What is MESFET?
MESFET is the acronym of Metal-Semiconductor Field Effect Transistor. It is similar to JFET but the difference is instead of p-n junction in JFET metal semiconductor Schottky junction is used. The conduction is controlled by the gate terminal and the depletion region below.
Symbol of MESFET :
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| Symbol of MESFET |
In the Depletion mode MESFET is normally “ON” and in the enhancement mode MESFET is normally “OFF”. In the depletion mode the size of the depletion layer increases and thus in the symbol gate terminal in the depletion mode is thicker. The arrow points outside in the P channel and arrow points inside in the N channel.
Structure of MESFET :
In the process of MESFET fabrication in addition to the silicon, materials like GaAs and InP are used. The process is same like fabricating JFET above semi insulating material GaAs. The channel or the conducting layer is very thin and lightly doped semiconductor layer. Since the mobility of the electrons are greater than the holes the conducting layer is made up of n type semiconductor.
A) Self aligned source and drain:
Here the length of the Gate is equal to the length of the channel which runs from source to drain. Gate is formed first then the source and drain are formed later. Gate material should be able to withstand the heat formed during the ion implantation of source and drain. So only some limited materials are used for gate terminal.
Source and drain terminals are taken from highly doped n type semiconductor layers. The source and the drain form the Ohmic contact and in between that two terminals Schottky contact is fabricated.
B) Non self aligned source and drain:
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| Non self aligned structure of MESFET |
Here in this type source and drain are formed first then the gate is formed. So gate is placed only on a section of the channel and it does not cover the whole channel.
Working of MESFET :
1) Enhancement mode:
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| Enhancement mode of MESFET |
In the enhancement mode normally MESFET is OFF and positive voltage is applied between the gate and source terminals. The size of the depletion layer is small so the charge carriers move from source to drain and thus current is formed. When positive voltage is applied Schottky junction is in forward biased condition and large current flows.
2) Depletion mode:
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| Depletion mode of MESFET |
In the depletion mode negative gate source voltage is applied, so the electrons get repelled and the width of the depletion region increases and thus it blocks the flow of charge carriers from source to drain.
V-I Characteristics of MESFET
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| V-I Characteristics of MESFET |
When the source drain voltage is applied the drain current increases linearly and when the applied voltage is increases further it reaches saturation region where it is almost constant. When the negative gate voltage is increased further the width of the depletion region increases and reaches pinch- off condition where the flow of current is totally blocked and very small current or no current flows. When the gate voltage is zero the current flow is maximum.
Advantages of MESFET:
- Used in microwave circuits and operates at high frequency.
- The mobility of electrons in GaAs is much higher than in silicon and the channel length is shorter in MESFET. So they operate at higher frequencies of about 1 to 10 gigahertz.
- High input impedance
- Lack of oxide traps since no oxide layer
- High level of geometry control
Disadvantages of MESFET:
- The turn-ON voltage for GaAs Schottky diode is 0.7V, so the threshold voltage should be lower than this turn-ON voltage. It is difficult to manufacture circuits in this range.
Applications of MESFET:
- Used in satellite communication
- Used in Radar and microwave circuits
- Used in RF amplifier devices
- Used in cellphones
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