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For loop is used where we already know about the number of times loop needs to be executed. Typically for a index used in iteration.
When VGS is increased beyond VGS (th) drain current starts flowing. For small values of VDS (VDS < (VGS – VGS (th)) ID is almost proportional to VDS. Consequently this mode of operation is called “ohmic mode” of operation. In power electronic applications a MOSFET is operated either in the cut off or in the ohmic mode. The slope of the VDS – ID characteristics in this mode is called the ON state resistance of the MOSFET .At still higher value of VDS (VDS > (VGS – VGS (th)) the ID – VDS characteristics deviates from the linear relationship of the ohmic region and for a given VGS, ID tends to saturate with increase in VDS. The exact mechanism behind this is rather complex. It will suffice to state that, at higher drain current the voltage drop across the channel resistance tends to decrease the channel width at the drain drift layer end. In addition, at large value of the electric field, produced by the large Drain – Source voltage, the drift velocity of free electrons in the channel tends to saturate. As a result the drain current becomes independent of VDS and determined solely by the gate – source voltage VGS. This is the active mode of operation of a MOSFET. Due to the presence of the anti parallel “body diode”, a MOSFET cannot block any reverse voltage. The body diode, however, can carry an RMS current equal to IDM. It also has a substantial surge current carrying capacity. When reverse biased it can block a voltage equal to VDSS.
For safe operation of a MOSFET, the maximum limit on the gate source voltage (VGS (Max)) must be observed. Exceeding this voltage limit will cause dielectric break down of the thin gate oxide layer and permanent failure of the device. It should be noted that even static charge inadvertently put on the gate oxide by careless handling may destroy it. The device user should ground him before handling any MOSFET to avoid any static charge related problem.
1)
What is the use of fprint function?
fprintf() function is used to
write the data to text file and its syntax is ” fprintf(FILE *fptr,const char
*format [argu1,argu2……]);. Here fptr is a file pointer of type file structure.
2) What is the use of fscanf() function?
It is used
to read data from stream and store the data according to the parameter format
into the location pointed by the additional argument.
3)
From “fgets()” and “gets()” which function is safe to use?
fgets() function is safe to used
because in it we can specify the size of the buffer into which the string
supplied will be store. So we use fgets() function mostly because it
is the safe.
4) What function we use for reading from the strings?
We used
“sscanf()” function to read from the string s file.
5)
What header file we use for performing memory formatting?
We use strstream.h / strstream
header file for performing memory formatting.
6) What is the difference
between 'while' loop and 'for' loop? What is an example of this?
Both for loop and while loop
are used to execute one or more lines of code certain number of times. The main
differences are as follows.
Syntax:
While loop:
while(condition) {
//statements to execute.
}
For loop:
for(initialization; condition; Increment or decrement){
// statements to be executed.
}
There are few variations we can do with for loop.
Such as all three parts of for loop are optional. That means you can also have a loop like this.
for(;;) which is nothing but an infinite loop.
For initialization there can be multiple statements.
for(i =0, j =0;i<20;i++)
Similarly for the third component there can be any expression and not necessarily increment or decrement.
for(i =0;i<10;i = i * 2)
Syntax:
While loop:
while(condition) {
//statements to execute.
}
For loop:
for(initialization; condition; Increment or decrement){
// statements to be executed.
}
There are few variations we can do with for loop.
Such as all three parts of for loop are optional. That means you can also have a loop like this.
for(;;) which is nothing but an infinite loop.
For initialization there can be multiple statements.
for(i =0, j =0;i<20;i++)
Similarly for the third component there can be any expression and not necessarily increment or decrement.
for(i =0;i<10;i = i * 2)
Working:
In while
loop first of all the condition expression is evaluated and if it is true then
the body of the loop is executed. Then control again evaluate the condition
expression. This goes on until condition becomes false.
In for loop
the initialization step is executed if it is there. It is never executed again.
After this condition expression is evaluated and if it is found true then body
of loop is executed after this control goes to third part i.e. increment or
decrement if it is defined. Then again control goes to check the condition
again and if found true then body gets executed and this goes on until
condition becomes false.
Use:
While loop is used in situations where we do not know how many times loop needs to be executed beforehand.
While loop is used in situations where we do not know how many times loop needs to be executed beforehand.
For loop is used where we already know about the number of times loop needs to be executed. Typically for a index used in iteration.
7) What is MOSFET?
MOSFET stands for Metal oxide Semiconductor field
effect transistor. A type of transistor that is controlled by voltage rather
than current. The power MOS field effect transistor (MOSFET) evolved from the
MOS integrated circuit technology. The new device promised extremely low input
power levels and no inherent limitation to the switching speed. Thus, it opened
up the possibility of increasing the operating frequency in power electronic
systems resulting in reduction in size and weight. At high frequency of
operation the required gate drive power becomes substantial. MOSFETs also have
comparatively higher on state resistance per unit area of the device cross
section which increases with the blocking voltage rating of the device.
MOSFET is a special type of field-effect
transistor ( FET ) that works by electronically varying the width of a channel
along which charge carriers flow. The wider the channel, the better the
device conducts. The charge carriers enter the channel at the source,
and exit via the drain. The width of the channel is controlled by the
voltage on an electrode called the gate, which is located
physically between the source and the drain and is insulated from the channel
by an extremely thin layer of metal oxide.
8)
Explain how MOSFET functions?
There are two ways in
which a MOSFET can function.
The first is known as depletion mode . When there is no
voltage on the gate, the channel exhibits its maximum conductance . As the
voltage on the gate increases (either positively or negatively, depending on
whether the channel is made of P-type or N-type semiconductor material), the
channel conductivity decreases.
The second way in which a MOSFET can operate is
called enhancement mode. When there is no voltage on the gate, there is in
effect no channel, and the device does not conduct. A channel is produced by
the application of a voltage to the gate. The greater the gate voltage, the
better the device conducts.
9) Explain constructional features of a MOSFET.
Power
MOSFET is a device that evolved from MOS integrated circuit technology. The
first attempts to develop high voltage MOSFETs were by redesigning lateral
MOSFET to increase their voltage blocking capacity. The resulting technology
was called lateral double defused MOS (DMOS).
However
it was soon realized that much larger breakdown voltage and current ratings could be achieved by
resorting to a vertically oriented structure. Since then, vertical DMOS (VDMOS)
structure has been adapted by virtually all manufacturers of Power MOSFET. A
power MOSFET using VDMOS technology has vertically oriented three layer
structure of alternating p type and n type semiconductors. A large number of
cells are connected in parallel to form a complete device.
The two
n+ end layers labelled “Source” and “Drain” are heavily doped to approximately
the same level. The p type middle layer is termed the body (or substrate) and
has moderate doping level (2 to 3 orders of magnitude lower than n+ regions on
both sides). The n- drain drift region has the lowest doping density. Thickness
of this region determines the breakdown voltage of the device. The gate
terminal is placed over the n- and p type regions of the cell structure and is
insulated from the semiconductor body be a thin layer of silicon dioxide (also
called the gate oxide). The source and the drain region of all cells on a wafer
are connected to the same metallic contacts to form the Source and the Drain
terminals of the complete device. Similarly all gate terminals are also
connected together. The source is constructed of many (thousands) small polygon
shaped areas that are surrounded by the gate regions. The geometric shape of
the source regions, to same extent, influences the ON state resistance of the
MOSFET.
One interesting feature of the MOSFET cell is that the alternating n+ n- p n+ structure embeds a parasitic BJT (with its base and emitter shorted by the source metallization) into each MOSFET cell. The nonzero resistance between the base and the emitter of the parasitic npn BJT arises due to the body spreading resistance of the p type substrate. In the design of the MOSFET cells special care is taken so that this resistance is minimized and switching operation of the parasitic BJT is suppressed. With an effective short circuit between the body and the source the BJT always remain in cut off and its collector-base junction is represented as an anti parallel diode (called the body diode) in the circuit symbol of a Power MOSFET.
One interesting feature of the MOSFET cell is that the alternating n+ n- p n+ structure embeds a parasitic BJT (with its base and emitter shorted by the source metallization) into each MOSFET cell. The nonzero resistance between the base and the emitter of the parasitic npn BJT arises due to the body spreading resistance of the p type substrate. In the design of the MOSFET cells special care is taken so that this resistance is minimized and switching operation of the parasitic BJT is suppressed. With an effective short circuit between the body and the source the BJT always remain in cut off and its collector-base junction is represented as an anti parallel diode (called the body diode) in the circuit symbol of a Power MOSFET.
10)
Explain the three regions of operation of a MOSFET.
Cut-off region: When VGS < Vt, no channel is induced and the
MOSFET will be in cut-off region. No current flows.
Triode region: When VGS ≥ Vt, a channel will be induced and
current starts flowing if VDS > 0. MOSFET will be in triode region as
long as VDS < VGS – Vt.
Saturation region: When VGS ≥ Vt, and VDS ≥ VGS – Vt, the
channel will be in saturation mode, where the current value saturates. There
will be little or no effect on MOSFET when VDS is further increased.
11) What are the main constructional differences between a MOSFET and a BJT?
What effect do they have on the current conduction mechanism of a MOSFET?
A
MOSFET like a BJT has alternating layers of p and n type semiconductors.
However, unlike BJT the p type body region of a MOSFET does not have an
external electrical connection. The gate terminal is insulated for the
semiconductor by a thin layer of SiO2. The body itself is shorted with n+ type
source by the source metallization. Thus minority carrier injection across the
source-body interface is prevented. Conduction in a MOSFET occurs due to
formation of a high density n type channel in the p type body region due to the
electric field produced by the gate-source voltage. This n type channel
connects n+ type source and drain regions. Current conduction takes place
between the drain and the source through this channel due to flow of electrons
only (majority carriers). Where as in a BJT, current conduction occurs due to
minority carrier injection across the Base-Emitter junction. Thus a MOSFET is a
voltage controlled majority carrier device while a BJT is a minority carrier
bipolar device.
12)
What are the advantages of MOSFET?
The MOSFET has certain advantages
over the conventional junction FET, or JFET. Because the gate is insulated
electrically from the channel, no current flows between the gate and the
channel, no matter what the gate voltage (as long as it does not become so
great that it causes physical breakdown of the metallic oxide layer). Thus, the
MOSFET has practically infinite impedance . This makes MOSFETs useful for power
amplifiers. The devices are also well suited to high-speed switching
applications. Some integrated circuits (ICs) contain tiny MOSFETs and are used
in computers. Because the oxide layer is so thin, the MOSFET is susceptible to
permanent damage by electrostatic charges. Even a small electrostatic build up
can destroy a MOSFET permanently. In weak-signal radio-frequency (RF) work,
MOSFET devices do not generally perform as well as other types of FET.
13) Draw I-V characteristic of MOSFET.
The MOSFET, like the BJT is a three terminal device
where the voltage on the gate terminal controls the flow of current between the
output terminals, Source and Drain. The source terminal is common between the
input and the output of a MOSFET. The output characteristics of a MOSFET is
then a plot of drain current (ID) as a function of the Drain –Source
voltage (VDS) with gate source voltage (VGS) as a
parameter.With gate-source voltage (VGS) below the threshold voltage
(VGS(th)) the MOSFET operates in the cut-off mode. No drain current
flows in this mode and the applied drain–source voltage (VDS) is
supported by the body-collector p-n junction. Therefore, the maximum applied
voltage should be below the avalanche break down voltage of this junction (VDSS)
to avoid destruction of the device.
When VGS is increased beyond VGS (th) drain current starts flowing. For small values of VDS (VDS < (VGS – VGS (th)) ID is almost proportional to VDS. Consequently this mode of operation is called “ohmic mode” of operation. In power electronic applications a MOSFET is operated either in the cut off or in the ohmic mode. The slope of the VDS – ID characteristics in this mode is called the ON state resistance of the MOSFET .At still higher value of VDS (VDS > (VGS – VGS (th)) the ID – VDS characteristics deviates from the linear relationship of the ohmic region and for a given VGS, ID tends to saturate with increase in VDS. The exact mechanism behind this is rather complex. It will suffice to state that, at higher drain current the voltage drop across the channel resistance tends to decrease the channel width at the drain drift layer end. In addition, at large value of the electric field, produced by the large Drain – Source voltage, the drift velocity of free electrons in the channel tends to saturate. As a result the drain current becomes independent of VDS and determined solely by the gate – source voltage VGS. This is the active mode of operation of a MOSFET. Due to the presence of the anti parallel “body diode”, a MOSFET cannot block any reverse voltage. The body diode, however, can carry an RMS current equal to IDM. It also has a substantial surge current carrying capacity. When reverse biased it can block a voltage equal to VDSS.
For safe operation of a MOSFET, the maximum limit on the gate source voltage (VGS (Max)) must be observed. Exceeding this voltage limit will cause dielectric break down of the thin gate oxide layer and permanent failure of the device. It should be noted that even static charge inadvertently put on the gate oxide by careless handling may destroy it. The device user should ground him before handling any MOSFET to avoid any static charge related problem.
14) What is Forward Transconductance?
It is
the ratio of
ID and (VGS – VGS (th)). In a MOSFET switching
circuit it determines the clamping voltage level of the gate – source voltage
and thus influences dVDS/dt during turn on and turns off.
15)
What does it mean “the channel is pinched off”?
For a MOSFET when VGS is
greater than Vt, a channel is induced. As we increase VDS current
starts flowing from Drain to Source (triode region). When we further increase VDS,
till the voltage between gate and channel at the drain end to become Vt, i.e. VGS –
VDS = Vt, the channel depth at Drain end decreases
almost to zero, and the channel is said to be pinched off. This is where a
MOSFET enters saturation region.
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