Unit – 2 Semiconductor Devices · 2019. 9. 11. · Semiconductor Devices Lecture_2.3 Bipolar Junction Transistor (BJT) Bipolar Junction Transistor (BJT) •3 terminal, active, electronic

Unit – 2Semiconductor Devices

Lecture_2.3

Bipolar Junction Transistor (BJT)

Bipolar Junction Transistor (BJT)

• 3 terminal, active, electronic device.

• Bipolar - Current is contributed both by majority and minority charge carriers.

• Similar to vacuum tubes but smaller in size.

• Used in computers, satellites, communication systems, etc..• Amplifier

• Oscillator

• Switch

Construction of BJT

• Silicon or germanium crystal• PNP thin layer of N-type material between 2 P-type materials.

• NPN thin layer of P-type material between 2 N-type materials.

• Emitter (E), Base (B) and Collector (C).• E is heavily doped, injects large carrier to B.

• B is lightly doped, very thin; passes carriers from E to C.

• C is moderately doped.

• Arrow on E specifies, direction of current flow.

Construction of BJT

Transistor Biasing

• EB junction is Forward Biased.

• CB junction is Reverse Biased.

• Forward biasing causes current flow from E to C through B.

• All emitter current flows through collector.

Operation of NPN Transistor

• EB junction is forward biased.

• e- from E crossover B.

• e- combines with holes in B which is lightly doped with P-type impurity.

• As a result base current 𝐼𝐵 decreases.

• Remaining e- constitutes collector current 𝐼𝐶 .

• Internal Emitter current, • 𝑰𝑬 = − 𝑰𝑪 + 𝑰𝑩

• External circuit Emitter current,• 𝑰𝑬 = 𝑰𝑪 + 𝑰𝑩

Operation of PNP Transistor

• EB junction is forward biased.

• Holes from E, cross B to reach C.

• Holes combines with e- in B which is doped lightly with N-type impurity.

• As a result base current 𝐼𝐵 decreases.

• Remaining holes constitutes collector current 𝐼𝐶 .

• Internal Emitter current, • 𝑰𝑬 = − 𝑰𝑪 + 𝑰𝑩

• External circuit Emitter current,• 𝑰𝑬 = 𝑰𝑪 + 𝑰𝑩

Types of transistor configuration

Configuration Common Terminal Input Terminal Output Terminal

CE Emitter (E) Base (B) Collector (C)

CB Base (B) Emitter (E) Collector (C)

CC Collector (C) Base (B) Emitter (E)

CB Configuration – Circuit diagram

CB Configuration – Input Characteristics

• Starting from zero, 𝑉𝐶𝐵 is kept constant.

• 𝐼𝐸 is measured by increasing 𝑉𝐸𝐵, functions as forward biased PN diode.

• Early effect – as reverse bias to CB junction is increased, base width decreases and space charge between C & E increases.

• Punch through – at larger biasing voltages, base width = 0; voltage breakdown of transistor occurs.

CB Configuration – Output Characteristics

• 𝐼𝐸 is kept constant by adjusting 𝑉𝐸𝐵.

• 𝐼𝐶 is measured by increasing 𝑉𝐶𝐵 for each value of 𝐼𝐸.

• 𝐼𝐶 is independent of 𝑉𝐶𝐵, 𝐼𝐶 flows for 𝑉𝐶𝐵 = 0.

CE Configuration – Circuit diagram

CE Configuration – Input Characteristics

• Starting from zero, 𝑉𝐶𝐸 is kept constant.

• 𝐼𝐵 is measured by increasing 𝑉𝐵𝐸, functions as forward biased PN diode.

CE Configuration – Output Characteristics

• 𝐼𝐵 is kept constant by adjusting 𝑉𝐵𝐸.

• 𝐼𝐶 is measured by increasing 𝑉𝐶𝐸 from zero.

• Saturation region, both junctions are FB; for increase in 𝐼𝐵, there is no large change in 𝐼𝐶 .

• Cut off region, below 𝐼𝐵 = 0, both junctions are RB; open circuit between C & E; off state.

• Active region, EB junction is FB & CB junction is RB; linear amplification; short circuit; on state.

CC Configuration – Circuit diagram

CC Configuration - Characteristics

Input Characteristics Output Characteristics

Characteristics are same as CE Configuration

Comparison of CB, CE & CC Configuration