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Hi, DG FET should stand for Dual Gate fet used mostly in radio-frequency amplifier where one gate (G1)is the signal gate and the other (G2) is the control gate making an easy matter to control the dynamic range of the device varying the bias on gate2; are used also on mixer, where one gate is used to connect to an incoming signal of a certain frequency and the other to a local oscillator: on the drain there will be the difference of the two signals making it (lower freq than the incoming one) simple to manipolate.
The gates are insulated from the rest of the device (drain & source) by a layer of very thin dielectric material then the resistance between G1&G2 and drain&source is extremely high cause the thin dielectric; typical vales of Rin are over million megOhms so the DG FET are susceptible to gate insulation damage by the electrostatic discharge of energy through the devices;
"Electrostatic discharge can occur in a Insulated-Gate Metal Oxide Semiconductor Field-Effect Transistor if a type with an unprotected gate is picked up and the static charge, built in the handler's body capacitance, is discarged through the device."
Is said 'unprotected gate' because at the beginning where so; today the gates are protected internally with diodes connected to source, adding a little to the total capacitance but handling and in-circuits transients are improved a lot.