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Other Transistor Topologies 30 March and 1 April 2015 JFET Junction Field Effect Transistor • The two gate terminals are tied together to form single gate connection; the source terminal is grounded • The flow of electric charge through a PN JFET is controlled by constricting the current-carrying channel; the width of the channel is controlled by the gate voltage through varying the depletion region at the PN junction at the interface between the gate and the channel • The current also depends on the electric field between source and drain 1 Structure of JFET N-channel JFET • a long channel of n-type (N-channel) or p-type (p-channel) semiconductor. Two ohmic contacts with each at one end of the channel: the source and the drain • The gate (control) terminal has doping opposite to that of the channel, so there is a PN junction at the interface between the junction and the channel. The contact from gate to outside is also ohmic. 2 3 4 I-V relationship of PN JFET Nonsaturation region (linear region) Saturation region 5 Other Features of PN JFET • JFET is unipolar device since only majority carriers transport in the channel • The source and drain region are interchangeable • N-channel devices have greater conductivity than p-channel types, since electrons have higher mobility than holes • The gate current is approximately zero since the PN junction is reverse biased Symbols of JFET (arrow represents the polarity of the PN junction) 6 Comparisons of Transistors BJT Structure NPN: n++p+n PNP: p++n+p Current transport Diffusion Carriers involved in current transport Bipolar: electrons and holes Current at terminals IC=βIB= (1/α) IE (Forward active mode) IC=f(VBE, VBC) MOSFET n(p)-type inversion layer structure as channel from S to D Drift Unipolar NMOS: electrons PMOS: holes IG=0 ID=f(VGS, VDS) JFET Inversed biased PN junction between the gate and channel from S to D Drift Unipolar: N-channel: electrons P-channel: holes IG≈0 ID=f(VGS, VDS) 7 Comparisons of Transistors (Cont’d) BJT MOSFET JFET Symbols NPN Applications PNP • Current-controlled current amplifier • Switch for digital signal • Discrete circuits NMOS PMOS • Voltage-controlled current amplifier • Switch for digital signal • IC circuits N-channel P-channel • Voltage-controlled current amplifier • Switch for digital signal • IC circuits • MESFET can be used in higherfrequency than PN-JFET due to higher electron mobility in GaAs 8 9 10 11 12