There are two main types of transistors: bipolar junction transistors (BJT) and field effect transistors (FET). BJTs use both holes and electrons as current carriers and include NPN and PNP types, while FETs use only one carrier type and include JFETs and MOSFETs. MOSFETs are particularly important as they can be easily integrated into circuits. MOSFETs operate in different modes depending on the voltage applied to the gate and include depletion, enhancement, linear, and saturation modes.

1. Different Types of
Transistors and Their
Functions
By Unsa Shakir

2. Introduction:
Different Types of Transistors and
Their Functions
 Transistor is an active component and that is establishing in
all over electronic circuits. They are used as amplifiers and
switching apparatus. There are low, medium and high
power transistors, for functioning with high and low
frequencies, for functioning with very high current and or
high voltages.

4. Bipolar Junction Transistor
(BJT)
 BJT transistors are built up of 3 regions, the base, the collector,
and the emitter.
 Bipolar Junction transistors are bi-polar current-controlled
devices.
 Bipolar junction transistors come in two major types, NPN and
PNP.
 A NPN transistor is one in which the majority current carrier
are electrons.
 A PNP transistor is one in which the majority current carrier are
holes.

5. Bipolar Junction Transistor
(BJT)

6. Transistor
Symbol

7. Field Effect
Transistor
 Field Effect Transistors are made up of 3 regions, a gate, a source,
and a drain.
 FETs are uni-polar voltage-controlled devices.
 The FET consists of a semiconductor channel with electrodes at
either end referred to as the drain and the source.
 A control electrode called the gate is placed in very close proximity
to the channel so that its electric charge is able to affect the
channel.
 In this way, the gate of the FET controls the flow of carriers
(electrons or holes) flowing from the source to drain.

8. Field Effect
Transistor
 Field Effect Transistors come in 2 main types: JFETs and
MOSFETs.

9. Junction FET
Transistor
 The Junction Field Effect Transistor has two PN-junctions.
 It has a narrow part of high resistivity semiconductor material
forming a “Channel” of either N-type or P-type silicon.
 The majority carriers to flow through with two ohmic electrical
connections at either end normally called the Drain and the
Source.
 There are a two basic configurations of junction field effect
transistor.
• The N-channel JFET and the P-channel JFET.

10. Junction FET Transistor

11. Metal Oxide Semiconductor Field Effect Transistor
• MOSFET is a unipolar transistor, which acts as a voltage-
controlled current device and in which current at drain and source
is controlled by the action of an electric field at gate, having in-
between semiconductor and metal very a thin metal oxide layer.
• The MOSFET is a core of integrated circuit and it can be designed
and fabricated in a single chip because of these very small sizes.
• The MOSFET is a four terminal device with source(S), gate (G),
drain (D) and body (B) terminals.

12. • The MOSFET works by electronically varying the width of a channel
along which charge carriers flow (electrons or holes).
• The charge carriers enter the channel at source and exit via the
drain.
• The width of the channel is controlled by the voltage on an
electrode is called gate which is located between source and drain.
• It is insulated from the channel near an extremely thin layer of
metal oxide. The MOS capacity present in the device is the main
part

13. The MOSFET can function in two ways
In metal oxide field effect transistor there are two major transistor
mode of operation, corresponding to whether the transistor is in
an ON state or an OFF state at zero gate-source voltage
• Depletion Mode
• Enhancement Mode

14. Depletion Mode
• When there is no voltage on the gate, the channel shows its
maximum conductance.
• In a depletion-mode MOSFET, the device is normally ON at zero
gate–source voltage
• Key points: In depletion mode (normally ON at VGS = 0, OFF on a
Voltage between Gate and Source)

15. Enhancement mode
• When there is no voltage on the gate the device does not conduct.
More is the voltage on the gate, the better the device can conduct.
• Enhancement-mode MOSFETs are the common switching elements in
most MOS. These devices are off at zero gate–source voltage
• Key point: In enhancement mode (normally OFF at VGS = 0, ON at a
Threshold Voltage VDS)

16. The MOSFET can be categorized into three separate modes when in
operation.
• The first is the sub-threshold or cut-off mode: VGS < Vt, where Vt is
the threshold voltage. In this mode the device is essentially OFF, and in
the ideal case there is no current flowing through the device.
• The second mode of operation is the linear region when VGS > Vt.
Essentially, the MOSFET operates similar to a resistor in this mode with
a linear relation between voltage and current.
• The third mode of operation is saturation mode when VGS > Vt. In this
mode the switch is on and conducting, however since drain voltage is
higher than the gate voltage, part of the channel is turned off. This
mode corresponds to the region to the right of the dotted line, which is
called the pinch-off voltage.
MOSFET Operation

17. • Pinch-off occurs when the MOSFET stops operating in the linear
region and saturation occurs.
• In digital circuits MOSFETS are only operated in the linear mode,
while the saturation region is reserved for analogue circuits.

18. Types of MOSFETS
n-channel
Enhancement Mode
(nMOSFET)
p-channel
Enhancement Mode
(pMOSFET)
n-channel
Depletion Mode
(nMOSFET)
p-channel
Depletion Mode
(pMOSFET)

19. P-Channel MOSFET
• The MOSFET having P-channel region between source and drain is
called as P-channel MOSFET.
• Here the source and drain terminals are heavily doped with P-type
material and the substrate is doped with N-type material.
• The current flow between source and drain is because of holes
concentration.
• The applied voltage at gate will controls the flow of current through
channel region.

21. N- Channel MOSFET
• The MOSFET having N-channel region between source and drain is
called N-channel MOSFET.
• Here the source and gate terminals are heavily doped with n-type
materials and substrate is doped with p-type semiconductor material.
• Here the current flow between source and drain is because of electrons.
• The gate voltage controls the current flow in the circuit.
• N-channel MOSFET is most preferable than P-channel MOSFET
because the mobility of electrons is high than mobility of holes.