J-FET (Junction Field Effect Transistor)

Introduction

The field-effect transistor (FET) controls the current between two points but it is different from bipolar transistor.  The FET operates by the effects of an electric field on the flow of electrons through a single type of semiconductor material.  This is why the FET is sometimes called a unipolar transistor.

J-FET (Junction Field Effect Transistor)

Introduction…cont

Current moves within the FET in a channel, from the source (S) connection to the drain (D) connection.  A gate (G) terminal generates an electric field that controls the current .

The channel is made of either N-type or P-type semiconductor material; an FET is specified as either an N-channel or P-channel device

Majority carriers flow from source to drain.

 In N-channel devices, electrons flow so the drain potential must be higher than that of the Source (VDS > O)- In P-channel devices, the flow of holes requires that VDS < 0

FET ( Field Effect Transistor)

1. Unipolar device i. e. operation depends on only one type of charge carriers (h or e)

2. Voltage controlled Device (gate voltage controls drain current)3. Very high input impedance (109-1012 )4. Source and drain are interchangeable in most Low-frequency

applications5. Low Voltage Low Current Operation is possible (Low-power

consumption)6. Less Noisy as Compared to BJT7. No minority carrier storage (Turn off is faster) 8. Self limiting device9. Very small in size, occupies very small space in ICs10. Low voltage low current operation is possible in MOSFETS 11. Zero temperature drift of out put is possible

Few important advantages of FET over conventional Transistors

Types of Field Effect Transistors (The Classification)

» JFET

MOSFET (IGFET)

n-Channel JFETp-Channel JFET

n-Channel EMOSFET

p-Channel EMOSFET

Enhancement MOSFET

Depletion MOSFET

n-Channel DMOSFET

p-Channel DMOSFET

FET

Figure: n-Channel JFET.

The Junction Field Effect Transistor (JFET)

Figure: n-Channel JFET and Biasing Circuit.

Biasing the JFET

Figure: The nonconductive depletion region becomes broader with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.)

Operation of JFET at Various Gate Bias Potentials

Figure: n-Channel FET for vGS = 0.

Simple Operation and Break down of n-Channel JFET

Figure: If vDG exceeds the breakdown voltage VB, drain current increases rapidly.

Break Down Region

N-Channel JFET Characteristics and Breakdown

Figure: Typical drain characteristics of an n-channel JFET.

VD-ID Characteristics of EMOS FET

Saturation or Pinch off Reg.

Locus of pts where PGSDS VVV

Figure: Transfer (or Mutual) Characteristics of n-Channel JFET

2

1

P

GSDSSDS V

VII

IDSS

VGS (off)=VP

Transfer (Mutual) Characteristics of n-Channel JFET

Figure: Circuit for drain characteristics of the n-channel JFET and its Drain characteristics.

Non-saturation (Ohmic) Region:

The drain current is given by

2

2 2

2DS

DSPGSP

DSSDS

VVVV

V

II

22 PGSP

DSSDS

VVV

II

2

1 and

P

GSDSSDS V

VII

Where, IDSS is the short circuit drain current, VP is the pinch off voltage

Output or Drain (VD-ID) Characteristics of n-JFET

Saturation (or Pinchoff) Region:

PGSDSVVV

PGSDSVVV

ELEC 121 18January 2004

MOSFETs

MOSFETs have characteristics similar to JFETs and additional characteristics that make then very useful

There are 2 types of MOSFET’s:• Depletion mode MOSFET (D-MOSFET)

• Operates in Depletion mode the same way as a JFET when VGS 0

• Operates in Enhancement mode like E-MOSFET when VGS > 0• Enhancement Mode MOSFET (E-MOSFET)

• Operates in Enhancement mode• IDSS = 0 until VGS > VT (threshold voltage)

• Importance for LSI/VLSI– Low fabrication cost– Small size– Low power consumption

• Applications– Microprocessors– Memories– Power Devices

• Basic Properties– Unipolar device– Very high input impedance– Capable of power gain– Two possible device types: enhancement mode;

depletion mode– Two possible channel types: n-channel; p-channel

Symbols

G

D

S

B G

D

S

B

p Channel MOSFET n Channel MOSFET

pn+n+

metal

sourceS

gateG drain

Dbody

B

oxide

+-

+++

++++++

VDS large

The saturation region is when the MOSFET experiences pinch-off.

Pinch-off occurs when VG - VD is less than VT.

Figure 5.46 n-Channel depletion MOSFET.

Depletion mode n-MOSFET

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Depletion Mode MOSFET Construction

The Drain (D) and Source (S) leads connect to the to n-doped regionsThese N-doped regions are connected via an n-channelThis 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