AIR UNIVERSITY

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

EXPERIMENT NO 1

Lab Title: Characteristics of Bipolar Junction Transistor (BJT) and MOSFET

Student Name: FAARAN AHMED Reg. No:190424

Objective: The objective is to determine transistor types terminals,to study

transistor behavior as a switch, to examine MOSEFT.

LAB ASSESSMENT:

Excellent Good Average Satisfactory Unsatisfactory

Attributes
(5) (4) (3) (2) (1)
Objective and
implementation
Troubleshooting and
measurement

conclusion

Total Marks: Obtained Marks:

LAB REPORT ASSESSMENT:

Excellent Good Average Satisfactory Unsatisfactory

Attributes
(5) (4) (3) (2) (1)

Data presentation

Experimental results

Conclusion

Total Marks: Obtained Marks:

Date: Signature:
 Experiment#01
Characteristics of Bipolar Junction Transistor (BJT) and
MOSFET

Objectives:
 To determine transistor type (npn, pnp), terminals, and material using a digital multimeter
(DMM).
 To study the behavior of transistor as a switch.
 To examine the properties of a MOSFET.
 To observe the effect of various biasing schemes.

Equipment:
 Transistor
 MOSFET
 Digital Multimeter
 Resistors

Discussion:

 Bipolar Junction Transistor (BJT):

Bipolar transistors are made of either Silicon (Si) or Germanium (Ge). Their structure
consists of two layers of n-type material separated by a layer of p-type material (npn), or of two
layers of p-material separated by a layer of n-material (pnp). In either case, the center layer forms
the base of the transistor, while the external layers form the collector and the emitter of the
transistor. It is this structure that determines the polarities of any voltages applied and the direction
of the electron or conventional current flow. The arrow at the emitter terminal of the transistor
symbol for either type of transistor points in the direction of conventional current flow. One part
of this experiment will demonstrate how you can determine the type of transistor, its material, and
identify its three terminals.
The relationships between the voltages and the currents associated with a bipolar junction
transistor under various operating conditions determine its performance. These relationships are
collectively known as the characteristics of the transistor. As such, they are published by the
manufacturer of a given transistor in a specification sheet.
It is one of the objectives of this lab to experimentally measure these characteristics and to
compare them with their published values.

The three terminals of a transistor are following:

 Collector
 Base
 Emitter
  NPN Transistor:

NPN is one of the two types of bipolar transistors, in which the letters "N" and "P" refer to
the majority charge carriers inside the different regions of the transistor. Most bipolar transistors
used today are NPN, because electron mobility is higher than hole mobility in semiconductors,
allowing greater currents and faster operation.
NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-
doped layers. A small current entering the base in common-emitter mode is amplified in the
collector output. In other terms, an NPN transistor is "on" when its base is pulled high relative to
the emitter.
The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of
the conventional current flow when the device is in forward active mode.

NPN BJT structure creates two p-n junctions. The junction between the n-type collector and
the p-type base is called the Collector-Base Junction (CBJ). Note for the CBJ, the anode is the
base, and the cathode is the collector. However, the junction between the n-type emitter and the p-
type base is called the Emitter-Base Junction (EBJ). Note for the EBJ, the anode is the base, and
the cathode is the emitter.
 PNP Transistor:

PNP transistors consist of a layer of N-doped semiconductor between two layers of P-

doped material. A small current leaving the base in common-emitter mode is amplified in the
collector output. In other terms, a PNP transistor is "on" when its base is pulled low relative to the
emitter.
 The arrow in the PNP transistor symbol is on the emitter leg and points in the direction of
the conventional current flow when the device is in forward active mode.

PNP BJT structure creates two p-n junctions. For the pnp BJT, the anode of the CBJ is the
collector, and the cathode of the CBJ is the base. Likewise, the anode of the EBJ is the emitter, and
the cathode of the EBJ is the base.
 MOSFET:
Previously, we have been considering BJTs, besides BJT, another more important three
terminal device is Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) or MOS.
MOSFETs are used more widely in industry for implementing digital designs. They are preferred
in industry because of their size (smaller), ease of manufacture and lesser power utilization.
MOSFET technology allows placement of approximately 2 billion transistors on a single IC and
therefore forms the backbone of very large-scale integration (VLSI).

 Basic Structure and Principle of Operation:

The n-type Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) consists of a
source and a drain, two highly conducting n-type semiconductor regions which are isolated from
the p-type substrate by reversed-biased p-n diodes. A metal (or polycrystalline) gate covers the
region between source and drain but is separated from the semiconductor by the gate oxide. The
basic structure of an n-type MOSFET and the corresponding circuit symbol are shown below.
 It can be seen in the figure that the source and the drain regions are identical. It is the
applied voltages which determine which n-type region provides the electrons and becomes the
source, while the other n-type region collects the electrons and becomes the drain. The voltages
applied to the drain and gate electrode as well as to the substrate by means of a back contact, are
referred to as the source potential, as also indicated in the figure.
A top view of the same MOSFET is shown below where the gate length, L, and gate width,
W, are identified. Note that the gate length does not equal the physical dimension of the gate, but
rather the distance between the source and drain regions underneath the gate. The overlap between
the gate, the source and drain region is required to ensure that, the inversion layer forms a
continuous conducting path between the source and drain region. Typically, this overlap is made
as small as possible in order to minimize its parasitic capacitance.
 The typical current versus voltage (I-V) characteristics of a MOSFET

The flow of electrons from the source to the drain is controlled by the voltage applied to
the gate. A positive voltage applied to the gate, attracts electrons to the interface between the gate
dielectric and the semiconductor. These electrons form a conducting channel between the source
and the drain, called the inversion layer. No gate current is required to maintain the inversion layer
at the interface since the gate oxide blocks any carrier flow. The net result is that the current
between drain and source is controlled by the voltage which is applied to the gate.

 Modes of Operation and Voltage Transfer Characteristic Curve:

 Three regions exist in VTC.
 VGS < Vt (Cut off)
 VOV = VGS – Vt < 0
 ID = 0
 Vout = VDD
 Vt < VGS < VDS + Vt (Saturation)
 VOV = VGS – Vt > 0
 ID = ½ kn (VGS – Vt)2
 VDS >> VOV
 Vout = VDD – IDRD
 VDS + Vt < VGS < VDD (Triode)
 VOV = VGS – Vt > 0
 ID = kn (VGS – Vt – VDS) VDS
 VDS > VOV
 Vout = VDD – IDRD

MOS transistor acts as an amplifier in Saturation region and as an inverter in Triode and Cut
off regions.
 Biasing a MOSFET:
For a MOSFET to act as an amplifier, a proper DC bias point is required. The DC bias
circuit is to ensure the MOSFET in saturation with a proper collector current I D. ID, however, is
dependent on a number of factors which also includes physical factors which can vary with
temperature and a number of factors.

Therefore, a number of biasing schemes have been presented. Two of which will be
covered in Lab.
(1) Biasing using Gate-to-Drain Feedback Resistor:
 In this type of biasing category, a single power supply is needed. There will be no current
between drain and gate. RG ensures the MOSFET in saturation (VGS=VDS). MOSFET operating point
is maintained.

The value of the feedback resistor RG affects the small-signal gain. RD acts as a negative
feedback resistor to stabilize the drain current.

(2) Biasing using Fixed VG with Source Resistor:

This configuration generates fix VG from the given power supply by making a voltage
divider. Therefore, according to the equation:

As VG is fixed, VGS would vary to keep ID fixed. In this configuration, gate resistance is
chosen to be very high to give high input impedance to the AC signal which is to be coupled to the
MOSFET. Drain resistance also need to be high in order to give high voltage gain, however, it
should not be such large that FET move away from saturation. Also, RS (Source Resistance) is kept
very high to nullify the effect of parametric changes.
  NMOS as an Inverter:
When VIN is logic 1, VOUT is logic 0. Constant nonzero current flows through the transistor.
Power is used, even though no new computation is being performed. When VIN changes to logic 0,
transistor gets cutoff. ID goes to 0. Resistor voltage goes to zero. VOUT “pulled up” to 5 V.
 Procedure:
 Bipolar Junction Transistor (BJT):
 Determination of Transistor’s Type, Terminals and Material:
The following procedure will determine the type, terminals and material of a transistor.
The procedure will utilize the diode testing scale found on many modern multimeters. If no such
scale is available, the resistance scales of the meter may be used.
a. Label the transistor terminals of below figure as 1, 2 and 3. Use the transistor without
terminal identification for this part of the experiment.

b. Set the selector switch of the multimeter to the diode scale (or to the 2kΩ range if the
diode scale is unavailable).
c. Connect the positive lead of meter to terminal 1 and the negative lead to terminal 2.
Record the reading the Table.
d. Reverse the leads and record your reading.
e. Connect the positive lead to terminal 1 and the negative lead to terminal 3. Record the
reading.
f. Reverse the lead and record the reading.
g. Connect the positive lead to terminal 2 and the negative lead to terminal 3. Record
your reading.
h. Reverse the leads and record your reading.

Multimeter Readings
Multimeter’s Probes Multimeter’s Readings

Red Probe Black Probe in Volts

1 2 0.674V
2 1 \
1 3 0.676V
3 1 \
2 3 \
3 2 \
 i. The meter readings between two of the terminals will read high (or higher resistance)
regardless of the polarity of the meter leads connected. Neither of these two terminals
will be the base. Based on the above calculations, record the number of the base terminal
in the table below.

j. Connect the negative lead to the base terminal and the positive lead to either of the
other terminals. If the meter reading is low (approximately 0.7 V for Si and 0.3 V for
Ge or lower resistance), the transistor type is pnp. If it is not so, then connect the
positive lead to the base terminal and the negative lead to either of the other terminals.
If the meter reading is low (approximately 0.7 V for Si and 0.3 V for Ge or lower
resistance), the transistor type is npn.
k. (1) For pnp Type: Connect the negative lead to the base terminal and the positive lead
alternatively to either of the other two terminals. The lower of the two readings obtained
indicates that the base and collector are connected. Thus, the other terminal is the
emitter. Record the terminals in the above table.
(2) For npn Type: Connect the positive lead to the base terminal and the negative lead
alternatively to either of the other two terminals. The lower of the two readings obtained
indicates that the base and collector are connected. Thus, the other terminal is the
emitter. Record the terminals in the above table.
l. If the readings in either (1) or (2) of (k) were approximately 700mV, the transistor
material is Silicon. If the readings were approximately 300mV, the material is
germanium. If the meter does not have a diode testing scale, the material cannot be
determined directly. Record the Type of Material in the above table.
 Transistor as a switch:
 Connect the circuit given below. Record the readings in the table.
 Vin IB IC VC
0V
5V

Table
 MOSFET:
 Biasing a MOSFET:
(1) Biasing using Gate-to-Drain Feedback Resistor:

VDD = ……12v……….
RD = ……2.2k………...
RG = ……1M………...

VG VD VGS VDS Mode

(2) Biasing using Fixed VG with Source Resistor:

VDD =.......12V...........
RD = ……10K……...
RG1 = ……6.8K…….
RG2 = ……15K…….
RS = ……10K………

VG VD VGS VDS Mode

 NMOS as an Inverter:

VIN VOUT
 SIMULATIONS TO PERFORM

 Determination of Transistor’s Type, Terminals and Material: 

Multimeter’s Probes Multimeter’s Readings

Red Probe Black probe Voltage in volts

1 2 0.69
2 1 -
1 3 0.69
3 1 -
2 3 -
3 2 -

 From above table we come to know that our transistor is of NPN type and we are having 1 as a
BASE, 2
as a COLLECTOR and 3 as a EMITTER .

 CHARACTERISTIC CURVE AND REJOIN OF BJT AND MOSFET:

TRANSFER CHARACTERISTICS CURVE OF BIPOLAR JUNCTION

TRANSISTOR(BJT)
By this simulation we get the graph of as below:

TRANSFER CHARACTERISTIC CURVE OF MOSFET:

As done in BJT now replacing BJT with a MOSFET and we get simulation like as given below:

After this simulation we get a graph which is as given below:

 Using Transistors as a switch: 
From the above simulations we get the result as below:

Vin IB IC VC
0V 0A 0A +5V
5V 0.42mA 0.92mA +0.05V

 BJT AS AMPLIFIER:
BJT amplifier oscilloscope output:

 MOSFET AS A SWITCH:
 MOSFET BIASING:

GATE-DRAIN feedback:
Fixed VG:
  Questions:
Q#1 What is pinch off voltage in MOSFETS?

Pinch off voltage is the drain to source voltage after which the drain to source current
becomes almost constant and JFET enters into saturation region and is defined only when gate
to source voltage is zero

Q#2 What is the difference between enhancement and depletion type

MOSFET?
Moving the gate voltage toward the drain voltage "enhances" the conduction in the channel,
so this defines the enhancement mode of operation, while moving the gate away from the
drain depletes the channel, so this defines depletion mode.

 Conclusion: -
In this lab we study about the structure of transistor and MOSFET. We learn that there are three modes of
MOSFET. We learn about the transfer curve of MOSFET. We perform task related to biasing of MOSFET