Microelectronic solutions for digital photography
Microelectronic solutions for digital photography

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

Microelectronic solutions for digital photography

1.2 MOS structures

Carefully designed metal–oxide–semiconductor (MOS) structures are a common building block in digital electronics, primarily intended to form part of a transistor-based switch. However, throughout the active regions of a microelectronic chip there will be secondary MOS structures that arise because metal tracks are insulated from the semiconductor substrate by a layer of oxide; equally careful design is necessary to ensure that these do not form part of a switch. The acronym is a mixture of materials classification and materials, but it is so well established that it's too late to argue that conductor–insulator–semiconductor is more generic, so MOS it is.

This text is written presuming that MOS will be pronounced ‘em-oh-ess’. The alternative pronunciation, ‘moss’, is reserved for use within the longer acronym CMOS: ‘see-moss’.

SAQ 1

Note down three important functions that can be performed by MOS structures.

Answer

MOS structures are central to the following functions:

  • electronic switches;

  • the detection of light;

  • electronic memory.

In practice, especially for mass-market consumer products, the semiconductor is silicon; the insulator is then silicon dioxide (see Box 1: Silicon and silicon dioxide). There are various options for the conductor: for several years it was aluminium; then it became expedient to use something more refractory such as heavily doped polycrystalline silicon, deposited from silane; the interconnecting tracks typically involve an exotic intermetallic compound layer followed by copper.

Box 1: Silicon and silicon dioxide

Solid-state electronics has made much use of silicon dioxide and silicon as archetypal insulator and semiconductor. They are excellent partners for the following reasons:

  • Silicon can be manufactured as wafers of extremely pure, single-crystal material.

  • Silicon of high purity can also be grown out of the vapour phase, for instance silicon tetrahydride (silane, SiH4), at elevated temperature.

  • The conductivity of silicon is readily manipulated locally by the addition of controlled quantities of dopant in combination with photolithography.

  • The oxide of silicon – namely silicon dioxide, SiO2 – can be readily grown onto a silicon surface on exposure to oxygen or steam at elevated temperature.

  • Silicon dioxide can also be grown out of the vapour phase at elevated temperature, so it can be incorporated over surfaces other than silicon.

Table 1 shows some bulk physical properties for three silicon-based materials and, as a comparator, aluminium. Exercise 1 will help you to appreciate the data.

Table 1 Comparisons of silicon, silicon dioxide and aluminium at 300 K

Si (intrinsic) Si (heavily doped polycrystalline) SiO2 Al
Structure diamond diamond amorphous FCC
Density / 103 kg m−3 2.3 2.3 2.2 2.7
Dielectric constant 12 12 4
Resistivity / Ω m 2.3 × 103 1 × 10−5 1 × 1015 2.7 × 10−8
Breakdown strength / V m−1 3 × 107 1 × 109
Melting temperature / °C 1415 1415 1600 660
Energy gap / eV 1.1 1.1 9.0
Atom density / 1028 atoms m−3 5 5 2.2 6

Exercise 1

Using data in Table 1, say:

  • (a) how much less conductive than aluminium is heavily doped polycrystalline silicon

  • (b) how much less conductive than aluminium is pure silicon

  • (c) how much less resistive than silicon dioxide is pure silicon.

Answer

  • (a) We want the inverse ratio of resistivities: heavily doped polycrystalline silicon is about 400 times less conductive than aluminium.

  • (b) We want the inverse ratio of resistivities: pure silicon is about 1011 times less conductive than aluminium.

  • (c) We want the ratio of resistivities: silicon is 4 × 1011 times less resistive than silicon dioxide.

There are two other things you should know about silicon and aluminium. First, they form a eutectic alloy at 11.7% Si which melts at 577 °C, so don't go thinking that process temperatures can approach 660 °C (see Table 1) once something has been made involving these two elements in contact. In fact, 450 °C is generally recognised as the safe upper limit. Second, aluminium – like gallium, boron and indium – is a well-known dopant capable of rendering silicon a p-type semiconductor.

T356_2

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus