Field Effect Transistors (FETs)场效应晶体管（FET）.pptVIP

Field Effect Transistors (FETs) In a field effect transistor, current flow through a semiconductor channel is controlled by the application of an electric field (voltage) perpendicular to the direction of current flow. We consider the MOSFET (MOST)- the metal oxide semiconductor (field effect) transistor. Field Effect Transistors (FETs) In a depletion mode MOSFET a channel is built in so that conduction occurs with no control voltage applied. We consider an n-channel enhancement mode device which has no ‘built-in’ conduction channel. N-channel enhancement mode MOSFET (schematic) Symbol for N-channel enhancement mode MOSFET Note connection to substrate shown Symbol for N-channel enhancement mode MOSFET Note connection to substrate shown N-channel enhancement mode MOSFET: outline of operation VGS =0, little or no current can flow S ? D, back to back p-n junctions. As a positive VGS is applied , holes in the p-region are repelled. As VGS increases further, electrons are attracted from the substrate towards the positive gate. An inversion layer of mobile electrons forms near the silicon surface. N-channel enhancement mode MOSFET with inversion layer N-channel enhancement mode MOSFET: outline of operation This inversion layer is effectively n-type. Electrons can carry current in an continuous n-type path (or channel) from source to drain. The conduction channel forms when VGS attains a threshold voltage, VT N-channel enhancement mode MOSFET The value of VT is determined by the device process and the level of the p-type (substrate) doping. Note that an n-channel device is formed with a p-type substrate. N-channel enhancement mode MOSFET As VGS increases further, more electrons are drawn into the inversion layer and the channel resistance decreases (The device operates as a voltage controlled resistor). However…… N-channel enhancement mode MOSFET The voltage between gate and channel varies from VGS at the source end to VGS -VDS at the drain end. Thus as the voltage at t