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Junction Field Effect Transistors

The field effect transistor was actually conceived before the more familiar bipolar transistor. Due to limited technology and later the rapid rise of the bipolar device it was not pursued until the early 1960Õs as a viable semiconductor alternative. At this time further investigation of the field effect transistor and advances in semiconductor process technology lead to the types in use today. | H-2 01 99 Junction Field Effect Transistors InterFET Application Notes Introduction The field effect transistor was actually conceived before the more familiar bipolar transistor. Due to limited technology and later the rapid rise of the bipolar device it was not pursued until the early 1960 s as a viable semiconductor alternative. At this time further investigation of the field effect transistor and advances in semiconductor process technology lead to the types in use today. Field effect transistors include the Junction FET JFET and the MOSFET. The MOSFET is a metaloxide semiconductor technology and is sometimes referred to as the IGFET or Insulated Gate FET. All field effect transistors are majority carrier devices. This means that current is conducted by the majority carrier species present in the channel of the FET. This majority carrier consists of hole for p-channel devices and electrons for n-channel devices. The JFET operates with current flow through a controlled channel in the semiconductor material. The MOSFET creates a channel under the insulated gate region which is produced by an electric field induced in the semiconductor by applying a voltage to the gate. The JFET is a depletion mode device whereas the MOSFET can operate as a depletion mode or an enhancement mode device. Depletion mode devices are controlled by depleting the current channel of charge carriers. Enhancement mode devices are controlled by enhancing the channel with additional charge carriers. The JFET The junction field effect transistor in its simplest form is essentially a voltage controlled resistor. The resistive element is usually a bar of silicon. For an N-channel JFET this bar is an N-type material sandwiched between two layers of P-type material. The two layers of P-type material are electrically connected together and are called the gate. One end of the N-type bar is called the source and the other is called the drain. Current is injected into the channel from the source .