1

Other Transistor Topologies30 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

JFETJunction Field Effect Transistor

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

3

4

5

I-V relationship of PN JFET

Nonsaturation region (linear region)

Saturation region

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

Comparisons of Transistors

BJT MOSFET JFET

StructureNPN: n++p+nPNP: p++n+p

n(p)-type inversion layer structure as channelfrom 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

UnipolarNMOS: electronsPMOS: holes

Unipolar:N-channel: electronsP-channel: holes

Current at terminals

IC=βIB= (1/α) IE

(Forward active mode)

IC=f(VBE, VBC)

IG=0

ID=f(VGS, VDS)IG≈0

ID=f(VGS, VDS)

Comparisons of Transistors (Cont’d)

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

NPN PNP NMOS PMOS N-channel P-channel

8

9

10

11

12