Transistors Flashcards
Basics of Transistors
What are transistors ?
A transistor is a semiconductor device that can amplify or switch electronic signals and electrical power. It consists of three layers of semiconductor material, forming two junctions. The three regions of a transistor are called the Emitter (E), Base (B), and Collector (C).
Types of Transistors
Bipolar Junction Transistor (BJT), Field-Effect Transistor (FET)
About BJT
Bipolar Junction Transistor (BJT):
Made up of three regions: Emitter, Base, and Collector.
Operates with two types of charge carriers: electrons and holes.
Comes in two varieties:
NPN Transistor:
Has a layer of p-type material sandwiched between two n-type materials.
PNP Transistor:
Has a layer of n-type material sandwiched between two p-type materials.
About FET
Field-Effect Transistor (FET):
Has three terminals: Source (S), Gate (G), and Drain (D).
Operates with a single type of charge carrier (electrons or holes).
Comes in two main types:
Junction FET (JFET):
Uses a PN junction to control the flow of current.
Metal-Oxide-Semiconductor FET (MOSFET):
Uses an insulated gate to control the flow of current.
Further divided into:
n-channel MOSFET: Majority carriers are electrons.
p-channel MOSFET: Majority carriers are holes.
Working of BJT
NPN Transistor Operation:
The Base-emitter junction is forward-biased (low voltage), and the Collector-base junction is reverse-biased (high voltage).
When a small current is applied to the Base, it controls a larger current flowing from the Collector to the Emitter.
The Base region is very thin and lightly doped, allowing most charge carriers to move from the Emitter to the Collector.
PNP Transistor Operation:
The Base-emitter junction is forward-biased, and the Collector-base junction is reverse-biased.
Current flows from the Emitter to the Collector when a small current is applied to the Base.
JFET Operation
The Gate-source voltage controls the current flowing from the Drain to the Source.
No current flows into the Gate in an ideal JFET; only a voltage controls the current.
When a voltage is applied to the Gate, it changes the width of the conductive channel between the Source and Drain.
MOSFET Operation
The Gate is insulated from the Channel by a thin oxide layer, which allows for very high input impedance.
Applying a voltage to the Gate controls the current flow from the Source to the Drain.
Two modes:
Enhancement Mode (requires a gate voltage to turn on) and Depletion Mode (conducts with zero gate voltage).
What is Current Gain (Ξ² or hFE) in BJT
Represents the ratio of Collector current (I_C) to Base current (I_B):
π½ = πΌπΆ / πΌπ΅
Indicates the amplification capability of the transistor.
Different regions in BJT
Saturation Region:
When the transistor is fully βon,β both the Base-Emitter and Base-Collector junctions are forward-biased.
The transistor behaves like a closed switch with minimal voltage drop across the Collector and Emitter.
Cutoff Region:
When the transistor is fully βoff,β no current flows through the Collector.
Both junctions are reverse-biased.
Active Region:
The transistor operates as an amplifier.
The Base-Emitter junction is forward-biased, while the Collector-Base junction is reverse-biased.
What is Transconductance (g_m) in FET
Measures the change in Drain current (I_D) resulting from a change in Gate-Source voltage (V_GS).
Defined as:
ππ = ππΌπ· / πππΊπ
