## 2.6 Summary

### 2.6.1 Summary to: 2. Semiconductor Materials and Products

| Structure and size matter! |  |
Typical Si wafer: | 300 mm diameter, 850 µm thick, perfect single crystal
Solar cell: Si | Single crystalline, bulk. Poly crystalline, large grain, bulk. Polycrystalline, micro grain, "thick" film Polycrystalline, nano grain, thin film. Amorphous (plus H), thin film
|  | Mostly we need single crystals, as perfect (and as large) as possible |
|  | Either in bulk, or thin films |
|  | If thin film, substrates matter. |
| For some applications (solar cell , LCD, ...) polycrystalline or amorphous semiconductors are used |
|  | "CIGS" or CdTe for solar cells. |
|  | Amorphous or poly-Si for LCD transistor matrix |
| Important elemental semiconductors are Si and marginally Ge |  |
Some important Properties | Remarks
Lattice type, lattice constant | Structure independent
Melting point, diffusion constants
Bandgap type and energy
Dielectric constant
Thermal expansion coefficient
Doping range | Structure dependent
Transport of electron / holes (mobility, life time, diffusion length, ..
Unwanted levels in bandgap
|  | Forget Se, C, P, As and b |
| Compound semiconductors are important |
|  | Group IV and compounds: SiGe, SiC |
|  | III-V compounds (Al, Ga, In) - (N, P , As, Sb). Important GaAs, GaxAl1-xAs, GaP, InP, .. |
|  | Chalkogenides AxBy(S, Se, Te)2. Important "CIGS" = CuInxGa1-xSe2 |
|  | "Newcomers" like organic semiconductors, Metal oxides (e.g. TiO2) |
| Properties matter! Some properties are rather independent of the structure (= defects), others can be structure sensitive |
| What counts in the end are products that sell and make a profit! |  |
Integrated circuits, Solar cells, Liquid crystal displays, Micro electronic and mechanical systems, Light emitting diodes, (Diode) Lasers, Sensors, ...
|  | Besides the direct semiconductor products, there are also products that contain semiconductors (PC's, Cars, TV's, any modern machine,...) and products that are needed to make semiconductor products (crystal growers, ovens, ion implanters, ..). |

| Silicon, and only Si, enables integrated circuits of amazing complexity, with billions of transistor on one chip |  |
|  | Two kinds of integrated transistors exist. MOS - the absolute majority bipolar - if speed counts |
|  | Wafers diameter are up to 300 mm (2007), smallest (lateral) structures on a wafer are in or below the 100 nm range. |
|  | Integrated circuits are packaged chips with some connections to the outside world |
| Besides integrated circuits, Si is increasingly used for other semiconductor products: |  |
|  | Solar cells based on Si consume more Si than IC's, and demand rapidly increasing Si production. The key point of Si solar cell technology is to have high efficiencies h at low prices. |
|  | Microelectronic and micro-mechanic (and micro-optics and micro-fluidic and...) = MEMS systems find increasing uses for many tasks. |

| III-V semiconducrors combine the group III elements Al, Ga, In) with the group V elements N, P , As, Sb; giving 12 possible combinations. |  |
Properties | Si | GaAs | InP | GaP | GaN | In0,53Ga0,47As
--- | --- | --- | --- | --- | --- | ---
Band gap [eV] | 1,12 | 1,42 | 1,35 | 2,26 | 3.39 | 0,75
Type | Indirect | Direct | Direct | Indirect | Direct | Direct
Lattice | fcc | fcc | fcc | fcc | hex | fcc
|  | The most important ones are probably GaAs, InP GaP and GaN |
|  | Band gap energies and types vyr,; lattice are wurtzite or zincblende (= fcc) and sphalerite ( = hex) |
| Ternary and quaternary (IIIxIII1-xVyV1-y) compounds are relatively easy to make. |  |
|  | Properties like band gap, lattice constant, refractive index then adjustable to some extent. |
|  | Main materials for optoelectronic products. Some high-speed and sensor applications. |
|  | "Master diagram" = bandgap vs. lattice constant: of elementary importance for semiconductor technology. |

| Germanium (Ge) and SiC |  |
|  | Germanium was almost "useless" but is experiencing some comeback now (2007) in conjunction with Si technology. |
|  | SiC is very difficult to obtain as a good single crystal (many polytypes) but has some desirable properties for high speed or high power devices |
| II-Vl semiconductors are objects of heavy research but hardly used for products at present. |
|  | The "hot" contenders CdTe used for solar cells and actually on the market, and, maybe ZnO in the near future. |  |
| "Chalcogenides", meaning compounds with "Chalcogens", i.e. S, Se, and Te as major elements are often semiconductors |  |
|  | Oxygen, in the same IIa group, forms "oxides"! |  |
|  | The most prominent representative of chalcogenides is "CIS" (CuInSe2) or better "CIGS" (CuInxGa1-xSe2) used for solar cells and actually on the market. |  |
| Organic semiconductors. A relatively recent addition to the club, organic semiconductors seem to have a bright future in optoelectronics |  |
|  | OLED's are on the market, in particular as part of a flat panel display; the first OLED based TV screen has been announced for 2008. |  |
|  | The big problem of OLED's is their sensitivity to oxygen. |  |
Exercise 2.6-1
All Questions to 2.

© H. Föll (Semiconductor Technology - Script)