Download Unit – iv

Unit – iv
TYPES OF FIELD EFFECT TRANSISTORS
Junction field effect transistor JFET
Metal Oxide semiconductor field effect transistor ( MOSFET )
Junction field effect transistor JFET
Is a three terminal semi conductor device in which current conduction is by one
type of carrier i.e electrons or holes. The output characteristics are controlled by input
voltage and not by input current.
CONSTRUCTION
A JFET consists of p type or N type silicon bar containing two PN junction at the
sides. the bar forms the conducting channel for the charge carriers.. If the bar is of n type
it is called n channel JFET. And if the bar is Ptype it is called p channel JFET. the two pn
junctions forming diodes are connected internally and common terminal called gate. is
taken out. other terminal are source and drain taken out from the bar.
WORKING PRINICIPLE
When a voltage Vds is applied between drain and source terminals and the
voltage on the gate is zero. the two pn junction at the side of the bar establish depletion
layers. The electrons will flow from source to drain through a channel between the
depletion layers. the size of these layers determined the width of the channel and hence
the current conduction through the bar.
When a reverse voltage VGS is applied between the Gate and source, width of the
depletion layer is increased this reduces width of conducting channel , their by increasing
the resistance of n type bar .the current from source to drain is decreased. if the reverse
voltage on the gate is decreased width of the depletion layer also decreases. this increases
the width of the conducting channel and hence source to drain current
OUTPUT CHARACTERISTICS OF JFET
The curve between drain current (Id) and drain source voltage (Vds)of a JFET at
constant gate source voltage (VGS) is known as output characteristics of JFET .keeping
Vgs fixed at some value 1 V the drain source voltage in steps. Corresponding to each
value of Vds the drain current Id is noted. A plot of these values gives the output
characteristics of JFET at Vgs = 1V repeating similar procedure output charaecteristics at
other gate –source voltages .
At first the drain current Id rises rapidly with drain surce voltage Vds but then becomes
constant. The drain source voltage above which drain current becomes constant is known
as pinch of f voltage.OA is the pinch off voltage.
Shorted gate drain current (Idss)
It is the drain current with source short circuited to gate Vgs =0 and drain voltage
Vds equal to pinch pff voltage.it is called zero bias current.
The drain current raise rapidly at first and then levels off at pinch off voltageVp.
The drain current has now reached the maximum value Idss when Vds is incresed beyond
Vp the depletion layers expand at the top of the channel.the channel now acts as current
limiter and holds drain current constant at Idss
Pinch off voltage Vp
It is the minimum drain source voltage at which the drain current essentially
becomes constant
Gate – source cut off voltage Vgs (off)
It is the gata source voltage where the channel is completely cut off and the drain
current becomes zero.
Expression for Drain Curent
The relation between Idss and Vp .We note that gate – source cut off voltage VGS
off on the transfer characteristics is equal to pinch off voltage vp on the drain
characteristics
Vp = VGS (off)
JFET has VGS = -4 V Vp =4
ID= Idss (1-VGS/VGS(off))2
Id =drain current at given Vgs
Idss = shorted -gate drain voltage
Vgs = gate – source voltage
Vgs (off) = gate –source cut off voltage
FET AS A VOLTAGE –VARIABLE RESISTOR (VVR):
FET is operated in the constant current portion of its output characteristics for the linear
applications .In the region before pinch off , where Vds is small the drain to source
resistance rd can be controlled by the bias voltage Vgs.The FET is useful as a voltage
variable resistor (VVR) or Voltage Dependent resistor.
In JFET the drain source conductance gd = Id/Vds for small values of Vds which may be
expressed as gd = gdo [ 1-( VgsVp)1/2 ] where gdo is the value of drain conductance
when the bias voltage Vgs is zero.
The variation of the rd with vgs can be closely approximated by rd = ro / 1- KVgs ro –
drain resistance at zero gate bias and K constant dependent upon FET type.
Small signal FET drain resistance rd varies with applied gate voltage Vgs and FET act
like a VARIABLE PASSIVE RESISTOR.
Advantagesof JFET
Very high input impedance order of 100 ohm
Operation of JFET depends on the bulk material current carriers that do not cross
junctions
Negative temperature coefficients
Very high power gain
Smaller size longer life and high efficiency
Ac drain resistance rd it is the ratio of change in drain – source voltage to the change in
drain current at constant gate source voltage
Transconductance it is the ratio of change in drain current to the change in gate source
voltageat constant drain source voltage
Amplification factor it is the ratio of change in drain source voltage to the change in gate
source voltage at constant drain current.
METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR
(MOSFET)
The input impedance of a mosfet is much more than that of a JFET because of
very small gate leakage current. The MOSFET can be used in any of the circuits
converted for the JFET.
CONSTRUCTION
N – CHANNEL JFET
There is only a single p region this region is called substrate A thin layer of metal
oxide is deposited over the left side of the channel. A metallic gate is deposited over the
oxide layer . Asilicon dioxide is an insulator ther fore gate is insulated from the channel
.it is called Insulated Gate FET Like JFET , MOSFET has three terminals SOURCE,
GATE, DRAIN.
Working principle
Instead of gate diode as in JFET here gate is formed as a small capacitor. One
plate of the capacitor is the gate and the other plate is channel with metal oxide as the
dielectric. when negative voltage is applied to the gate electrodes accumulate it. These
electrons repel the conduction band electrons in the n channel. lesser number of
conduction electron made available for current conduction through the channel. The
grater the negative voltage on the gate the lessor is current conduction from source to
drain. If the gate is given positive voltage more electrons are made available in the n
channel.