Bipolar junction transistor - Basics Introduction Walter Brattain, John Bardeen, and William Shockley invented the bipolar junction transistor (BJT) in 1949, while working for Bell Telephone Laboratories. This revolutionary invention changed the world. The invention of the BJT followed the invention of the point-contact transistor by Walter Brattain and John Bardeen. The point-contact transistor has several problems that prevented it from becoming a viable device. BJT is a three-terminal device. BJT is used as amplifier and switch.

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

1

Circuit diagram of pnp transistor consisting of two diodes

The two n-type regions merge to form a very thin base. EB junction: Forward bias CB junction: Reverse bias

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

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Band diagram (PNP)

Junction bias? Major current flows?

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

3

Basic amplifier circuits Common-base configuration

α =

IC IE

(1)

α = current amplification in common base circuit Typical values: α > 0.99 (for state-of-the-art transistor)

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

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Common-emitter configuration

β = amplification in common-emitter circuit

β =

IC IB

=

IC IE − IC

⎛1 ⎞ = ⎜ − 1⎟ ⎠ ⎝α

−1

=

α 1− α

(2)

β > 100 for state-of-the art transistors

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

5

Common-collector configuration

IE IB

=

IC / α IB

=

β α

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

(3)

6

Nature of bipolar transistor BJT is a current amplifier (not a voltage amplifier). BJT is current-controlled current source. BJT base current controls the emitter current and thereby the collector current.

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

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Qualitative discussion of pnp transistor

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

8

Basic ideas EB junction is asymmetric:

I Ep

>>

I En

(4)

The emitter hole current is controlled by EB junction. The base width is small.

WB > WB BC junction currents (BC is reversely biased) (5) Electron minority carrier current from C to B (6) Hole minority carrier current from B to C We know that current (5) and (6) can be neglected for most practical purposes.

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

11

Basic equations What is the fraction of the emitter hole current that reaches the collector?

IC

= B I Ep

(6)

B = Base transport factor B = Probability that a hole injected into B reaches C B≤1

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

12

What fraction of total emitter current is emitter hole current?

EEp

= γ IE

(

= γ I En + I Ep

γ

= Emitter Efficiency

γ

= Ratio of I Ep to I E

)

(7)

γ ≤ 1 γ

=

I Ep I En + I Ep

⎞ ⎛ I En ⎟ = ⎜1 + ⎟ ⎜ I Ep ⎠ ⎝

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

−1

I En ≈ 1 − I Ep

(8)

13

Current amplification α

α =

IC IE

=

B

I Ep IE

=

Bγ

(9)

We will later calculate B and γ in two ways: 1. Approximate calculation 2. Exact calculation

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

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Approximate hole distribution in base (PNP) Long base (WB >> Lp)

δp ( xn ) = ∆p e − xn / LP

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

(10)

15

Short base (WB > ND

(23)

That is, Emitter doping >> Base doping

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

22

Example: Problem: Assume a PNP transistor with the following parameters: Emitter doping: NA = 1 × 1018 cm–3 Base doping: ND = 1 × 1017 cm–3 Dp = Dn WB = 100 nm Ln = 1 µm Calculate emitter efficiency. Solution:

γ

Dn WB N D = 1 − Dp Ln N A

1 = 1 − 100

= 0.99

The problem assumed reasonable parameters. For such reasonable parameters, we obtain a high current gain.

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

23

Approximate calculation: Base transport factor (PNP)

Thought experiment: Let’s assume that the BC junction would not influence the hole distribution. Warning: Strictly speaking, this is incorrect assumption!

© E. F. Schubert, Rensselaer Polytechnic Institute, 2003

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In this case, the following hole distribution would be obtained:

Base recombination current ∝ Q1 / τ ∝ ∆p WB

(24)

Collector current ∝ Q2 / τ ∝ ∆p Lp

(25)

(Note: In Eqs. 24 and 25, we use that WB