1. 1 Other Transistor Topologies 30 March and 1 April 2015 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 JFET Junction Field Effect Transistor

2. 2 Structure of 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. N-channel JFET

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5. 5 I-V relationship of PN JFET Nonsaturation region (linear region) Saturation region

6. 6 Other Features of PN JFET Symbols of JFET (arrow represents the polarity of the PN junction) 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

7. 7 Comparisons of Transistors BJTMOSFETJFET Structure NPN: n ++ p + n PNP: p ++ n + p n(p)-type inversion layer structure as channel from S to D Inversed biased PN junction between the gate and channel from S to D Current transport Diffusion Drift Carriers involved in current transport Bipolar: electrons and holes Unipolar NMOS: electrons PMOS: holes Unipolar: N-channel: electrons P-channel: holes Current at terminals I C =βI B = (1/α) I E (Forward active mode) I C =f(V BE, V BC ) I G =0 I D =f(V GS, V DS ) I G ≈0 I D =f(V GS, V DS )

8. Comparisons of Transistors (Cont’d) BJTMOSFETJFET Symbols Applications Current-controlled current amplifier Switch for digital signal Discrete circuits Voltage-controlled current amplifier Switch for digital signal IC circuits Voltage-controlled current amplifier Switch for digital signal IC circuits MESFET can be used in higher- frequency than PN-JFET due to higher electron mobility in GaAs NPN PNP NMOSPMOS N-channel P-channel 8

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