ECE321 Electronics I

Transcription

1 ECE321 Electronics I Lecture 4: Physics of Semiconductor iodes Payman Zarkesh-Ha Office: ECE Bldg. 230B Office hours: Tuesday 2:00-3:00PM or by appointment pzarkesh.unm.edu Slide: 1

2 Review of Last Lecture Electrical Property of Materials Energy Band iagrams Semiconductor Materials n-type and p-type Semiconductor Materials Mass ction Law Slide: 2

3 Today s Lecture Carrier Transport in Semiconductors rift Current iffusion Current P Junction epletion Region Reverse Biased P Junction Forward Biased P Junction Slide: 3

4 rift Current ue to Electric Field This is what happens in a typical resistors irection of electrons and holes under externally applied electric field (E) is shown below Slide: 4

5 rift Current Equations in Semiconductor (c) What is e resistivity of e material? (c) 216 = 10 / ρ ρ = 46.3 mω cm Ref: ρ cu = 1.7 µω cm Slide: 5

6 iffusion Current ue to ensity Gradient This happens when e carrier concentration is different from one side to e oer side n analogous case happens in gas container below, where e gas molecules diffuses from higher density to lower density Slide: 6

7 iffusion Current Equations n and p are e electron and hole diffusion constants Slide: 7

8 P Junction What does happen when P-Type semiconductor meets -Type Semiconductor? P P Slide: 8

9 P Junction Some electrons will cross e junction and fill holes. pair of ions is created each time is happens. s is ion charge builds up, it prevents furer charge migration across e junction. The junction goes into equilibrium when e barrier potential prevents furer diffusion. t 25 degrees C, e barrier potential for a silicon pn junction is about 0.5 to 0.7 volts. P epletion Layer Slide: 9

10 epletion Region in Equilibrium The barrier potential across deletion region is computed as: bi Ln n 2 i kt q The wid of depletion region is computed as: W 2 q si bi P epletion Layer Slide: 10

11 Example: epletion Region Calculate e built-in potential and depletion wid at 300K in a diode if n i =1.062x10 10 cm -3, ε si = 1.04x10-12 F/cm, =10 16 cm -3, and = cm bi Ln 0.026Ln n 10 i W si bi cm 0. um q P epletion Layer um Slide: 11

12 epletion Region Phenomena The depletion region is so named because it is formed by removal of all free charge carriers leaving none to carry a current. epletion region is erefore an insulator region wiin a conductive material. Many modern semiconductor devices function based on depletion region phenomena. Example: iodes, Solar cells, Bipolar Transistors, and MOS Transistors, ariable Capacitance iodes Slide: 12

13 P Junction Under pplied oltage The applied external voltage,, on e diode will directly affects e built-in potential In reverse bias, e external voltage adds up to e built-in potential increasing e effect of bi In forward bias, e external voltage is against e built-in potential reducing e effect of bi epletion region wid also depends on e voltage applied to e diode. Review: P - bi + W 2 si q bi epletion Layer bi Ln 2 ni kt q + - Slide: 13

14 Reverse Biased P Junction In CMOS transistors, at are currently used in semiconductor industry, all P junctions are normally in reverse bias condition. When <0 e wid of depletion region increases which results in no conduction. I () Reverse bias ( < 0) () Slide: 14

15 Forward Biased P Junction When 0< < bi e wid of depletion region decreases. Once >= bi, en depletion region disappears and e diode starts conducting. The current in a diode can en be approximated as: I I e 1 S I () kt q Forward bias ( > 0) () Slide: 15

16 iode s Parasitic Capacitance The depletion region, which act as an insulator, behaves like a capacitor in a P junction. The junction capacitance can be calculated as: C j si W 2 si q si bi q si 2 1 bi bi C j0 1 bi Where: P - bi + bi Ln 2 ni kt q epletion Layer + - Slide: 16

17 ariable Capacitance iode (aractor) Generally, e junction capacitance in a diode is undesirable because it slows down e switching of e diode. Sometimes, we make a good use of is parasitic capacitance. The capacitance of a reverse bias diode can be adjusted by e amount of bias on a diode. If e C input signal is small enough, e reverse bias diode can be seen as a capacitor, whose value can be adjusted by its reverse bias voltage. 1MΩ bias signal ariable Capacitance iode Slide: 17

18 Small Signal Model for iodes I I S e 1 I kt q ynamic (Small Signal) Conductance g d di d Q I S e Q I Q ynamic (Small Signal) Resistance r d 1 g d I Q Slide: 18