6.2 Diodes & rectification.
The Diode is the simplest type of semiconductor device. There are, in fact, lots of different kinds of diodes. Here we will just consider the most common type.
This is called a PN-Junction, made by joining together two bits of semiconducting material — one P-Type, the other N-Type — as illustrated in figure 6.3a. When we bring the materials together two things happen. The -ve acceptor atoms in the P-type material repel the mobile electrons on the N-type side of the join (Junction). The +ve donors in the N-type material also repel the holes on the P-type side of the junction.
The donor & acceptor atoms (shown as small squares) are fixed, but the mobile electrons & holes (shown as circles) can move & hence they are driven away from the region near the junction. As a result a depletion zone forms around the junction. Any charges which can move have moved away from this zone, leaving it empty.
The free electrons inside the N-type material need some extra energy to overcome the repulsion of the P-type's acceptor atoms. If they don't have enough energy, they can't cross the depletion zone & reach the P-type material. If they do manage to get past this energy barrier some of their kinetic energy will have been converted into potential energy, but — once well clear of the depletion zone — they can move around OK unless they ‘fall into a hole’.
This action is usually described using a conventional ‘energy level diagram’ of the sort shown in figure 6.3b. The electrons can roll around the ‘flat’ parts of the energy diagram, but need extra energy to roll up the step and move from N-type to P-type across the junction. Coming the other way they'd ‘drop down’ and zip into the N-type material with extra kinetic energy. The size of this energy barrier can be defined in terms of a junction voltage, 
.
This means the amount of energy converted from kinetic to potential form (or vice versa) when an electron crosses the depletion zone is 
where e is the charge on a single electron.
A similar argument applies to the free holes in the P-type material. However, this is more complicated to understand because holes have the ‘strange’ property that their energy increases as they go ‘down’ the energy level diagram. Their behaviour is similar in some ways to a bubble underwater or a hydrogen balloon in air.This is because a hole is the absence of an electron. (To create a hole at a lower level we have to lift an electron up to the conduction band from further down — this takes more energy.) As a result, the moveable holes in the P-type material find it difficult to ‘roll down’ the barrier and get into the N-type material. They keep trying to ‘bob up’ to the top of the band.
The electrical properties of the diode can now be understood as consequences of the formation of this energy barrier & depletion zone around the PN Junction. The first thing to note is that the depletion zone is free of charge carriers & the electrons/holes find it difficult to cross this zone. As a result, we can expect very little current to flow when we connect an external voltage supply & apply a small potential difference between the two pieces of semiconducting material. (Here, ‘small’ mean small compared with 
, which an electron requires to get over the potential barrier.)
The effect of applying a bigger voltage depends upon which way around it's connected. When we make the N-type side more +ve (i.e. drag some of the free electrons out of it) & the P-type more -ve (drag holes out of it) we increase the difference in potential across the barrier. This makes it even harder for a stray electron or hole to cross the barrier.
When we make to N-type side more -ve & the P-type side more +ve we force lots of extra electrons & holes into the two pieces of material. To understand the effect of this, let's concentrate on the electrons.
The electrons in the N-type region near the junction are repelled by the fixed acceptor atoms in the P-type. However, they're also repelled by the other electrons drifting around inside the N-type material.
When we shove extra electrons into the N-type material (i.e. make it more -ve) we increase the number of electrons ‘pushing’ from this side. This works a bit like pressure in a gas. Electrons near the junction are helped across the junction by being shoved from behind. The effect is to reduce the amount of extra energy required to cross the barrier — i.e. the barrier height reduces. As a result, the barrier is reduced or removed and free electrons/holes can move freely from one material to the other.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)