Other Transistor Topologies 18 and 20 March 2020 Discuss Rest of Semester 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 1 • The current also depends on the electric field between source and drain 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 MOSFET JFET n(p)-type inversion layer NPN: n++p+n Inversed biased PN junction Structure structure as channel between the gate and channel PNP: p++n+p from S to D from S to D Current Drift Drift transport Diffusion Carriers Unipolar Unipolar: involved in Bipolar: electrons NMOS: electrons N-channel: electrons current and holes transport PMOS: holes P-channel: holes IC=βIB= (1/α) IE Current at (Forward active IG=0 IG≈0 terminals mode) ID=f(VGS, VDS) ID=f(VGS, VDS) IC=f(VBE, VBC) 7 Comparisons of Transistors (Cont’d) BJT MOSFET JFET Symbols N-channel NPN PNP NMOS PMOS P-channel • Voltage-controlled current amplifier • Current-controlled • Voltage-controlled • Switch for digital signal current amplifier current amplifier • IC circuits Applications • Switch for digital • Switch for digital • MESFET can be used in higher- signal signal frequency than PN-JFET due to higher • Discrete circuits • IC circuits electron mobility in GaAs 8 9 10 11 12 Esaki (Tunnel Diode) is obsolete but illustrates an early application of quantum effects. Gunn (Transferred Electron Device) Essentially Obsolete Better Approaches to Microwave Signal Generation L IMPATT (IMPact Ionization Avalanche Transit Time Essentially Obsolete Better Approaches to Microwave Signal Generation (f=v/2L) .