Ever wondered which makes the best – when it comes to build and amplifier ? A Mosfet or a Bipolar transistor ? Lets go through this article which compares both pros and cons of this dilemma. Most people in general never care about this as long as they get a satisfactory output performance. Lets first go through some Mosfet amplifiers – thats a list of mosfet amplifier circuits for many output variations, say 10 watts, 18 watts and 50 watts. Okay! Now lets go through another list of  Transistor amplifiers - which lists many amplifier circuits built using bipolar transistors. Now that should give you a rough idea and freshen up the concepts. Lets come to the point then, we are talking about a comparison between Mosfet and Bipolar designs in amplifiers. Well as per many read more

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The drain characteristics of enhancement type MOSFET is given below.This depicts the variation of drain current(ID)with drain to source voltage(VDS)for different values of gate to source voltage(VGS). The lower most curve is for VGS(Th).When VGS < VGS(Th)drain current is almost zero.When VGS >VGS(Th) the device is ON.As in the case of other FET,the device can operate in the ohmic,active or cutoff(break down)region.The rising part of curve (fromVDS=0 to VDS=few volts)is the ohmic region.The device behaves as a resistor,when operated in this region.The drain current is almost constant when the device operates in the active region.when VDS exceeds the rated value,avalanche breakdown occurs and the device is in the breakdown region. Transconductance curve of enhancement MOSFET is read more

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This type of power MOSFET is widely used in digital computers. The construction of enhancement type power MOSFET is shown in figure above .This is an n-channel device.However as seen in the figure,the p-substrate extends right up to the silicon dioxide layer.Thus there is no n-channel between drain and source. The above figure shows the connections.When the gate voltage is zero,drain battery tries to push free electrons from the source to the drain.However the p-substrate has only a few thermally produced free electrons and some electrons due to surface leakage.Therefore the drain current is almost zero.When gate is positive(VGS>0)the gate attracts free electrons into the p-substrate region these free electrons recombine with holes near silicon dioxide layer.When VGS is large read more

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Drain characteristics of an n-channel MOSFET is shown in figure below. VGS can be positive and negative.The characteristics for p-channel are similar to the above figure excepts signs of current and voltage are reversed. The transconductance curve(transfer characteristics)of depletion type MOSFET is shown in figure below. This characteristcs shows the variation of ID with VGS.IDSS denoteS the drain current with shorted gate.The curve extends on both sides ie VGS can be negative as well as positive.Since VGS can be positive also IDSS is not maximum value of drain current. This device has three regions.The ohmic region,active region and breakdown region.The rising position of the drain characteristics is the ohmic region.The device acts as resistor.The drain current is nearly constant read more

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i) Negative Gate Operation In the figure given below a negative bias is applied to the gate The negative voltage depletes the conducting channel of majority carriers electrons and controls their flow. The gate and channel forms a parallel plate capacitor with silicon dioxide layer as dielectric.The negative gate bias causes concentration of electrons on the gate.These electrons repel the conduction band electrons in the n-channel leaving a positive ions layer as shown.More negative gate voltage,the greater is the depletion of electrons in n-channel. ii) Positive Gate Operation A depletion mode MOSFET when gate bias is positive is shown in figure below. The gate and channel can again be thought of as a capacitor positive charge on gate induce negative charge in n-channel.These negative read more

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Power MOSFET is a voltage controlled device and it requires only small input current.It has extremely high input impedance and is widely used in switching devices.The switching is high and the switching time is in the order of nanoseconds.They are used in low power high frequency converters.MOSFETs are of two types. i) Depletion Type MOSFET ii) Enhancement type MOSFET i) Depletion Type MOSFET A depletion type n-channel MOSFET has a lightly doped p-type substrate into which two highly doped n-region(n+) are diffused.These two regions acts as drain and source.A thin insulating silicon dioxide layer is grown across the semiconductor surface. The two holes cut into the oxide layer allow contact with the source and drain.A metal layer is then deposited on the oxide.This layer forms the read more

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