12. Applications Flashcards
Where is the fermi level relative to the band gap for insulators?
In the middle of the band gap
What is Poole-Frenkel conduction?
Poole–Frenkel conduction is a means by which an electrical insulator can conduct electricity.
Electrons can move (slowly) through an insulator by the following method. The electrons are generally trapped in localized states (loosely speaking, they are “stuck” to a single atom, and not free to move around the crystal). Occasionally, random thermal fluctuations will give that electron enough energy to get out of its localized state, and move to the conduction band. Once there, the electron can move through the crystal, for a brief amount of time, before relaxing into another localized state (in other words, “sticking” to a different atom). The Poole–Frenkel effect describes how, in a large electric field, the electron doesn’t need as much thermal energy to get into the conduction band (because part of this energy comes from being pulled by the electric field), so it does not need as large a thermal fluctuation and will be able to move more frequently.
Name mechanisms through which insulators can conduct
- Poole-Frenkel conduction (large electric field can move e- into the conduction band
- Tunnelling (e- can tunnel through a sufficiently small barrier)
- Hot carrier injection (carrier gains enough KE under a high field that there’s a probability of thermal excitation over the barrier, or to tunnel through the barrier)
How does flash memory work?
Flash memory consists of an FET with an extra, electrically isolated floating gate between the external/control gate and the channel. The floating gate stores charge, by electrons tunnelling across the tunnel oxide, to be stored in the floating gate. The charge shifts the threshold (switch-on) voltage of the FET controlled by the control gate to more positive voltages (less negative). This charge can be retained for years. The charge is emptied through the oxide through application of a large reverse field.
What is SRAM?
Static random-access memory (static RAM or SRAM) is a type of semiconductor memory that uses bistable latching circuitry (flip-flop) to store each bit. SRAM exhibits data remanence, but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered.
What is DRAM?
Dynamic random-access memory (DRAM) is a type of random access semiconductor memory that stores each bit of data in a separate tiny capacitor within an integrated circuit. The capacitor can either be charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1. The electric charge on the capacitors slowly leaks off, so without intervention the data on the chip would soon be lost. To prevent this, DRAM requires an external memory refresh circuit which periodically rewrites the data in the capacitors, restoring them to their original charge. This refresh process is the defining characteristic of dynamic random-access memory, in contrast to static random-access memory (SRAM) which does not require data to be refreshed.
What is the difference between SRAM and DRAM?
SRAM does not need to be a periodic refresh of its memory, where as DRAM does
What is the name for the law which describes the exponential increase in transistor numbers on a chip?
Moore’s law
What is the common method for transistor scaling?
Constant field scaling
It ensures all materials run at the same drive field, so not breakdown will occur due to increasing fields and electron velocity is the same.
What limits the device structures and thus number of transistors that can be put on a chip (from a manufacturing standpoint)? (and describe this process)
It is limited by photolithography, a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask/optical mask to a photosensitive chemical photoresist on the substrate. A series of chemical treatments then either etches the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist.
What places a limit on the resolution limit of photolithography?
The limit is set by the ‘Young’s slits’ resolution of light:
d = λ / NA, where λ is the lights wavelength and NA is the numerical aperture of the lens. Therefore smaller wavelength light and greater numerical aperture (using lenses) increases the resolution of photolithography.
What is the current method for introducing dopants into a material, and what is a downside to this?
Ion implantation - allows the dopants to be placed in precisely defined patterns and depths, as depth is controlled by the ion energy. Downside is that ion bombardment creates damage + vacancies.
How can defects/vacancies by removed from a semiconductor?
By annealing, called ‘Dopant Activation’. After implantation heat up to ~1000C for 5s and the vacancies and interstitials move out to the edge of the sample and annihilate.
What is the limit of SiO2 as a gate oxide?
AS the channel gets smaller, the gate oxide thickness also gets smaller, and it is now at a value where the oxide is thin enough to allow direct quantum mechanical tunnelling of electrons across it, so it is not an insulator any more. This leads to an unacceptably high gate leakage current and thus excessive static power dissipation in the device.
How does gate leakage current scale with decreasing oxide thickness?
the current increases exponentially
What value for a oxide determines how good of an insulator it will be for the gate?
Having a higher dielectric constant ε
