Basics of Semiconductors

1

Outline

Basic knowledge Carrier generation Carrier recombination Carrier transport P-N junction Solar cells

2

Basic knowledge

3

Crystal structure Diamond lattice: group IV

elements (e.g. C, Si, Ge etc)

4

Crystal structure

5

Zincblende lattice: IIIV compound (e.g. GaAs) and IIVI compound (e.g. CdTe)

6

7Basic of Energy Gap (Carbon)

Isolated carbon atoms contain six electrons, which occupy the 1s, 2s and 2p orbital in pairs.

As the lattice constant is reduced, there is an overlap of the electron wavefunctions occupying adjacent atoms.

This leads to a splitting of the energy levels consistent with the Pauli exclusion principle. The splitting results in an energy band.

At zero Kelvin, the lower band is completely filled with electrons and labeled as the valence band. The upper band is empty and labeled as the conduction band.

8Basic of Energy Gap

A simplified energy band diagram used to describe semiconductors. Shown are the valence and conduction band as indicated by the valence band edge, Ev, and the conduction band edge, Ec. The vacuum level, Evacuum, and the electron affinity, qX, are also indicated on the figure.

Energy gap variation with temperature

10

Doping of Si

11

Atoms with one more valence electron than silicon are used to produce "n-type" semiconductor material, which adds electrons to the conduction band and hence increases the number of electrons. Atoms with one less valence electron result in "p-type" material.

In doped material, there is always more of one type of carrier than the other and the type of carrier with the higher concentration is called a "majority carrier", while the lower concentration carrier is called a "minority carrier."

Doping of semiconductor

12

Before doping After doping

Doping and PV Work-1

13

Carrier generation by photons

14

Photoelectric Effect

Incoming photons on the left strike a metal plate (bottom), and eject electrons, depicted as flying off to the right.

The first photovoltaic device was demonstrated in 1839 by Edmond Becquerel

Absorption of light

by H.Y. Yu / Jan. 2010 15

Absorption of light

16

Absorption of light

by H.Y. Yu / Jan. 2010 17

Absorption Coefficient The absorption coefficient

determines how far into a material light of a particular wavelength can penetrate before it is absorbed.

In a material with a low absorption coefficient, light is only poorly absorbed, and if the material is thin enough, it will appear transparent to that wavelength.

18

Absorption Depth

dependence of on wavelength The absorption depth is given by the inverse of the

absorption coefficient, or -1.19

Carrier generation

where N0 = photon flux at the surface (photons/unit-area.sec.); = absorption coefficient; and x = distance into the material

20

Carrier generation The net (total)

generation is the sum of the generation for each wavelength.

(Fig. left) Generation rate of electron-hole pairs in a piece of silicon as a function of distance into the cell. The cell front surface is at 0 m and is where most of the high energy blue light is absorbed.

21

22

Carrier recombination

by H.Y. Yu / Jan. 2010 23

24

Different types of recombination Recombination can be classified as: Unavoidable:

a. photon (radiative recombination), b. kinetic energy to another carrier such as phonon

(lattice), like in Si and Ge. Avoidable:

Traps assistant recombination (SRH), Traps includes bulk trap and surface and interface traps.

Radiative (Band-to-Band) Recombination

Dominates in direct bandgap semiconductors

25

Auger Recombination

Auger recombination takes place in non-direct band-gap semiconductor, and it is the dominant recombination mechanism in Si and Ge (high doping) 26

Shockley-Read-Hall or SRH recombination

Recombination through defect

27

Carrier Lifetime

is the minority carrier lifetime, n is the excess minority carriers concentration and R is the recombinaton rate.

A measure of how long a carrier is likely to stay around for before recombining.

A silicon wafer has a long lifetime" usually means minority carriers generated in the bulk of the wafer by light or other means will persist for a long lifetime before recombining.

28

Diffusion Length

L is the diffusion length in metres, D is the diffusivity in m/s and is the lifetime in seconds.

In silicon, the lifetime can be as high as 1 msec. For a single crystalline silicon solar cell, the

diffusion length is typically 100-300 m.

29

Surface Recombination

High recombination rate at a surface depletes this region of minority carriers. A localised region of low carrier concentration causes carriers to flow into this region from the surrounding, higher concentration regions (diffusion). Therefore, the surface recombination rate is limited by the rate at which minority carriers move towards the surface.

30

Since the surface of the solar cell represents a severe disruption of the crystal lattice, the surfaces of the solar cell are a site of particularly high recombination.

Surface Recombination

A parameter called the "surface recombination velocity", in units of cm/sec, is used to specify the recombination at a surface.

In a surface with no recombination, the movement of carriers towards the surface is zero, and hence the surface recombination velocity is zero. In a surface with infinitely fast recombination, the movement of carriers towards this surface is limited by the maximum velocity they can attain, and for most semiconductors in on the order of 1 x 107 cm/sec.

31

To achieve a high efficiency solar cell: To maximize charge generation To minimize carrier recombination

32

Diffusion

Diffusion in semiconductors is the motion of charge carriers due to their concentration gradient

Jn and Jp are the diffusion current densities, q -electron charge, Dn and Dp - diffusion coefficients for electrons and holes, n and p - electron and hole concentrations.

33

Drift Transport due to the

movement of carriers in an electric field is called "drift transport".

Jx is the current density in the x-direction, Ex : electric field applied in the x-direction, q : electron charge, n and p : electron and hole concentrations, n and p : electron and hole mobilities. 34

P-N junction

35

P-n junction diodes form the basis not only for solar cells, but for many other electronic devices such as LEDs, lasers, photodiodes and bipolar junction transistors (BJTs).

A p-n junction aggregates the recombination, generation, diffusion and drift effects.

Formation of P-N junction

P-n junctions are formed by joining n-type and p-type semiconductor materials. Since the n-type region has a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type side to the n-type side.

36

P-N junction On the n-type side, positive

ion cores are exposed. On the p-type side, negative ion cores are exposed.

37

An electric field forms between positive ion cores in the n-type material and negative ion cores in the p-type material.

This region is called the "depletion region" (no free carriers). A "built in" potential Vbi due to is formed at the junction.

P-N junction

by H.Y. Yu / Jan. 2010 38

Carrier Movement in Equilibrium

In equilibrium, the net current from the device is zero.

The electron/hole drift current and the electron/hole diffusion current exactly balance out.

by H.Y. Yu / Jan. 2010 39

Three operation modes Thermal Equilibrium: no external inputs such as light or

applied voltage. The currents balance each other out so there is no net current within the device.

Steady State: external inputs such as light or applied voltage, and the conditions do not change with time. Devices typically operate in steady state and are either in forward or reverse bias.

Transient State: not important to solar cells.

40

Diodes under Forward Bias

By applying a positive voltage to the p-type material and a negative voltage to the n-type material.

Reducing the net electric field in the depletion region, and reducing the diffusion barrier and increasing the diffusion current.

41

Diodes under Reverse Bias

In reverse bias, the electric field at the junction increases.

The higher electric field in the depletion region decreases the diffusion current.

42P-N Junction and Diode

Ideal Diode Law

I = the net current flowing through the diode; I0 = "dark saturation current", in the absence of light; V = applied voltage across the terminals of the diode; q = electron charge; k = Boltzmann's constant; and T = absolute temperature (K).

The "dark saturation current" (I0) is a measure of the recombination in a device. A diode with a larger recombination will have a larger I0.

I0 increases as T increases; and I0 decreases as material quality increases. At 300K, kT/q = 25.85 mV, the "thermal voltage".

43

Non-Ideal Diodes

For actual diodes where:

n = ideality factor, a number between 1 and 2 which typically increases as the current decreases.

44

The diode law for silicon -current changes with voltage and temperature.

Basics of SemiconductorsOutlineBasic knowledgeCrystal structureCrystal structureSlide Number 6Slide Number 7Slide Number 8Slide Number 9Energy gap variation with temperatureDoping of SiDoping of semiconductorSlide Number 13Slide Number 14Absorption of lightAbsorption of lightAbsorption of lightAbsorption CoefficientAbsorption DepthCarrier generationCarrier generationCarrier recombinationSlide Number 23Different types of recombinationRadiative (Band-to-Band) RecombinationAuger RecombinationShockley-Read-Hall or SRH recombinationCarrier Lifetime Diffusion LengthSurface Recombination Surface Recombination Slide Number 32DiffusionDriftP-N junctionFormation of P-N junctionP-N junctionP-N junctionCarrier Movement in EquilibriumThree operation modes Diodes under Forward Bias Diodes under Reverse Bias Ideal Diode LawNon-Ideal Diodes