MOS Capacitors#
Structure#
A metal-oxide-semiconductor capacitor (MOS capacitor) is an electronic component made up of three superimposed layers:
- The top layer is the metal layer also known as the M layer or the gate. Historically, it was made of aluminium, but the modern standard is heavily doped silicon. However, for cutting-edge components on the scale of a few nanometers, manufacturers again use metals like tantalum.
- The middle layer is the insulation layer or oxide layer. It is usually made up of silicon dioxide \(\mathrm{SiO}_2\).
- The bulk layer or substrate layer is the third layer and is made up of doped semiconductor, most commonly silicon.
Operation#
Applying a voltage across a MOS capacitor influences the charge distribution inside its substrate and its behavior.
Accumulation#
Applying a negative voltage \(V_{\text{GB}} \lt 0\) to the gate of a MOS capacitor with a p-type substrate or a positive voltage \(V_{\text{GB}} \gt 0\) to the gate of a MOS capacitor with an n-type substrate causes it to enter accumulation mode.
The holes / free electrons are attracted to the gate by the voltage and accumulate under it.
Depletion#
Applying a positive voltage \(V_{\text{GB}} \gt 0\) to the gate of a MOS capacitor with a p-type substrate or a negative voltage \(V_{\text{GB}} \lt 0\) to the gate of a MOS capacitor with an n-type substrate causes it to enter depletion mode as long as this voltage does not exceed a particular threshold \(V_{\text{th}}\).
The holes / free electrons are pushed away from the gate and a region devoid of mobile charge carriers forms under it.
Inversion#
If the applied voltage exceeds the threshold (\(|V_{\text{GB}}| \gt |V_{\text{th}}|\)), it becomes strong enough to not only push away the characteristic charge carriers of each doped semiconductor but to also attract the small number of thermically released charge carriers of the opposite type.
A region of mobile electrons forms under the gate of a MOS capacitor with a p-type substrate and a region of mobile holes forms under the gate of a MOS capacitor with an n-type substrate. Within this region, the semiconductor essentially behaves as if it were doped in the opposite way.