Bipolar Junction Transistors (BJT)

• Bipolar Junction Transistors (BJT) are three-terminal semiconductor devices. The terminals are Emitter (E), Base (B), and Collector (C).
• Bipolar in BJT signifies that both majority and minority carriers play a role in current conduction.
• A junction forms when two different types (P & N) of semiconductors meet.
• A transistor transfers resistance, transforming signals from low to high resistance. Low input resistance results in high output resistance.
• A BJT is a current-controlled device. The base current (I_B) controls the collector current (I_C).
• Three types of currents exist in the BJT: Emitter Current (I_E), Base Current (I_B), and Collector Current (I_C)
• The relationship between these currents is I_E = I_B + I_C.
• In terms of doping levels within a BJT: Base < Collector < Emitter.
• In terms of area within BJT regions: Base < Emitter < Collector.
• Two junctions exist in a BJT: the Emitter-Base (E-B) junction and the Collector-Base (C-B) junction. Both form PN junctions.
• BJTs are of two types: NPN and PNP transistors.

Transistor Construction

• A transistor consists of two PN junctions formed by sandwiching either P-type or N-type semiconductor material between a pair of opposite types.

NPN Transistors

• Two layers of N-type semiconductor are separated by a thin layer of P-type semiconductor

PNP Transistors

• A transistor in which two layers of P-type semiconductors are separated by a thin layer of N-type semiconductor.

Transistor Symbols

• There are two types of transistor symbols

NPN Transistors

• On the emitter terminal, the arrow points outwards

PNP Transistors

• On the emitter terminal, the arrow points inwards
• The arrow is always placed on the emitter terminal and its direction indicates the direction of conventional current flow.
• E = Emitter, B = Base, C = Collector.

Unbiased Transistors

• Occurs when no external supply is connected . Depletion regions are formed from the diffusion of charge carriers.
• The width of the depletion region is uneven on both sides of the junction due to different doping levels.
• The depletion region penetrates more in lightly doped regions and less in heavily doped regions.
• No current conduction occurs in this state

Transistor Action

• This involves applying external voltages (biasing).

Transistor Biasing and Applications:

• Forward-biased Emitter-Base junction, Reverse-biased Collector-Base junction: Active region for Amplification.
• Forward-biased Emitter-Base junction, Forward-biased Collector-Base junction: Saturation region for a Closed Switch.
• Reverse-biased Emitter-Base junction, Reverse-biased Collector-Base junction: Cut-off region for an Open Switch.
• Reverse-biased Emitter-Base junction, Forward-biased Collector-Base junction: Inverted, with no intended application as the transistor won't work.

Operation of an NPN Transistor

• Forward biasing causes electrons from the N-type emitter to flow towards the P-type base, forming the emitter current (I_E).
• Due to the thin, lightly doped base region, only a few injected electrons recombine with holes, creating a small base current (I_B).
• Most electrons cross the base region and move to the collector region, drawn to the positive terminal of the DC source. This constitutes the collector current (I_C).
• Emitter current is the sum of collector and base currents. I_E = I_B + I_C.
• Transistor works in the 'active' region during normal operation

Transistor Configurations

• A transistor needs four terminals in a circuit (two for input, two for output).
• This is achieved by making one terminal common to both the input and output. The configuration of a transistor depends on which terminal is common

Common Base (CB) Configuration

• The base terminal is common to the input and output.
• Input is applied between the emitter and base
• Output is taken between the collector and base.

Common Emitter (CE) Configuration

• The emitter terminal is common to the input and output.
• Input is applied between the base and emitter
• Output is taken between the collector and emitter.

Common Collector (CC) Configuration

• On configurations, he collector terminal is common to the input and output.
• Input is applied between the base and collector
• Output is taken between the emitter and collector.

Common Base Configuration Analysis (DC)

• Input is applied between the base and emitter; output is taken between the collector and base. The base is common to both input and output
• DC current gain (α_DC): Defined as the ratio of collector current (I_C) to emitter current (I_E).
• α = I_C / I_E
• α is less than 1. Ranges from 0.9 to 0.99

AC Current Gain

• The ratio of change in collector current (ΔI_C) to the change in emitter current (ΔI_E) at constant collector-base voltage (V_CB)
• α_AC = ΔI_C / ΔI_E at constant V_CB
• α_AC is less than 1

Leakage Current

• The emitter terminal is open, there is no emitter current, hence no base or collector current.
• The collector-base junction is reverse-biased, but minority carriers produce a small leakage current (I_CBO or I_CO).
• Total collector current = I_C = αI_E + I_CBO
• I_C = α (I_B + I_C) + I_CBO
• Which simplifies to I_C = (αI_B / (1 - α)) + (I_CBO / (1 - α))

Common Emitter Configuration (Analysis)

• Input is applied between base and emitter, and the output is taken from collector and emitter, with the emitter common.
• DC current gain (β or β_DC): Ratio of collector current (I_C) to base current (I_B).
• β = I_C / I_B
• β >> 1

AC Current Gain

• The ratio of change in collector current (ΔI_C) to the change in base current (ΔI_B), keeping collector-emitter voltage (V_CE) constant.
• β_AC = ΔI_C / ΔI_B at constant V_CE
• β_AC is much greater than 1
• Total collector current =I_C(total)=I_Cmajority++I_Cminority
• I_C = βI_B + I_CEO

Common Collector Configuration (Analysis)

• The input is applied between the base and collector. The output is taken between the emitter and collector. The collector of the transistor is common in this configuration. DC current gain of Common Collector configuration:
• The gain is the ratio of the emitter current (I_E) to the base current (I_B)
• γ = I_E / I_B

AC Current Gain

• The gain is the ratio of the change in the emitter current (ΔI_E) to the change in the base current (ΔI_B) while the emitter to collector voltage (V_EC) is kept constant.
• γ = ΔI_E / ΔI_B at constant V_EC

Relationships between transistor gain measurements.

α = I_C / I_E , β = I_C / I_B , γ = I_E / I_B

• αγ = β

Relationship Between α and β:

• γ = 1 + β = (1 / (1 - α))

Relationship Between α and γ

• β = α / (1- α)

Relation Between Leakage Currents of CB and CE Configuration:

• The leakage current (I_CBO) for the common base configuration and finding I_CEO and the link with the currents
• Using an derived relation; I_CEO = I_CBO/ (1- α)

V-I Characteristics of a Transistor

• The behavior of a transistor is observed through its V-I characteristics. There are two types of characteristics: input and output.

Input Characteristics

• The input voltage and the input current relationship for a constant output voltage.

Output Characteristics

• Output voltage and output current relationship for a constant input current.

V-I Characteristics in CB Configuration:

• Two types:
• Input
• Output

Input Characteristics

• Drawn between emitter and base voltage (V_EB) and emitter current (I_E) for a constant collector-base voltage (V_CB).

Output Characteristics

• For NPN transistor can be drawn between collector and base voltage (V_CB) and collector current (I_C) for a constant emitter current (I_E).

V-I Characteristics for CE Configuration

• Characteristics namely, input and output characteristics

Input characteristics

• Drawn between base to emitter voltage and base current at constant collector to emitter voltage.

Output characteristics

• Drawn between collector to emitter voltage and collector current for the constant base current.

V-I Characteristics for Common Collector (CC)

• The input characteristics will be drawn between VCB and IB for some constant VCE
• The output characteristics will be drawn between VCE and IE for a constant IB.

Field-Effect Transistor (FET or JFET)

• Is a three-terminal semiconductor device

FET Terminals

• Drain (D), Gate (G), and Source (S).
• FET are voltage-controlled device. Gate-source voltage (V_GS) controls drain current (I_D).
• FET are unipolar device. Current is carried by only one type of charge particle, either electrons or holes.
• FET has very high input impedance
• Temperature is more stable compared to the BJT.

Principle of Operation

• Voltage applied between the gate and source (V_GS) controls drain current (ID).

Classification of FET

• Junction Field-Effect Transistor (JFET):
• N Channel JFET.
• P Channel JFET.
• Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET).
  • Depletion-type MOSFET (D-MOSFET):
  • N Channel D-MOSFET.
  • P Channel D-MOSFET.
  • Enhancement-type MOSFET (E-MOSFET):
    • P Channel E-MOSFET.

N Channel D MOSFET

• N Channel EMOSFET

Current Controlled Device

• Bipolar junction transistor (BJT).
• Collector current (I_C).
• Base current (I_B).

Voltage Controlled Device

• Field effect transistor (FET).
• Drain current (I_D).
• Gate to source voltage (V_GS).

N Channel JFET Construction

• A silicon bar is constructed in which two P-regions are introduced side-by-side (two P-N junctions)
• Two ohmic contacts are drawn from the top and bottom region (drain/source). The third terminal is internally connected to the P-regions.

N Channel JFET Symbol

• Arrow points inwards towards the channel line

P Channel JFET Symbol

• Arrow points outwards from the channel line

JFET Characteristics

• There are two kinds i.e. drain and output, and transfer characteristics

JFET Drain characteristics

• The drain characteristics are derived from the drain to the source voltage and drain current for the constant flow from the gate to source voltage.

JFET Transfer characteristics

• The transfer characteristics are derived from the gate to the source voltage and drain current for the constant flow from the drain to the source voltage.

JFET characteristics Equation

The ID = IDSS (1 - V_GS/V_p)^2

JFET Definitions and Relations

Pinch-Off Voltage (Vp)

• The point at which Drain to Source Voltage(VDS) will lead to the current that will be remaining in the constant at fixed Gate settings

Maximum Drain Saturation Current(IDSS)

• which exists at V_GS = 0V point
• AC Drain Resistance
• Defined under the ratio of drain to source voltage with specific Gate to Source voltage

Transconductance

Ratio of change in current and to change in Gate source voltage

Amplification factors

• the radio for the change in output voltage and change in drain current

Relationships

• That M = Rdxjm (prove )
• That jm = is given through the transconductance equation

Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

• This is important Semiconductor device and widely used in many circuit applications
• The voltage controlled device, in the gate control
• the unipolar device, the current conduction is only due to some carriers
• Mainly is some three-terminal type
• Is the input impedance in MOSFET

The Basis of some construction types

• The two typical Types the depletion and Enhance In the enhancement type MOSFET the channel is of physically is not there

MOSFET

• The channel of P type semiconductor MOSFET can some type work in the depletion
• Enhancement mode

N-Channel D-MOSFET

P channel D-MOSFET, etc

Construction of MOSFET

• You have different types like N&P channel enhancement

N-Channel Depletion-Type MOSFET Construction

• We construct a channel of N type between the two doped regions now A silicon dioxide layer will generated just above to two Terminal Drain source is taken
• Out via contact form Then to Gate is insulted from by insulator.

N-Channel Enhancement-Type MOSFET Construction

• See that there is the current that is not the presence.

MOSFET N Channel depletion

The modes

• Depletion Mode
• Gate voltage is less than the value
• Enhancement mode
• Voltage is higher than 0 In a Depletion Mode of the Voltage then is the the electron that will repel that the electron will flow out .

EE MOSFET work in two ways

• Both are Depletion and Enhancement modes.

More on MOSFETS

• Only in the type of EE voltage
• At some negative level of some Gate voltages there is will knowns there is the region that will is the for it is the
• Where there is some the minor carriers

Working Channel type EE MOSFET

• In work that in the channel of the new EE voltage because that MOSFETS In the work In some Depletion channels
• Where it is the Source where there are Some gate voltages
• Only runs for some EE voltages

The Modes

• There is no formed between that doped the region in no drain the current flows From the the Drain the source enhance that that that.