Bel 06 field effect transistor

FEATURES of FETs

 Field Effect Transistors ( FET ) are three terminal devices and
its current controlled mechanism is based on an electric field
established by the voltage applied to the controlled terminal.

 Just as there are npn and pnp BJTs, there are n-channel and
p-channel FETs

 The current is conducted by only one type of carrier
( electrons or holes ) depending on the type of FET ( n-
channel or p-channel ), which gives the FET another name,
the unipolar transistors.

 The primary difference between BJTs and FETs is the fact that
BJT is a current controlled device while FET is a voltage
controlled device.
S. Kal, IIT-Kharagpur

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FEATURES of FETs

 The most important characteristics of FET is its high input
impedance (meg Ohms) compared to that of BJT (K Ohms).

 FETs are more temperature stable than BJTs.

 FETs are usually smaller in construction than BJTs, making
them more useful in IC chips

 Two types of FETs : (1) Junction Field Effect transistor
( JFET ) and (2) Metal Oxide Semiconductor Field Effect
Transistors ( MOSFET )

 Three terminals of FETs are known as Source, Drain and
Gate

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JUNCTION FIELD EFFECT TRANSISTOR (JFET)
JFET is a three terminal device with one terminal capable of
controlling the current between the other two.
The major part of the structure
is the n-type material that forms
the channel between two p-type
layers which are connected to
form the Gate ( G ) terminal. The
two ends of the channel are
connected via Ohmic contact to
form Drain (D) and Source (S).
In the absence of any applied
potential, the two p-n junctions
form depletion region – void of
free carriers and unable to
support conduction through the
region.
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A positive voltage VDS is applied across the channel and
the gate has been connected directly to the source such
that VGS = 0 V.

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 As the voltage VDS is increased from 0 to a few volts, the
current will increase as determined by Ohm’s law ; as V DS
approaches VP, depletion region widens causing reduction in
channel width and channel resistance increases. The
resistance approaches infinite Ohms in the horizontal region.
The level of VDS at which two
depletion region touches, is
known as pinch-off voltage and
is denoted by VP. ID maintains a
saturation level defined as IDSS -
the maximum value of drain
current for a JFET.
Once VDS > VP, the JFET has
the characteristics of a current
source. S. Kal, IIT-Kharagpur
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N-Channel J-FET Characteristics
 The level of VGS that results in ID = 0 mA is defined by VGS = VP,
with VP being a negative voltage for n-channel devices and +ve
voltage for p-channel JFETs.

 When VGS< 0, depletion
regions will be
established similar to
those obtained at VGS = 0
but at a lower level of
VDS.

 Pinch-off voltage
continues to drop in a
parabolic manner as VGS
becomes more and more
negative. S. Kal, IIT-Kharagpur

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TRANSFER CHARACTERISTICS OF A JFET

The transfer curve [plot of output (or drain) current versus input
controlling quantity (gate-source voltage) ] has extended use in
JFET amplifiers.

The input and output relationship of a JFET is not linear. The
relationship between ID and VGS is defined by Schockley’s equation:

 The squared term of the equation will result in a nonlinear
relationship between ID and VGS, producing a curve that grows
exponentially with increasing values of VGS.
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TRANSFER CHARACTERISTICS OF A JFET
 The transfer curve can be obtained using Shockley’s
equation or from the output characteristics as shown
below.
 When VGS = 0, ID = IDSS, and VGS = VP, ID = 0


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Metal-Oxide-Semiconductor FET (MOSFET)
Since late 1970s, MOSFET has been extremely popular.
 Compared to BJT, MOS transistor can be made quiet small and
their manufacturing process is relatively simple.
 Analog, digital and combined analog-digital circuits (VLSI) are
designed efficiently using MOS transistors.

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Metal-Oxide-Semiconductor FET (MOSFET)
• Two classes of MOSFETs are available – Enhancement type
and Depletion type

 Heavily doped n-type regions, indicated as n+ source and n+
drain are created in the p-substrate. A thin (0.02 – 1 µm ) layer
of SiO2, which is an insulator covers the area between source
and drain. Metal is deposited on top of the oxide layer to form
gate electrode. Metal contact is also made to source, drain and
substrate, which is known as body. The gate electrode is
electrically insulated from the device body which causes the
current in the gate terminal to be extremely small ( ~ 10 -15 A )
 Current – voltage relation in the saturation region of NMOS is
given by
ID = ½ µ 0 Cox (W/L) (VGS – VT)2

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Enhancement MOSFET

(a) (b)
(a) Physical structure of the enhancement-type NMOS transistor
(b) iD-vDS characteristics of NMOSFET for VGS > Vt

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Enhancement MOSFET

(a) (b)
(a) Enhancement NMOS as VDS is increased. The induced channel
acquires a tapered shape and its resistance increases. V GS is
kept constant at a value > Vt
(b) ID – VDS (Drain) characteristics of a typical NMOS transistor
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N-Channel Depletion-Mode MOSFET

• Circuit symbol and physical structure of typical
depletion-type MOSFET

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Depletion Mode NMOS Transistor

Drain (ID – VDS) and Transfer (ID – VGS) characteristics of a typical
depletion-mode MOSFET

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Complementary MOSFET

Cross-section schematic diagram of a CMOS transistor (a
well also called a tub, is produced by an extra diffusion
step).

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Complementary MOS (CMOS)

 Complementary devices makes possible many powerful
circuit – design possibilities and is the most useful of all the
IC MOS technologies

 It is a combination of PMOS and NMOS transistor

 NMOS is implemented directly in the p-type substrate, PMOS
is fabricated in a specially created n region, called n well

 The two devices are isolated from each by a thick region of
oxide that function as an insulator. This oxide is known as
field oxide

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Bel 06 field effect transistor

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