KKK3201

1. KKK3201 Lecture 2

2. EXTRINSIC MATERIAL • The characteristic of semiconductor can be altered by adding impurity through doping process (extrinsic material) • Two type: • N-type • P-type

3. N-TYPE • N-type is created by introducing impurity elements that have five valence electrons (pentavalent) – antimony, arsenic, phosphorus • Note that four covalent bonds are still present, however there is additional fifth electron due to impurity atom • The remaining electron is free to move within the newly formed n-type material • Diffused impurities with five valence electrons are called donor atoms Figure 1.9 Antimony impurity in n-type material

4. P-TYPE • P-type is created by doping with impurity atoms having three valence electrons – boron gallium, indium • Note that there are insufficient number of electrons to complete covalent bonds resulting a hole • This hole is ready to accept a free electron • The diffused impurities with three valence electrons are called acceptor atoms. Figure 1.11 Boron impurity in p-type material

5. Electron versus Hole Flow

6. Majority and Minority Carriers • In an n-type material - electron is called majority carrier and hole the minority carrier • In a p-type material – hole is majority carrier and electron is the minority carrier

7. Semiconductor Diode • Diode is formed by bringing these two material together p- and n-type • Electrons and holes at joined region will combine, resulting in a lack of carriers in the region near the junction (depletion region)

8. Since the diode is two-terminal device, the application of a voltage across its terminals leaves three possibilities: • No bias (VD = 0V) • Foreard bias (VD > 0V) • Reversed bias (VD < 0V) • Each condition will result in a response

9. No Applied Bias (VD = 0V) • Under no-bias conditions, any minority carries (holes) in the n-type material find themselves within the depletion region will pass directly into p-type material • Majority carriers (electrons) of n-type material must overcome the attractive forces of the layer of positive ions in n-type material and the shield of negative ions in p-type material to migrate into the area beyond the depletion region of p-type material. • In the absence of an applied bias voltage, the net flow of charge in any one direction for semiconductor diode is zero

10. Figure 1.14 p-n junction with no external bias

12. Reverse-Bias Condition (VD < 0V) • The number of uncovered positive ions in the depletion region of n-type will increase due to large number of free electrons drawn to the positive potential • The number of uncovered negative ions will increase in p-type resulting widening of depletion region • This region established great barrier for the majority carriers to overcome – resulting Imajority = 0 • The number pf minority carriers find themselves entering the depletion region will not change resulting in minority-carrier flow vectors of the same magnitude • The current exists under reverse-bias conditions is called the reverse saturation current and represented by Is • Therefore, ID= -Is

13. Figure 1-16 Reverse-biased p-n junction

15. Forward-Bias Condition (VD = 0V) • A semiconductor diode is forward-biased when the association p-type and positive and n-type and negative has been established • The application of forward-bias potential will pressure the electrons in n-type and hole in p-type to recombine with ions near the boundary and reduce the width of depletion region • The resulting minority-carrier flow of electrons from p-type to n-type has not changed in magnitude, but the reduction in width of depletion region has resulted in a heavy majority flow across the junction

16. Figure 1.18 Forward-biased p-n junction

17. (1.4) For Forward-bias and Reverse-bias

18. Figure 1.19 Silicon semiconductor diode characteristics