1. Microelectronics, BSc course
Field effect transistors 1: The JFET

2. The abstraction level of our study:
SYSTEM +
MODULE
GATE
Vout
Vin
CIRCUIT
DEVICE n+ S D G
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

3. Most important parameter:
The JFET
FET = Field Effect Transistor – the flow of charge carriers is influenced by electric field
transversal field is used to control
Flow
Channel
depletion layer
JUNCTION FET: depletion layers of pn-junctions close the channel
Most important parameter:
U0 pinch-off voltage
Unipolar device: current is conducted by majority carriers
Power needed for controlling the device  0
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

4. The JFET – possible realization:
n type epi-layer
The width of the closed PN junction controls the conductivity of the channel
PN junction  junction FET
channel
depletion layer
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

5. The JFET – a "normally on" device
Symbols:
Characteristics:
n channel
p channel
controlled resistor (see later the triode region of MOSFETs)
no saturation
saturation
pinch off voltage
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

6. Calculation of the pinch off voltage
U(x) W d
geom. width of channel S
width of depl. layer
dh(x) L x
Pinch off: d geom. width = 2 x width of depletion layer
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

7. Problem
Determine the pinch off voltage of a Si JFET, if the channel width is d = 4 m and the doping is Nd = 1015/cm3 !
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

8. The characteristic width of depletion layer geom. width of channel
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

9. The characteristic width of depletion layer geom. width of channel
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

10. The characteristic Channel conductivity 27-10-2015
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

11. The characteristic Current constant For the triode region! 27-10-2015
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

12. The characteristic For all regions! Only in saturation: 27-10-2015
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET

13. Small signal parameters, equivalent
Slope / transconductance In
Out
Output conductance
Voltage gain
Microelectronics BSc course, The JFETs © András Poppe & Vladimír Székely, BME-EET