@@ -46,15 +46,15 @@ _(based on chapters 17–18 of the book)_
Up until this point, we focused on calculating and understanding the band structures.
However, the dispersion of a band is only part of the story.
An empty band is not gonna lead to any interesting physical properties no matter how sophisticated it is.
An empty band is not going to lead to any interesting physical properties no matter how sophisticated it is.
Therefore, it is also important *how* bands are filled by the particles.
By carefully controlling the distribution of particles in the bands, we are able to engineer material properties that we require.
Without a doubt, the greatest example is *semiconductors* - the bedrock of all electronics.
Without a doubt, the greatest example is *semiconductors*—the bedrock of modern electronics.
In this lecture, we shall grasp the basics of semiconductors by learning how to treat bands at different levels of filling.
## Review of band structure properties
Before proceeding further, it is vital to remind ourselves some of the concepts in band structures.
Before proceeding further, let us remind ourselves of important band structure properties.
* Group velocity $v=\hbar^{-1}\partial E(k)/\partial k$.
Descibes how quickly electrons move within the lattice.
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@@ -66,91 +66,94 @@ The quantity is vital in order to calculate any bulk property of the material su
In order to check that everything makes sense, we apply the concepts to the free electron model:
$$H = \hbar^2 k^2/2m$$
$$H = \frac{\hbar^2 k^2}{2m}$$
The velocity is $\hbar^{-1}\partial E(k)/\partial k = \hbar k / m \equiv p/m$.
The effective mass is $m_{eff} = \hbar^2\left(d^2 E(k)/dk^2\right)^{-1} = m$.
So in the simplest case the definitions match the usual expressions.
So in this simplest case the definitions match the usual expressions.
## Filled vs empty bands
We distinguish three different band filling types: filled, empty and partially filled.
Let us start with the most extreme cases - filled and empty bands.
We treat these two cases together, because a completely filled band is very similar to a completely empty band.
For example, both filled and empty bands lead to zero current:
Despite being two opposite extreme extreme cases, filled and empty bands are very similar.
For example, both filled and empty bands carry no electric current:
$$
\begin{align}
j = 2e \frac{1}{2\pi} \int_{-\pi/a}^{\pi/a} v(k) dk = 2e \frac{1}{2\pi \hbar} \int_{-\pi/a}^{\pi/a} \frac{dE}{dk} \times dk = \\
2e \frac{1}{2\pi \hbar} [E(-\pi/a) - E(\pi/a)] = 0.
\end{align}
$$
An empty band has no electrons and thus no current.
On the other hand, a filled band has an equal number of electrons going forwards and backwards which thus cancel and lead to zero current.
Similar results apply to many other physical quantities such as heat capacity and magnetisation.
Similar results apply to many other physical quantities such as heat capacity and magnetization.
Therefore, filled and empty bands do not affect most physical properties and can be disregarded.
As a result, rather than to consider thousands of bands that a material contains, we neglect most of them and just focus on partially filled bands around Fermi level.
As a result, rather than to consider hundreds of bands that a material contains, we neglect most of them and just focus on the handful of partially filled bands around Fermi level.
## From electrons to holes
In order to understand partial filling, we begin with a simple analogy.
Let's say we have 100 chairs: 99 are occupied and 1 is empty.
To keep track which chair is occupied/empty, we could write down the occupation of each and every chair.
However, that would require a lot of unnecessary writing!
Instead, we only need to write down which chair is empty, because we know the rest wil be occupied.
The same philosophy is applied to band filling.
Because completely filled or completely empty bands have simple properties, we may search for a convenient way to describe a band that only has a few electrons missing or extra.
While keeping track of a few electrons has no tricks, even a few electrons missing from a band seem to require considering all the other electrons in a band.
A more efficient approach to describing a nearly filled band is motivated by the following analogy.
Let us say we have 100 boxes: 99 are occupied and 1 is empty.
To keep track which box is occupied/empty, we could write down the numbers of all 99 occupied boxes.
If, on the other hand, we only keep track which single box is empty, we solve the problem with a lot less book-keeping.
The same approach applies to band filling.
Instead of describing a lot of electrons that are present in an almost filled band, we focus on those that are absent.
The absence of an electron is called a **hole**: a state of a completely filled band with one particle missing.
In this schematic we can either say that 8×2 electron states are occupied (the system has 8×2 electrons counting spin), or 10×2 hole states are occupied.
A useful analogy to remember: glass half-full or glass half-empty.
Electron and hole pictures correspond to two different, but equivalent ways of describing the occupation of a band.
In practice, we choose to deal with electrons whenever a band is almost empty and holes when a band is almost full.
Naturally, dealing with electrons is more convenient whenever a band is almost empty and with holes when a band is almost full.
## Properties of holes
Let us compare the properties of electrons and holes.
Let us compare the properties of an electron and a hole obtained by removing that electron.
Since removing an electron reduces the total energy of the system, the hole's energy is opposite to that of an electron $E_h = -E$.
The probability for an electron state to be occupied in equilibrium is given by $f(E)$:
$$f(E) = \frac{1}{e^{(E-E_F)/kT} + 1}.$$
On the other hand, since a hole is a missing electron, the probability for a hole state to be occupied is
Since a hole is a missing electron, the probability for a hole state to be occupied is
therefore for holes both energy $E_h= -E$ and $E_{F,h} = -E_F$.
which is the Fermi distribution of particles with energy $E_h= -E$ and $E_{F,h} = -E_F$.
The **momentum** $p_h$ of a hole should give the correct total momentum of a partially filled band if one sums momenta of all holes.
Therefore $p_h = -\hbar k$, where $k$ is the wave vector of the electron.
Similarly, the total **charge** should be the same regardless of whether we count electrons or holes, so holes have a positive charge $+e$ (electrons have $-e$).
Similarly, the total **charge** should be the same regardless of whether we count electrons or holes, so holes have a positive charge $+e$ (electrons having $-e$).
On the other hand, the velocity of a hole is **the same**:
On the other hand, hole's velocity is **the same** as that of an electron:
Finally, we derive the hole effective mass from the equations of motion:
$$m_{h,i}\frac{d v}{d t} = +e (E + v\times B).$$
$$m_h\frac{d v}{d t} = +e (E + v\times B).$$
Comparing with
$$m_{e,i}\frac{d v}{d t} = -e (E + v\times B),$$
$$m_e\frac{d v}{d t} = -e (E + v\times B),$$
we get $m_{h,i} = -m_{e,i}$.
An additional band index $i$ was introduced to clarify that the electron/hole mass relation only applies to particles **within the same band**.
we get $m_h = -m_e$ (we could also obtain this by differentiating the hole's velocity).
## Semiconductors: materials with two bands.
In semiconductors the Fermi level is between two bands.
The unoccupied band is the **conduction band**, the occupied one is the **valence band**.
In the conduction band the **charge carriers** (particles carrying electric current) are electrons, in the valence band they are holes.
Semiconductors are materials with all bands either nearly occupied or almost empty.
Unlike in insulators, however, the band gap in semiconductors is sufficiently small, for it to be possible to create a few electrons in the lowest unoccupied band or the highest filled band.
Because in the unoccupied band the **charge carriers** (particles carrying electric current) are electrons, it is called **conduction band**, while in the almost occupied **valence band** the charge carriers are holes.
!!! note "Holes in semiconductors"
When introducting holes, we discussed holes obtained by removing *any* electron.
From this point on we will only speak of holes in valence band and electrons in conduction band.
The occupation of the bands is dictated by the Fermi level.
We can control the position of the Fermi level (or create additional excitations) making semiconductors conduct when needed.
The easiest way to control the Fermi level is through temperature by inducing thermal excitations.
In most semiconductors, the temperature is always smaller than the size of the band gap $k_b T \ll E_G$.
The occupation of the two bands is dictated by the Fermi distribution.
Furthermore, the Fermi level of a semiconductor lies between the conduction and the valence bands, and the band gap $E_G \gg k_B T$ in most materials.
As a result, only the bottom of the conduction band has electrons and the top of the valence band has holes.
Therefore we can approximate the dispersion relation of both bands as parabolic.
