Dual Gate MOSFET, Construction, Working, V-I Characteristics, Advantages, Challenges and Applications

 What is Dual gate MOSFET?


Dual Gate MOSFET
Dual Gate MOSFET

In dual gate MOSFET as the name says two gate terminals are used. The second gate terminal  is used to control and reduce the feedback capacitance between input and output and thus the amplifiers become more stable. It is mostly used in high frequency RF amplifier applications. The disadvantage of normal FET is high capacitance so that it cannot be used at higher frequencies.

Symbol of Dual gate MOSFET:


Symbol of Dual Gate MOSFET
Symbol of Dual Gate MOSFET

In the above diagram the symbol of depletion and enhancement dual gate MOSFET is given. N channel and P channel in each mode is given. But P channel is not used mostly because the mobility of holes is very low than the mobility of electrons. 

Construction of Dual gate MOSFET:

Construction of Dual Gate MOSFET
Construction of Dual Gate MOSFET


Double Gate MOSFET is manufactured using Silicon On Insulator method. Silicon channel is grown over silicon oxide layer. It is like two FETs connected in series. The highly doped N middle block acts as the drain for the first FET and source for the second FET. The gate oxide between gate and the channel is the dielectric layer and the metal used for gate is conductor and the channel is also conductor, so all these three layers together form the capacitor. So it is like two capacitors connected in series. The total capacitance connected in series is lesser than the individual capacitance. Two gates are used to reduce the feedback capacitance at high frequencies. The voltages at both the gates controls the flow of current in the channel.

Working of Dual gate MOSFET:


Working of Dual Gate MOSFET
Working of Dual Gate MOSFET

In the above diagram there are two gates and two FETs. They are top gate, bottom gate and top FET and bottom FET. In the top gate voltage is applied and kept constant and the bottom FET is analyzed. It is found that when the applied voltage is above 3V that is the threshold voltage to the top gate, it remains in the saturation region.

V-I Characteristics of Dual gate MOSFET:

V-I Characteristics of Dual Gate MOSFET
V-I Characteristics of Dual Gate MOSFET

The voltage applied at the first gate is kept at 3V and kept as constant. The applied voltage at the second gate is varied and and the drain and source voltage is varied and the drain current is observed. The threshold voltage is 3v. the drain current increases and remains in the saturation region where the current is almost constant.

Advantages of Dual gate MOSFET:

1) Higher drive currents at lower supply voltage and threshold voltage.

2) Reduced channel and gate leakage current at off state which saves power.

3) Separate gate control on voltage saves power and chip area.

Challenges in Dual gate MOSFET:

1) Identically sized gates

2) Self alignment of source and drain to both gates

3) Alignment of both gates to each other

4) Connecting two gates with low resistance path

Application of Dual gate MOSFET:

1) Mixers

2) Demodulators

3) Cascade amplifiers

4) RF amplifier

5) AGC amplifier

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