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40 Series Photo-MOSFET Relay Photo MOSFET Relay Reference Data 0.0 0.2 0.4 0.6 0.8 1.0-40 -15 1035 60 85 (ms) Ambient temperature (℃) Turn on time Vs. Ambient temperature I F =10mA, I L =45mA 0 10 20 30 40 50-20 0 20 40 60 80 100 Ambient temperature (℃) Load Current Vs. 8 5 50 100 150 200 250 300-40 -15 10 60 85 e (Ω) Ambient temperature. IGBT and MOSFET Driver, Optocouplers/Isolators manufactured by Vishay, a global leader for semiconductors and passive electronic components. 45 Series Photo-MOSFET Relay Photo-MOSFET Relay Dimensions (Tolerances acc. To ISO 2768-mp) SMD4 SMD6 SMD8 Photo-MOSFET Relay Dimensions (Tolerances acc. To ISO 2768-mp) SOP4 SOP8 Photo MOSFET Relay Pin-Out (Top View) 4 PIN 6 PIN 8 PIN.
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FETs have a few disadvantages like high drain resistance, moderate input impedance and slower operation. To overcome these disadvantages, the MOSFET which is an advanced FET is invented.
MOSFET stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. This is also called as IGFET meaning Insulated Gate Field Effect Transistor. The FET is operated in both depletion and enhancement modes of operation. The following figure shows how a practical MOSFET looks like.
Construction of a MOSFET
The construction of a MOSFET is a bit similar to the FET. An oxide layer is deposited on the substrate to which the gate terminal is connected. This oxide layer acts as an insulator (sio2 insulates from the substrate), and hence the MOSFET has another name as IGFET. In the construction of MOSFET, a lightly doped substrate, is diffused with a heavily doped region. Depending upon the substrate used, they are called as P-type and N-type MOSFETs.
The following figure shows the construction of a MOSFET.
The voltage at gate controls the operation of the MOSFET. In this case, both positive and negative voltages can be applied on the gate as it is insulated from the channel. With negative gate bias voltage, it acts as depletion MOSFET while with positive gate bias voltage it acts as an Enhancement MOSFET.
Classification of MOSFETs
Depending upon the type of materials used in the construction, and the type of operation, the MOSFETs are classified as in the following figure.
After the classification, let us go through the symbols of MOSFET.
The N-channel MOSFETs are simply called as NMOS. The symbols for N-channel MOSFET are as given below.
The P-channel MOSFETs are simply called as PMOS. The symbols for P-channel MOSFET are as given below.
Now, let us go through the constructional details of an N-channel MOSFET. Usually an NChannel MOSFET is considered for explanation as this one is mostly used. Also, there is no need to mention that the study of one type explains the other too.
Construction of N- Channel MOSFET
Let us consider an N-channel MOSFET to understand its working. A lightly doped P-type substrate is taken into which two heavily doped N-type regions are diffused, which act as source and drain. Between these two N+ regions, there occurs diffusion to form an Nchannel, connecting drain and source.
A thin layer of Silicon dioxide (SiO2) is grown over the entire surface and holes are made to draw ohmic contacts for drain and source terminals. A conducting layer of aluminum is laid over the entire channel, upon this SiO2 layer from source to drain which constitutes the gate. The SiO2 substrate is connected to the common or ground terminals.
Because of its construction, the MOSFET has a very less chip area than BJT, which is 5% of the occupancy when compared to bipolar junction transistor. This device can be operated in modes. They are depletion and enhancement modes. Let us try to get into the details.
Working of N - Channel (depletion mode) MOSFET
For now, we have an idea that there is no PN junction present between gate and channel in this, unlike a FET. We can also observe that, the diffused channel N (between two N+ regions), the insulating dielectric SiO2 and the aluminum metal layer of the gate together form a parallel plate capacitor.
If the NMOS has to be worked in depletion mode, the gate terminal should be at negative potential while drain is at positive potential, as shown in the following figure.
When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. Let some negative voltage is applied at VGG. Then the minority carriers i.e. holes, get attracted and settle near SiO2 layer. But the majority carriers, i.e., electrons get repelled.
With some amount of negative potential at VGG a certain amount of drain current ID flows through source to drain. When this negative potential is further increased, the electrons get depleted and the current ID decreases. Hence the more negative the applied VGG, the lesser the value of drain current ID will be.
The channel nearer to drain gets more depleted than at source (like in FET) and the current flow decreases due to this effect. Hence it is called as depletion mode MOSFET.
Photo Transistor Mosfet
Working of N-Channel MOSFET (Enhancement Mode)
The same MOSFET can be worked in enhancement mode, if we can change the polarities of the voltage VGG. So, let us consider the MOSFET with gate source voltage VGG being positive as shown in the following figure.
When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. Let some positive voltage is applied at VGG. Then the minority carriers i.e. holes, get repelled and the majority carriers i.e. electrons gets attracted towards the SiO2 layer.
With some amount of positive potential at VGG a certain amount of drain current ID flows through source to drain. When this positive potential is further increased, the current ID increases due to the flow of electrons from source and these are pushed further due to the voltage applied at VGG. Hence the more positive the applied VGG, the more the value of drain current ID will be. The current flow gets enhanced due to the increase in electron flow better than in depletion mode. Hence this mode is termed as Enhanced Mode MOSFET.
Photo Mosfet Relais
P - Channel MOSFET
The construction and working of a PMOS is same as NMOS. A lightly doped n-substrate is taken into which two heavily doped P+ regions are diffused. These two P+ regions act as source and drain. A thin layer of SiO2 is grown over the surface. Holes are cut through this layer to make contacts with P+ regions, as shown in the following figure.
Working of PMOS
When the gate terminal is given a negative potential at VGG than the drain source voltage VDD, then due to the P+ regions present, the hole current is increased through the diffused P channel and the PMOS works in Enhancement Mode.
Photovoltaic Mosfet Driver
When the gate terminal is given a positive potential at VGG than the drain source voltage VDD, then due to the repulsion, the depletion occurs due to which the flow of current reduces. Thus PMOS works in Depletion Mode. Though the construction differs, the working is similar in both the type of MOSFETs. Hence with the change in voltage polarity both of the types can be used in both the modes.
This can be better understood by having an idea on the drain characteristics curve.
The drain characteristics of a MOSFET are drawn between the drain current ID and the drain source voltage VDS. The characteristic curve is as shown below for different values of inputs.
Photo Mosfet Relay
Actually when VDS is increased, the drain current ID should increase, but due to the applied VGS, the drain current is controlled at certain level. Hence the gate current controls the output drain current.
Transfer characteristics define the change in the value of VDS with the change in ID and VGS in both depletion and enhancement modes. The below transfer characteristic curve is drawn for drain current versus gate to source voltage.
Comparison between BJT, FET and MOSFET
Now that we have discussed all the above three, let us try to compare some of their properties.
|Device type||Current controlled||Voltage controlled||Voltage Controlled|
|Operational modes||No modes||Depletion mode only||Both Enhancement and Depletion modes|
|Input impedance||Low||High||Very high|
So far, we have discussed various electronic components and their types along with their construction and working. All of these components have various uses in the electronics field. To have a practical knowledge on how these components are used in practical circuits, please refer to the ELECTRONIC CIRCUITS tutorial.
A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a semiconductor device. A MOSFET is most commonly used in the field of power electronics. A semiconductor is made of manufactured material that acts neither like a insulator nor a conductor. An insulator is a natural material that will not conduct electricity, such as a dry piece of wood. A conductor is a natural material that conducts or passes electricity. Metals are the most common examples of conductors. Semiconductor material from which devices like a MOSFET are made exhibit both insulation like properties and conduction like properties. Most importantly, semiconductors are designed such that the conduction or insulation properties can be controlled.
The transistor is perhaps the best known semiconductor device. Early transistors use a technology referred to as bi-polar material. Pure silicon can be doctored or 'corrupted'--a process that is referred to as 'doping'. It is possible to make either p type (positive) material or n type (negative) material depending upon material used to 'dope' or corrupt the pure silicon. If you combine p type material and n type material, you have a bipolar device. The transistor is a basic example of a bipolar device. The transistor has three terminals, the collector, the emitter, and the base. The current in the base terminal is used to control the flow of current between the emitter and the collector.
Photo Mosfet Relay
MOSFET technology is an enhancement on bipolar technology. Both n and p type material are still used but metal oxide insulators are added to provide some performance enhancements. There are still typically only three terminals but they now have the following names, the source, the drain, and the gate. The field effect portion of the name refers to the method used to control the electron or current flow through the device. The current is proportional to the electrical field developed between the gate and the drain.
Photo Mosfet Driver
One other very significant enhancement over bipolar technology is that a MOSFET has a positive temperature co-efficient. This means that as the temperature of the device increases its tendency to conduct current decreases. This feature allows the designer to easily use it in parallel to increase the system's capacity. A bipolar deice has the opposite effect. With MOSFET technology, devices in parallel will naturally share current between them. If one device tries to conduct more than its share it will heat up and the tendency to conduct current will decrease causing the current through the device to decrease until all devices are again sharing evenly. Bipolar devices in parallel, on the other hand, increase in temperature if one device starts to conduct more current. This means more current will switch to this device which will result in a further increase in temperature, and a further increase in current. This is a runaway condition that quickly destroys the device. For this reason it is much more difficult to connect bipolar devices in parallel and the reason MOSFET devices are now the most popular power semiconductor type transistor.