Chapter 4. MOS SystemsTotal 3 hours.

The Adventure of CarriersThe description must now borrow a picture from the classical books of adventure. To place a negative electrode against a semiconductor with negative carriers - this is like bringing a ship up to a quay in the Orient, with the yellow flag of the plague hoisted. The place becomes deserted by its carriers. Unloading - current - is blocked. But exchange that negative flag of pestilence for a positive sign and the carriers will return, the contact becoming conducting. Electrically this is called rectification. In those seafaring tales it was perhaps possible to induce the carriers to return, without striking the flag, merely by throwing some gold coins on the quay, thus positively destroying the insulation. It is possible to destroy the blockade in the semiconductor in a similar fashion by throwing in some positive holes around which the negative carriers will gather. This is transistor action. It is a fine thing that the carriers' strike can be broken up by rather few holes, which do not cost much energy.

Energy Levels

Equilibrium Band DiagramEquilibrium implies the existence of a path for charge flow between the metal and the silicon other than the oxide.But a barrier for electron flow along both directions

Voltage Drop across OxideDepends upon specific oxide properties.

Flat-Band Condition

Accumulation

Depletion

Inversion

Onset of Strong Inversion

MOS Capacitor

Capacitance of MOS Systems

CapacitanceAccumulation: Determined by oxideFlat band: Debye length (no surface potential)Depletion: Width of the surface depletion layerInversionLow frequency: Inversion layer population can followHigh frequency: Inversion layer cannot respond; charge modulated by the movement of carriers at the far edge of the depletion layerDeep depletion: Inversion layer cannot form at all

Charge in OxideFlat band

Charge-Coupled DevicesHow is an image stored digitally?

How to acquire an image? What are the basic principles of charge-coupled devices (CCDs)?pixels, or picture elements

Charge-Coupled DeviceCCDs consist of almost entirely close spaced array of MOS capacitors. In a MOS capacitor, the electrical signal is represented by the channel (inversion layer) charge, which can be transferred to the channel region of the adjacent device.

The nine-stage CCD illustrates how information is transferred from left to right, by changing gate voltages.