Other Transistor Topologies

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Other Transistor Topologies 23 and 28 March 2016
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

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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.

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I-V relationship of PN JFET
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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)
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Comparisons of Transistors
BJT MOSFET JFET
Structure NPN npn PNP pnp 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 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 ICßIB (1/a) IE (Forward active mode) ICf(VBE, VBC) IG0 IDf(VGS, VDS) IG0 IDf(VGS, VDS)
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Comparisons of Transistors (Contd)
BJT MOSFET JFET
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
N-channel
P-channel
NPN
PNP
NMOS
PMOS
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Esaki (Tunnel Diode) is obsolete but illustrates
an early application of quantum effects.
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Gunn (Transferred Electron Device) Essentially
Obsolete Better Approaches to Microwave Signal
Generation
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L
IMPATT (IMPact Ionization Avalanche Transit Time
Essentially Obsolete Better Approaches to
Microwave Signal Generation (fv/2L)
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