summery for chapter 1, Study notes of Calculus for Engineers

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ELECTRONIC

CIRCUITS

(ELTL 322)

Chapter 1 :

Semiconductor Diode

Diode Applications

Objectives: ❖ Use a diode in common applications. ❖ Analyze the V-I characteristic of a diode. ❖ Explain how the three diode approximations differ. ❖ Explain and analyze the operation of half - wave rectifiers. ❖ Explain and analyze the operation of full - wave rectifiers. ❖ Explain and analyze power supply filters and regulators. ❖ Explain and analyze the operation of diode limiters and clampers.

The Semiconductor Diode Figure 2(a) shows the typical diode packages with terminal identif ication. Figure 2(b) shows the Sur face - Mount Diode Packages. Figure 2

Operation with Open-Circuit Terminals Figure 3 shows a pn junction under open - circuit conditions (the external terminals are open). The charges on both sides of the depletion region cause an electric field E to be established across the region in the direction indicated in Fig.3. Figure 3

E n g. Ta l a l & S h a k e r A l h a r b i 7 Because the concentration of holes is high in the p region and low in the n region, holes diffuse across the junction from the p side to the n side. Similarly, electrons diffuse across the junction from the n side to the p side. These two current components add together to form the diffusion current ID, whose direction is shown in Fig. 3.

The Diffusion Current lD

E n g. Ta l a l & S h a k e r A l h a r b i 8 Moving electrons by drif t from p to n and moving holes by drif t from n to p will form the drif t current IS whose direction is shown in Fig. 3. Since the current IS is carried by thermally generated minority carriers, its value is strongly dependent on temperature ; however, it is independent of the value of the depletion voltage Vo. Under open-circuit conditions no external current exists; thus the two opposite currents across the junction must be equal in magnitude:

The Drift Current ls and Equilibrium

ID = IS

This equilibrium condition is maintained by the barrier voltage Vo.

Example 1.

Consider a pn junction in equilibrium at room temperature (T= 300 K) with doping

concentrations of NA= 10

18

/cm

3

and ND= 10

16

/cm

3

. Assume the Boltzmann’s

constant k= 1. 38 x 10

• 23

J/K, q= 1. 6 ╳ 10

• 19

C, and ni= 1. 5 ╳ 10

10

/cm

Calculate

the following:

a. The thermal voltage VT.

b. The built-in voltage Vo.

Solution 1. a.

Vo = VT ln

NA ND

n

i

Vo = 25. 9 × 10

ln

× 10

1. 5 × 10

= 0. 814 V

VT =

k T

q

1. 38 × 10

× 300

−

≈ 25. 9 × 10

= 25. 9 mV

b.

The requirements of forward bias are first the negative side of VBIAS is connected to the n region of the diode and the positive side is connected to the p region and a second requirement is that the bias voltage, VBIAS, must be greater than the barrier voltage. what happens when a diode is forward-biased is shown in Figure 6. More holes to diffuse from p to n and more electrons to diffuse from n to p. Thus, the diffusion current ID increases substantially. Figure 6

ID

Resulting in a reduced barrier voltage (Vo — Vbias) across the depletion region. This will be accompanied by reduced depletion-region charge and correspondingly narrower depletion-region width W. Thus, the diffusion current ID increases substantially and can become larger than the drif t current Is as in Figure 7. The current I in the external circuit is the difference between ID and Is. Its value is determined by the forward-bias voltage Vbias (VF). Figure 7

what happens when a diode is reverse-biased is shown in Figure 9. Because unlike charges attract, the positive side of the bias-voltage source “pulls” the free electrons away from the pn junction. As the electrons flow toward the positive side of the voltage source, additional holes are created at the depletion region. This results in a widening of the depletion region and fewer majority carriers. a reverse-bias voltage is sufficient to cause ID ≃ 0 , and the current across the junction and through the external circuit will be equal to Is. Figure 9

IS

Reverse Current:

The extremely small current that exists in reverse bias as in Figure 10. Figure 10

Voltage-Current Characteristic of a Diode

V-I Characteristic for Forward Bias

Voltage-Current Characteristic of a Diode Figure 11

V-I Characteristic for Forward Bias

As you gradually increase the forward-bias voltage, the forward current and the voltage across the diode gradually increase, as shown in Figure 11. When the forward-bias voltage is increased to a value where the voltage across the diode reaches approximately 0. 7 V (barrier potential), the forward current begins to increase rapidly.