1. 1 Concepts of electrons and holes in semiconductors

2. 2 Forward and reverse bias in a p-n junction Under forward bias the width of the depletion region decreases. Current increases exponentially. Under reverse bias the width of the depletion region increases. Very low current flow (leakage current Is)

3. 3 Chapter 4: Bipolar Junction Transistor 4.1 Basic Operation of the npn Bipolar Junction Transistor Figure 4.1 The npn BJT Basic Operation in the Active Region: An npn transistor (CE configuration) with variable voltage sources operating in the active region:  V BE ≈ 0.6 V to forward bias the BE junction  V CE >V BE - the base collector junction is reverse biased We will apply the Shockley equation: Here, the emission coefficient n = 1 (usually the case for ideal p-n junctions) ……… (4.1)

4. 4 Basic Operation in the Active Region – Cont’d First-Order Common- Emitter Characteristics Fig. 4.3 Note: The current flowing in a BJT is mostly due to electrons moving from the emitter through the base to the collector Base current consist of two components: (i) holes crossing from the base into the emitter, and (ii) holes recombining with the electrons injected into the base Usually we desire the base current (i/p current ) to be much lower than the collector current (o/p current). We define a current gain constant called β = I C /I B. Typically β ranges from 10 to 1000

5. 5 Device Equations – Cont’d From Eq. 4.3 and 4.4 we have:

6. 6 Example Figure 4.4

7. 7 Load line analysis – Input circuit Fig. 4.11 The slope of the load line is -1/R B From Kirchoff’s voltage law applied to the input circuit: Three major steps for analyzing BJT: 1.Solve the input circuit to determine the base current 2.Determine output characteristics corresponding to the base current 3.Solve the output circuit to determine the V CE and I C

8. 8 Load line analysis – Output circuit Fig. 4.11 As I B changes with input voltage V B, the output current and voltage changes according to the transistor characteristics. Almost always an amplification can be obtained. For each value of i b, there is specific i c vs. V CE characteristic. We have to draw the load line and find the equilibrium point for that load line to find the equilibrium i c and V CE From Kirchoff’s voltage law applied to the output circuit: