618 326

1. 618 326 Physics of electronic materials and devices I Lecture 12

2. Unipolar Transistor • An unipolar transistor is well known as “junction field effect transistor (JFET)”. • It is called unipolar since the current is transported by only majority carriers unlike in the case of bipolar transistor (BJT). • This current flow is controlled by an externally applied electric field. • A JFET can be used as a switch or an amplifier.

3. Unipolar Transistor • The active region of the JFET consists of a lightly doped n-type channel flanked by two heavily doped p+-gate regions. • Ohmic contacts attached to the two ends of the channel are known as the drain and source terminal through which the channel current flows.

4. Unipolar Transistor

5. Unipolar Transistor • Under normal operating conditions, a reverse bias is applied across the gates. • This makes the space-charge regions extending into the channel so that the channel resistance is increased. • Therefore, the current flow between the source and the drain is modulated by the gate voltage.

6. Unipolar Transistor • Consider at VG = 0, as VD increased, current ID increases and the depletion regions are increased into the channel near the drain due to the large value of Vx near the drain compared to that near the source.

7. Unipolar Transistor • By increasing VD into the point where depletion regions meet near the drain, this condition is called ‘pinch-off’. • The current ID at this point becomes saturated (IDSS) and the applied voltage is called ‘pinch-off voltage’.

8. Unipolar Transistor • Without a gate bias voltage, the transistor has a conducting channel between the source and drain terminals. • This is an ON state and the transistor is called ‘normally on’ JFET.

9. Unipolar Transistor • To calculate the pinch-off voltage, assume that the active channel is symmetric so that we can examine it only a half of its area. The depletion region width can be found as (1)

10. Unipolar Transistor • When a gate is reverse biased (2) • The depletion layer width at the pinch-off is equal to the channel height (h = a) and VDG = VP(pinch-off voltage). (3)

11. Unipolar Transistor = internal pinch-off voltage The drain current consists of electron-drift component only as (4) where 2(a-h)W is a cross-section area.

12. Unipolar Transistor • The depletion layer width as a function of x may now be written as (5)

13. Unipolar Transistor Substituting (4) into (5) and integrating, it yields (6)

14. Unipolar Transistor

15. Example 1 • An n-channel Si-JFET with ND = 1017 cm-3 and gates NA = 1020 cm-3. Half channel width a = 0.25 μm. Calculate (a) Vbi (b) drain voltage for pinch-off at VG = 0. Given: T = 300 K, ni = 1.45 x 1010 cm-3, and εr = 11.8

16. Example 1 • Soln

17. MIS junctions • MIS = Metal-Insulator-Semiconductor (MIS) junctions • If the insulator is oxide, MIS is then called “MOS” (metal-oxide-semiconductor). • We will study the MOS diode since it is an important device for advanced integrated circuits e.g. MOSFETs and CCDs

18. MIS junctions

19. MIS junctions • As seen from the figure that the energy difference between the metal work function qm and the semiconductor work function qs is zero. • The only charges existing in the diode are those in the semiconductor and in the metal with opposite sign. • There is no carrier transportthrough the oxide since the resistivity of the oxide is very high.

20. MIS junctions • When an ideal MOS diode is biased, there are 3 cases to be considered at the semiconductor surface. • For the case of p-type semiconductor, when a negative voltage (V < 0) is applied to the metal plate, the majority carriers (holes) will be induced at the oxide-semiconductor interface. • The bands near the semiconductor surface are bent upward but the Fermi level remains the same due to no current flow.

21. MIS junctions • The hole density can be found by (7) • The bending of the energy band causes an increase of Ei – EF. • This causes an increase of hole density near the oxide-semiconductor surface and this is called the ‘accumulation case’.

22. MIS junctions • Accumulation Depletion

23. MIS junctions • When a small positive voltage (V >0) is applied across the metal and semiconductor, the energy band will bend downward, the holes are depleted. • This is called the ‘depletion case’.

24. MIS junctions Inversion • If we keep increasing the applied positive voltage, the energy band will bend downward even more.

25. MIS junctions • Thus, the intrinsic level Ei at the surface can cross over the Fermi level (Ei< EF). • This means there is a large electron concentration in the conduction band at the oxide-semiconductor interface.

26. MIS junctions • This means there is a large electron concentration in the conduction band at the oxide-semiconductor interface. This electron concentration is given by (8)

27. MIS junctions • In this case, (EF – Ei) > 0, this implies the electron concentration np at the interface is large than niand the holes given by (7) is less than ni. • Therefore, the number of minority carriers (electrons) at the interface is greater than that of majority carriers (holes). • This is called the ‘inversion case’.

28. MIS junctions • At the strong inversion where the conduction band edge comes close to the Fermi level, the depletion layer width reaches the maximum (Wm). • The charge per unit area in the semiconductor can be found by

29. MIS junctions • The surface potential  = s where the carrier densities can be found by (10)

30. MIS junctions • s < 0Accumulation of holes (bands bend upward) • s = 0 Flat-band condition • B > s > 0Depletion of holes (bands bend downward) • s = B Midgap with ns = np = ni • s >B Inversion (bands bend downward)

31. MIS junctions • A criterion for the strong inversion is that the electron concentration at the surface nsequals to that of the substrate impurity concentration NA. Since , from (10), this yields (11)

32. MIS junctions • The maximum width of the surface depletion can be expressed by (12)

33. Example 2 • For an ideal metal-SiO2-Si diode having NA = 1017 cm-3, calculate the maximum width of the surface depletion region.

34. MIS junctions • The MOS diode is the very important device for advanced integrated circuits such as MOSFET or CCD (charge-coupled devices). • There are many kinds of FETs, such as MESFET (metal-semiconductor FET), but their output characteristics are similar. • They all have a linear region at low-drain biases. • As the bias increases, the output current eventually saturates.

35. Dielectric Materials • Dielectric material is an insulator by definition of having no transport occurring. • It also has a very wide energy gap, so there is no free charge for conduction. Electric field Current Metal or Semiconductor Rearrangement of charge (polarization of material) Electric field Dielectric Material

36. Dielectric Materials • There are 2 important dielectric materials: Piezoelectric materials and Ferroelectric materials. • Piezoelectric materials are dielectric materials which produce internal electric field (E) in response to applied external force (strain) and vice versa. • They are widely used in applications such as transducers, microphones, and surface acoustic wave (SAW) devices.

37. Dielectric Materials • While Ferroelectric materials are dielectric which exhibit inherent dipole moment that displays hysteresis under applied electric field. • They also have permanent dipole moment and even there is no applied electric field, their polarization are not necessarily zero.

38. Dielectric Materials • Ferroelectric materials are such as LiNbO3 (Lithium Niobate), LiTaO3 (Lithium Tantalate), or BaTiO3 (Barium Titanate). • Examples of applications for dielectric materials are optical fibers and electrooptic devices.

39. Optical fibers • An optical fiber is a waveguide at optical frequencies. • It is flexible and can guide optical signals over distance of many kilometers to a receiver, similar to the way a coaxial cable transmits electrical signals.

40. Optical fibers • It consists of 2 concentric cylinders made of purified silica (quartz). • The inner cylinder has larger refractive index than that of the outer. The inner cylinder is called ‘core’ and the outer one is called ‘cladding’.

41. Optical fibers • Light propagation is confined to higher index core region with n 0.005 - 0.01. • Most optical fibers are used as media for high capacity-high speed channels for transmission.

42. Electrooptic devices • The electrooptic devices are components produced in Ferrroelectric material that allow manipulating light propagation by electrical signals. • Similarly to fibers, they require high index region to confine the light.

43. Electrooptic devices • By means of applied external voltage, the internal electric field E causes a change in polarization which induces index change n and allows control of the phase for the output light. • Appications: • light modulators • light switches • light filters.

44. Photonic devices • Photonic devices are semiconductor optical devices with the photon playing a major role. • We will consider three kinds of photonic devices: • light-emitting diodes(LEDs). • laser diodes. • photodiodes.

45. Photonic Devices

46. Photonic Devices • There are three mechanisms for interaction between a photon and an electron in a solid: • Absorption. • Spontaneous emission. • Stimulated emission.

47. Photonic Devices a) Absorption. b) Spontaneous emission. c) Stimulated emission. • Any transition between these two states involves the emission or absorption of a photon with frequency 12 given by h12 = E2 – E1.

48. Light-Emitting Diodes (LEDs) • The dominant operating process for LEDs is spontaneous emission. • They can emit spontaneous radiation in ultraviolet, visible, or infrared regions.

49. Light-Emitting Diodes (LEDs) • The maximum sensitivity of the eye is at 0.555 μm. • The eye response falls to nearly zero at the spectrum about 0.4 μm and 0.7 μm. • For normal vision at the peak response of the eye, 1 W of radiant energy is equivalent to 683 lumen.

50. Light-Emitting Diodes (LEDs) • (a) conventional diode header • (b) a package suited for a transparent semiconductor such as GaP.