En este laboratorio se realiza el diseño, simulación y montaje de dos circuitos con transistores de efecto de campo metal-óxido-semiconductor (MOSFET), uno con tipo N y el otro con tipo P, c…Full description
Este archivo muestra la caracterización del mosfet CD4007Descripción completa
Descrição: Motorola Power Transistor Mosfet
Descripción: amplificación de pequeñas señales por medio de MOSFET
En este laboratorio se realiza el diseño, simulación y montaje de dos circuitos con transistores de efecto de campo metal-óxido-semiconductor (MOSFET), uno con tipo N y el otro con tipo P, c…Descripción completa
El transistor de efecto de campo metal-óxido-semiconductor o MOSFET (en inglés Metal-oxide-semiconductor Field-effect transistor) es un transistor utilizado para amplificar o conmutar señales elect...Full description
Descripción: Resumen de estructura, simbologia y transconductancia del mosfet de empobrecimiento.
Descripción: Ejercicio sobre transistores mosfet
Relatório descrevendo a experiência com o uso dos transistores JFET e MOSFET.
What is MOSFET? MOSFET stands for metal-oxide semiconductor field-effect transistor. It is a special type of field-effect transistor (FET).
Unlike BJT which is „current controlled‟, the MOSFET is a voltage controlled device. The MOSFET has “gate“, “Drain” and “Source” terminals instead of a “base”, “collector”, and “emitter” terminals in a bipolar transistor. By applying voltage at the gate, it generates an electrical field to control the current flow through the channel between drain and source, and there is no current flow from the gate into the MOSFET.
A MOSFET may be thought of as a variable resistor, where the Gate-Source voltage difference can control the DrainSource Resistance. When there is no applying voltage between the Gate-Source , the Drain-Source resistance is very high, which is almost like a open circuit, so no current may flow through the Drain-Source. When Gate-Source potential difference is applied, the Drain-Source resistance is reduced, and there will be current flowing through Drain-Source, which is now a open circuit.
In a nutshell, a FET is controlled by the Gate-Source voltage applied (which regulates the electrical field across a channel), like pinching or opening a straw and stopping or allowing current flowing. Because of this property, FETs are great for large current flow, and the MOSFET is commonly used as a switch.
Okay, let me summarize the differences between BJT and MOSFET.
Unlike bipolar transistors, MOSFET is voltage controlled. While BJT is current controlled, the base resistor needs to be carefully calculated according to the amount of current being switched. Not so with a MOSFET. Just apply enough voltage to the gate and the switch operates.
Because they are voltage controlled, MOSFET have a very high input impedance, so just about anything can drive them.
MOSFET has high input impedence.
How to use MOSFET as a switch? To use a MOSFET as a switch, you have to have its gate voltage (Vgs) higher than the source. If you connect the gate to the source (Vgs=0) it is turned off.
For example we have a IRFZ44N which is a “standard” MOSFET and only turns on when Vgs=10V – 20V. But usually we try not to push it too hard so 10V-15V is common for Vgs for this type of MOSFET.
However if you want to drive this from an Arduino which is running at 5V, you will need a “logic-level” MOSFET that can be turned on at 5V (Vgs = 5V). For example, the ST STP55NF06L. You should also have a resistor in series with the Arduino output to limit the current, since the gate is highly capacitive and can draw a big instantaneous current when you try to turn it on. Around 220 ohms is a good value.
This page shows some detail explanation how a MOSFET works as a switch. This page shows some advanced usage of MOSFET.
Types of MOSFET MOSFETs come in four different types. There are three main categories we need to know.
N-Channel (NMOS) or P-Channel (PMOS) Enhancement or Depletion mode Logic-Level or Normal MOSFET
N-Channel – For an N-Channel MOSFET, the source is connected to ground. To turn the MOSFET on, we need to raise the voltage on the gate. To turn it off we need to connect the gate to ground.
P-Channel – The source is connected to the power rail (Vcc). In order to allow current to flow the Gate needs to be pulled to ground. To turn it off the gate needs to be pulled to Vcc.
Depletion Mode – It requires the Gate-Source voltage ( Vgs ) applied to switch the device “OFF”.
Enhancement Mode – The transistor requires a Gate-Source voltage ( Vgs ) applied to switch the device “ON”.
Despite the variety, the most commonly used type is N-channel enhancement mode.
There are also Logic-Level and Normal MOSFET, but the only difference is the Gate-Source potential level required to drive the MOSFET.
How does it work in theory? I will try to explain it in the simplest way I can, for more detail or if you are in doubt, check the references and links I provide at the bottom of the post.
MOSFET is a voltage controlled field effect transistor that differs from a JFET. The Gate electrode is electrically insulated from the main semiconductor by a thin layer of insulating material (glass, seriously!). This insulated metal gate is like a plate of a capacitor which has an extremely high input resistance (as high as almost infinite!). Because of the isolation of the Gate there is no current flow into the MOSFET from Gate.
When voltage is applied at the gate, it changes the width of the Drain-Source channel along which charge carriers flow (electron or hole). The wider the channel, the better the device conducts.
The MOSFET are used differently compared to the conventional junction FET.
The infinite high input impedance makes MOSFETs useful for power amplifiers. The devices are also well suited to high-speed switching applications. Some integrated circuits contain tiny MOSFETs and are used in computers.
Because the oxide layer is so thin, the MOSFET can be damaged by built up electrostatic charges. In weaksignal radio-frequency work, MOSFET devices do not generally perform as well as other types of FET.
FAQ Where to put the load to a MOSFET? Source or Drain? Because load has resistance, which is basically a resitor. For N-channel MOSFET the reason we usually put the load at the Drain side is because of the Source is usually connected to GND.
If load is connected at the source side, the Vgs will needs to be higher in order to switch the MOSFET, or there will be insufficient current flow between source and drain than expected.
Heat Sink connected to the Drain? Typically the heat sink on the back of a MOSFET is connected to the Drain! If you mount multiple MOSFETs on a heat sink, they must be electrically isolated from the heat sink! It‟s good practice to isolate regardless in case the heat sink is bolted to a grounding frame.
What is the Body Diode For? MOSFETs also have an internal diode which may allow current to flow unintentionally. The body diode will also limit switching speed. You don‟t have to worry about it if you are operating under 1Mhz.
The MOSFET – Metal Oxide FET As well as the Junction Field Effect Transistor (JFET), there is another type of Field Effect Transistor available whose Gate input is electrically insulated from the main current carrying channel and is therefore called an Insulated Gate Field Effect Transistor or IGFET. The most common type of insulated gate FET which is used in many different types of electronic circuits is called the Metal Oxide Semiconductor Field Effect Transistor or MOSFETfor short. The IGFET or MOSFET is a voltage controlled field effect transistor that differs from a JFET in that it has a “Metal Oxide” Gate electrode which is electrically insulated from the main semiconductor N-channel or P-channel by a very thin layer of insulating material usually silicon dioxide, commonly known as glass.
This ultra thin insulated metal gate electrode can be thought of as one plate of a capacitor. The isolation of the controlling Gate makes the input resistance of the MOSFET extremely high way up in the Megaohms ( MΩ ) region thereby making it almost infinite. As the Gate terminal is isolated from the main current carrying channel “NO current flows into the gate” and just like the JFET, the MOSFET also acts like a voltage controlled resistor were the current flowing through the main channel between the Drain and Source is proportional to the input voltage. Also like the JFET, this very high input resistance can easily accumulate large amounts of static charge resulting in the MOSFET becoming easily damaged unless carefully handled or protected. Like the previous JFET tutorial, MOSFETs are three terminal devices with a Gate, Drain and Sourceand both P-channel (PMOS) and N-channel (NMOS) MOSFETs are available. The main difference this time is that MOSFETs are available in two basic forms:
1. Depletion Type – the transistor requires the Gate-Source voltage, ( VGS ) to switch the device “OFF”. The depletion mode MOSFET is equivalent to a “Normally Closed” switch.
2. Enhancement Type – the transistor requires a Gate-Source voltage, ( VGS ) to switch the device “ON”. The enhancement mode MOSFET is equivalent to a “Normally Open” switch.
The symbols and basic construction for both configurations of MOSFETs are shown below
The four MOSFET symbols above show an additional terminal called the Substrate and is not normally used as either an input or an output connection but instead it is used for grounding the substrate. It connects to the main semiconductive channel through a diode junction to the body or metal tab of the MOSFET. Usually in discrete type MOSFETs, this substrate lead is connected internally to the source terminal. When this is the case, as in enhancement types it is omitted from the symbol for clarification.
The line between the drain and source connections represents the semiconductive channel. If this is a solid unbroken line then this represents a “Depletion” (normally closed) type MOSFET and if the channel line is shown dotted or broken it is an “Enhancement” (normally open) type MOSFET. The direction of the arrow indicates either a P-channel or an N-channel device.
Basic MOSFET Structure and Symbol
The construction of the Metal Oxide Semiconductor FET is very different to that of the Junction FET. Both the Depletion and Enhancement type MOSFETs use an electrical field produced by a gate voltage to alter the flow of charge carriers, electrons for N-channel or holes for P-channel, through the semiconductive drain-source channel. The gate electrode is placed on top of a very thin insulating layer and there are a pair of small N-type regions just under the drain and source electrodes. We saw in the previous tutorial, that the gate of a junction field effect transistor, JFET must be biased in such a way as to reverse-bias the PN-junction. With a insulated gate MOSFET device no such limitations apply so it is possible to bias the gate of a MOSFET in either polarity, positive (+ve) or negative (-ve). This makes the MOSFET device especially valuable as electronic switches or to make logic gates because with no bias they are normally non-conducting and this high gate input resistance means that very little or no control current is needed as MOSFETs are voltage controlled devices. Both the P-channel and the N-channel MOSFETs are available in two basic forms, the Enhancement type and the Depletion type.
Depletion-mode MOSFET The Depletion-mode MOSFET, which is less common than the enhancement types is normally switched “ON” without the application of a gate bias voltage making it a “normally-closed” device. However, a gate to source voltage ( VGS ) will switch the device “OFF”. Similar to the JFET types. For an N-channel
MOSFET, a “positive” gate voltage widens the channel, increasing the flow of the drain current and decreasing the drain current as the gate voltage goes more negative. In other words, for an N-channel depletion mode MOSFET: +VGS means more electrons and more current. While a -VGS means less electrons and less current. The opposite is also true for the P-channel types. Then the depletion mode MOSFET is equivalent to a “normally-closed” switch.
Depletion-mode N-Channel MOSFET and circuit Symbols
The depletion-mode MOSFET is constructed in a similar way to their JFET transistor counterparts were the drain-source channel is inherently conductive with the electrons and holes already present within the N-type or P-type channel. This doping of the channel produces a conducting path of low resistance between the Drain and Source with zero Gate bias.
Enhancement-mode MOSFET The more common Enhancement-mode MOSFET is the reverse of the depletion-mode type. Here the conducting channel is lightly doped or even undoped making it non-conductive. This results in the device being normally “OFF” when the gate bias voltage is equal to zero. A drain current will only flow when a gate voltage ( VGS ) is applied to the gate terminal greater than the threshold voltage ( VTH ) level in which conductance takes place making it a transconductance device. This positive +ve gate voltage pushes away the holes within the channel attracting electrons towards the
oxide layer and thereby increasing the thickness of the channel allowing current to flow. This is why this kind of transistor is called an enhancement mode device as the gate voltage enhances the channel. Increasing this positive gate voltage will cause the channel resistance to decrease further causing an increase in the drain current, ID through the channel. In other words, for an N-channel enhancement mode MOSFET: +VGS turns the transistor “ON”, while a zero or -VGS turns the transistor “OFF”. Then, the enhancement-mode MOSFET is equivalent to a “normally-open” switch.
Enhancement-mode N-Channel MOSFET and circuit Symbols
Enhancement-mode MOSFETs make excellent electronics switches due to their low “ON” resistance and extremely high “OFF” resistance as well as their infinitely high gate resistance. Enhancement-mode MOSFETs are used in integrated circuits to produce CMOS type Logic Gates and power switching circuits in the form of as PMOS (P-channel) and NMOS (N-channel) gates. CMOS actually stands for Complementary MOS meaning that the logic device has both PMOS and NMOS within its design.
The MOSFET Amplifier Just like the previous Junction Field Effect transistor, MOSFETs can be used to make single stage class “A” amplifier circuits with the Enhancement mode N-channel MOSFET common source amplifier being the most popular circuit. The depletion mode MOSFET amplifiers are very similar to the JFET amplifiers, except that the MOSFET has a much higher input impedance. This high input impedance is controlled by the gate biasing resistive network formed by R1 and R2. Also, the output signal for the enhancement mode common source MOSFET amplifier is inverted because when VG is low the transistor is switched “OFF” and VD (Vout) is high. When VG is high the transistor is switched “ON” and VD (Vout) is low as shown.
Enhancement-mode N-Channel MOSFET Amplifier
The DC biasing of this common source (CS) MOSFET amplifier circuit is virtually identical to the JFET amplifier. The MOSFET circuit is biased in class A mode by the voltage divider network formed by resistors R1 and R2. The AC input resistance is given as RIN = RG = 1MΩ. Metal Oxide Semiconductor Field Effect Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The MOSFETs ability to change between these two states enables it to have two basic functions: “switching” (digital electronics) or “amplification” (analogue electronics). Then MOSFETs have the ability to operate within three different regions:
1. Cut-off Region – with VGS < Vthreshold the gate-source voltage is lower than the threshold voltage so the MOSFET transistor is switched “fully-OFF” and IDS = 0, the transistor acts as an open circuit
2. Linear (Ohmic) Region – with VGS > Vthreshold and VDS > VGS the transistor is in its constant resistance region and acts like a variable resistor whose value is determined by the gate voltage, VGS
3. Saturation Region – with VGS > Vthreshold the transistor is in its constant current region and is switched “fully-ON”. The current IDS = maximum as the transistor acts as a closed circuit
MOSFET Summary The Metal Oxide Semiconductor Field Effect Transistor, or MOSFETfor short, has an extremely high input gate resistance with the current flowing through the channel between the source and drain being controlled by the gate voltage. Because of this high input impedance and gain, MOSFETs can be easily damaged by static electricity if not carefully protected or handled. MOSFET’s are ideal for use as electronic switches or as common-source amplifiers as their power consumption is very small. Typical applications for metal oxide semiconductor field effect transistors are in Microprocessors, Memories, Calculators and Logic CMOS Gates etc. Also, notice that a dotted or broken line within the symbol indicates a normally “OFF” enhancement type showing that “NO” current can flow through the channel when zero gate-source voltage VGS is applied. A continuous unbroken line within the symbol indicates a normally “ON” Depletion type showing that current “CAN” flow through the channel with zero gate voltage. For P-channel types the symbols are exactly the same for both types except that the arrow points outwards. This can be summarised in the following switching table.
VGS = +ve
VGS = 0
VGS = -ve
So for N-channel enhancement type MOSFETs, a positive gate voltage turns “ON” the transistor and with zero gate voltage, the transistor will be “OFF”. For a P-channel enhancement type MOSFET, a negative gate voltage will turn “ON” the transistor and with zero gate voltage, the transistor will be “OFF”. The voltage point at which the MOSFET starts to pass current through the channel is determined by the threshold voltage VTH of the device and is typical around 0.5V to 0.7V for an N-channel device and -0.5V to -0.8V for a P-channel device. In the next tutorial about Field Effect Transistors instead of using the transistor as an amplifying device, we will look at the operation of the transistor in its saturation and cut-off regions when used as a solidstate switch. Field effect transistor switches are used in many applications to switch a DC current “ON” or “OFF” such as LED’s which require only a few milliamps at low DC voltages, or motors which require higher currents at higher voltages.
Depletion Mode MOSFET Fig 5.1 Depletion Mode N Channel MOSFET
The depletion mode MOSFET shown as a N channel device (P channel is also available) in Fig 5.1 is more usually made as a discrete component, i.e. a single transistor rather than IC form. In this device a thin layer of N type silicon is deposited just below the gate−insulating layer, and forms a conducting channel between source and drain. Therefore when the gate source voltage VGS is zero, current (in the form of free electrons) can flow between source and drain. Note that the gate is totally insulated from the channel by the layer of silicon dioxide. Now that a conducting channel is present the gate does not need to cover the full width between source and drain. Because the gate is totally insulated from the rest of the transistor this device, like other IGFETs, has a very high input resistance.
Fig. 5.2 Operation of a Depletion Mode MOSFET
In the N channel device, shown in Fig. 5.2 the gate is made negative with respect to the source, which has the effect of creating a depletion area, free from charge carriers, beneath the gate. This restricts the depth of the conducting channel, so increasing channel resistance and reducing current flow through the device. Depletion mode MOSFETS are also available in which the gate extends the full width of the channel (from source to drain). In this case it is also possible to operate the transistor in enhancement mode. This is done by making the gate positive instead of negative. The positive voltage on the gate attracts more free electrons into the conducing channel, while at the same time repelling holes down into the P type substrate. The more positive the gate potential, the deeper, and lower resistance is the channel. Increasing positive bias therefore increases current flow. This useful depletion/enhancement version has the disadvantage that, as the gate area is increased, the gate capacitance is also larger than true depletion types. This can present difficulties at higher frequencies.
Enhancement Mode MOSFET The Insulated Gate FET (IGFET). The Metal Oxide Silicon FET (MOSFET) or Metal Oxide Silicon Transistor (M.O.S.T.) has an even higher input 12 15 resistance (typically 10 to 10 ohms) than that of the JFET. In the MOSFET device the gate is completely insulated from the rest of the transistor by a very thin layer of metal oxide (Silicon dioxide SiO 2). Hence the general name applied to any device of this type, is the IGFET or Insulated Gate FET.
Planar Technology. There are several ways in which an insulated gate transistor may be constructed. All the methods used however, make use of planar technology in which the various parts of the device are laid down as planes or layers on the upper surface of a "SUBSTRATE" in a similar way to that shown on the Planar Transistors page in the BJT section. The layers are laid down one by one, by diffusing various semiconductor materials with suitable doping levels, as well as layers of insulation into the surface of the device, under carefully controlled conditions at high temperatures. Parts of a layer may be removed by etching, using photographic masks to make the required pattern of the electrodes etc. before the next layer is added. The insulating layers are made by laying down very thin layers of silicon dioxide and conductors are created by evaporating a metal, such as aluminium on to the surface. The transistors produced in this way have a much higher quality than is possible using other methods, and many transistors can be produced at one time on a single slice of silicon, before the silicon slice is cut up into individual transistors or integrated circuits.
Fig. 4.1 Construction of a N Channel Enhancement Mode MOSFET
The basic construction of a MOSFET is shown in Fig. 4.1. A body or substrate of P type silicon is used, then two heavily doped N type regions are diffused into the upper surface, to form a pair of closely spaced strips. −4 A very thin (about 10 mm) layer of silicon dioxide is then evaporated onto the top surface forming an insulating layer. Parts of this layer are then etched away above the N type regions using a photographic mask to leave these regions uncovered. On top of the insulating layer, between the two N type regions, a layer of aluminium is deposited. This acts as the GATE electrode. Metal contacts are also deposited on the N type regions, which act as the SOURCE and DRAIN connectors
Fig 4.2 Enhancement Mode Operation.
The gate has a voltage applied to it that makes it positive with respect to the source. This causes holes in the P type layer close to the silicon dioxide layer beneath the gate to be repelled down into the P type substrate, and at the same time this positive potential on the gate attracts free electrons from the surrounding substrate material. These free electrons form a thin layer of charge carriers beneath the gate electrode (they can't reach the gate because of the insulating silicon dioxide layer) bridging the gap between the heavily doped source and drain areas. This layer is sometimes called an "inversion layer" because applying the gate voltage has caused the P type material immediately under the gate to firstly become "intrinsic" (with hardly any charge carriers) and then an N type layer within the P type substrate. Any further increase in the gate voltage attracts more charge carriers into the inversion layer, so reducing its resistance, and increasing current flow between source and drain. Reducing the gate source voltage reduces current flow. When the power is switched off, the area beneath the gate reverts to P type once more. As well as the type described above, devices having N type substrates and P type (inversion layer) channels are also available. Operation is identical, but of course the polarity of the gate voltage is reversed. This method of operation is called "ENHANCEMENT MODE" as the application of gate source voltage makes a conducting channel "grow", therefore it enhances the channel. Other devices are available in which the application of a bias voltage reduces or "depletes" the conducting channel. These are described on the Depletion Mode MOSFET page.