Abstract: Based on the D-S effect, it is feasibility to build a terahertz source with the structure of MOSFET. The MOSFET biased by a voltage source act as a resonant cavity, and a small signal of 1 mV in the channel oscillates. When the bias voltage is fixed to 5V, the frequency of plasma signal is 2.15 THz and the peak value of voltage is 2mV. Besides, the frequency can be tuned from 0.96 to 4.30THz with the increasing bias voltage (1～20V). When the MOSFET is biased by a 5V voltage source and a 5 A current source, the plasma signal is amplified in an exponential manner. Due to channel pinched off, the maximum voltage output is 20V, the voltage gain up to 86dB, and the maximum power output is 200W. In the allowable range, the greater the bias voltage is, the higher the frequency will be; a great bias current will shorten the time of starting oscillation. Besides, the bias current cause a small frequency offset.