts the silicon dioxide layer is converted to N-type. This thin layer, which changes conductivity type, called the inversion layer N-type (inversion layer).
(A)
Gate
Abductor
Figure 4.6.7 Symbols thickening type MOSFET (a) N-channel and (b) P-channel
The minimum gate-source voltage, which causes the inversion layer is called N-type voltage threshold
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(Threshold voltage) and is denoted by VGS (th). So when the gate voltage is less than the threshold voltage MOSFET is open. Conversely, when the gate voltage is greater than the threshold voltage, the MOSFET conducts.
Figure 4.6.6 shows the typical drain current versus drain-voltage source for different values of gate voltage. The lower curve corresponds located at gate voltage equal to the threshold voltage (VGS (th). So when the gate voltage is below the threshold voltage is the drain current of almost zero. However, when the gate voltage is greater than the threshold voltage of the MOSFET conducts and the current drain is determined by the gate voltage. the characteristic shape (4.6.6.a) we observe a behavior similar to that of the MOSFET dilution, ie
an area where the current increases sharply (ohmic region) and a
area where the current remains almost constant (active area or power source). At this point we must be careful that no condensation on the MOSFET is compressive stress, since in these areas there are no cargo space, which unite, but reversal region. For this reason, use different symbols, the VIK, the voltage drain ohmic separating the active region. This trend is calculated by
In figure (4.6.6v) shows the transconductance characteristic, which in the case of densification MOSFET is part of a parabola. The parable begins, as expected from the above point by VGS (th). Here we must pay special attention to the fact that there is no thickening MOSFET IDSS. Instead, the manufacturers give a table of each device drain current of ID (on) for a given VGS (on) of a point, which is well above the threshold voltage. Thus, the densification MOSFET parameters IDSS and VGS (th) of the MOSFET is substituted for the dilution
parameters ^ (onK VGS (on) and VGS (th).
In the situation shorted gate-type MOSFET is in the thickening off, because there is no channel between source and drain contacts. This results in the circuit symbol
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the channel is indicated by a dotted line and, "This way, different from that of the MOSFET dilution (Figure 4.6.7). Figure 4.6.7 is given and the circuit symbol for a MOSFET P-channel densification. In this case the substrate is N-RS-set, the negative threshold voltage and current drain is flowing in the opposite direction than that of MOSFET N-channel.
In each type MOSFET, dilution <densification is inserted between the gate and channel a thin layer of silicon dioxide. Since the thickness of silicon dioxide dielectric strength material (107 V / cm) implies that the voltage applied to the gate can vary in some limits, which if overcome the MOSFET is destroyed. The gate voltage limits defined by the VGS (max) in the case of VN10 is VG ^ = ± 40 V.
The insulating layer of silicon dioxide gate can be destroyed by other less obvious reasons than the imposition of high voltage. The placement or removal of a MOSFET circuit, which is already powered, can lead to the development of transient voltages higher than the VGS (max). Also electrostatic charges deposited in a MOSFET gate when the catch, can develop a voltage exceeding the VGS (max). For this reason the protected MOSFET with a conductive ring, which is removed after installation or transported in conductive bag and placed by technicians
are standing on conductive floors or grounded conductive bracelet bearing which is also grounded.
