1. Field Effect Transistors

2. Introduction Two main types of FET:
- JFET –Junction field effects transistor
MOSFET – Metal oxide semiconductor field effect transistor
D-MOSFET ~ Depletion MOSFET
E-MOSFET ~ Enhancement MOSFET
Similarities:
-Amplifiers
-Switching devices
-Impedance matching circuits
Differences:
-FET’s are voltage controlled devices whereas BJT’s are current
controlled devices.
-FET’s also have a higher input impedance, but BJT’s have higher
gains.
-FET’s are less sensitive to temperature variations and more easily
integrated on IC’s.
- FET’s are also generally more static sensitive than BJT’s.

3. Introduction types of FET:
JFET –Junction field effects transistor
JFET is a unipolar-transistor, which acts as a voltage controlled current device and is a device in which current at two electrodes is controlled by the action of an electric field at a reversed biased p-n junction.

4. Construction and characteristics of JFET
N-channel device will appear as the prominent device with paragraph and section devoted to the impact of using a p-channel.
Major part of structure is n-type material.
Top of the n-type channel is connected through an ohmic contact to a terminal referred to as the drain (D)
The lower end-connected through an ohmic contact to a terminal referred as source (S)
P-type materials are connected together and to the gate (G) terminal.
JFET has two p-n junctions under no-bias conditions.

5. Construction and characteristics of JFET
N-Channel JFET Circuit Layout

6. JFET Operating Characteristics
There are three basic operating conditions for a JFET:
VGS = 0, VDS increasing to some positive
value
B. VGS < 0, VDS at some positive value
C. Voltage-Controlled Resistor

7. Voltage-Controlled Resistor
The region to the left of the pinch-off point is called the ohmic region.
The JFET can be used as a variable resistor, where VGS controls the drain-source resistance (rd). As VGS becomes more negative, the resistance (rd) increases.

8. p-Channel JFETS p-Channel JFET acts the same as the n-channel JFET,
except the polarities and currents are reversed.

9. P-Channel JFET Characteristics
As VGS increases more positively:
the depletion zone increases
ID decreases (ID < IDSS)
eventually ID = 0A
Also note that at high levels of
VDS the JFET reaches a
breakdown situation. ID
increases uncontrollably if
VDS> VDSmax.

10. JFET Symbols

11. MOSFETs
MOSFET Field effect transistor is a unipolar transistor, which acts as a voltage-controlled current device and is a device in which current at two electrodes drain and source is controlled by the action of an electric field at another electrode gate having in-between semiconductor and metal very a thin metal oxide layer

12. MOSFETs MOSFETs have characteristics similar to JFETs and additional
characteristics that make then
very useful.
There are 2 types:
Depletion-Type MOSFET
Enhancement-Type MOSFET

13. Depletion-Type MOSFET Construction
The Drain (D) and Source (S) connect to the to n-doped regions. These N-doped regions are connected via an n-channel. This n-channel is connected to the Gate (G) via a thin insulating layer of SiO2. The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS.

14. Depletion-type MOSFET in Depletion Mode
The characteristics are similar to the JFET.
When VGS = 0V, ID = IDSS
When VGS < 0V, ID < IDSS
The formula used to plot the Transfer Curve still applies:

15. Symbols

16. Enhancement-Type MOSFET Construction
The Drain (D) and Source (S) connect to the to n-doped regions.
These n-doped regions are connected via an n-channel. The
Gate (G) connects to the p-doped substrate via a thin insulating
layer of SiO2. There is no channel. The n-doped material lies
on a p-doped substrate that may have an additional terminal
connection called SS.

17. Enhancement-Type MOSFET Construction
VGS=0, VDS some value, the absence of an n-channel will result in a current of effectively 0A
VDS some positive voltage, VGS=0V, and terminal SS is directly connected to the source, there are in fact 2 reversed-biased p-n junction between the n-doped regions and p substrate to oppose any significant flow between drain and source.
VDS and VGS have been set at some positive voltage greater than 0V, establishing the D and G at a positive potential with respect to the source
The positive potential at the gate will pressure the holes in the p substrate along the edge of the SiO2 layer to leave the area and enter deeper regions of the p-substrate
The result is a depletion region near the SiO2 insulating layer void of holes

18. Symbols

20. VMOS VMOS – Vertical MOSFET increases the surface area of the device.
Advantage:
This allows the device to handle higher currents by providing it more surface area to dissipate the heat.
VMOSs also have faster switching times.

21. CMOS
CMOS – Complementary MOSFET p-channel and n-channel MOSFET on the same substrate.
Advantage:
Useful in logic circuit designs
Higher input impedance
Faster switching speeds
Lower operating power levels