Analog systems and applications, lecture — IV
“PN junction diodes: biased and unbiased diodes.”
This article belongs to a series of lectures on analog electronics, the paper goes by the name “Analog Systems and Applications” for the physics honors degree class. All lectures of this series will be found here. This is the fourth lecture of this series. This particular lecture was delivered on 9th January 2018.
Today we will discuss about what are PN junction diodes and various conditions they can be subjected to, viz. the forward and reverse bias. We will discuss what is a depletion layer and whats a built-in potential barrier.
In order to keep things in a perspective and help the concepts be apprehended better, you might wanna read through the first 3 lectures of the analog electronics series, these are linked here.
- semiconductors and charge carriers lecture-I
- semiconductors and charge carriers lecture-II
- conductivity and mobility in semiconductors lecture-III
In our previous lecture we saw what are extrinsic semiconductors. We discussed that they are of two types, viz. p-type and n-type. By themselves the p-type and n-type semiconductors are not so useful.
But when crystals are doped so that one-half of the same is p-type and the other-half is n-type they serve very important purposes. They are now called PN-junction diode. The border or interface between the p-type part and n-type part is known as PN-junction. The PN junction finds application in almost all sorts of electronics through diodes, transistors and integrated circuits.
The following diagram depicts p and n type semiconductors in terms of the ions and free charge carriers they consist of. For the p-type semiconductor + sign depicts the holes, – sign in a circle depicts the trivalent atom. Similarly for the n-type semiconductor – sign depicts (free) electrons, and + sign in a circle indicates pentavalent atoms. A pn junction is simply a juxtaposition of the two individual types.
A pn crystal is also known as a pn junction diode or junction diode. Diode comes from contraction of two words into one: Diode = Di + Electrode.
What happens in a pn junction diode?
i. There is repulsion among free electrons, on the n-side.
ii. The free electrons diffuse over to the p-side, across the junction.
iii. A free electron which diffused from n-side where it was a majority carrier, is now a minority carrier on p-side.
iv. The electron on the p-side is surrounded by large number of holes. It has a short life-time.
v. The electron recombines with one of the holes, the hole disappears and the conduction electron becomes a valence electron.
vi. Diffusion of electron across the junction creates a pair of ions. On n-side pentavalent atoms are devoid of one electron and become positive (+ve) ions. On p-side the electron recombines with with trivalent atom (the hole), which makes the latter a negative ion. These ions are fixed at lattice sites and are immobile.
vii. Each +ve and -ve ion pair at the junction forms a dipole. This represents the deficiency of one pair of hole and electron. The region around the pn junction which is devoid of free carriers is known as “depletion layer”.
Barrier potential (builtin)
Each dipole created in the vicinity of the pn junction due to creation of pairs of ions produces an electric potential which is directed from the n-side to the p-side. i.e. it prevents free electrons in the n-side, from diffusing across the junction to the p-side.
At equilibrium the diffusion of electrons stops completely and this determines the final value of the potential across the depletion layer. This potential is known as Barrier Potential (or builtin barrier) as it stops electrons from moving across the junction from n-side. The barrier potential is about 0.3 V for Ge and 0.7 V for Si, at 25 degree centigrade.
i. When DC -ve potential is connected to the ‘n’ type semiconductor and +ve potential is connected to the ‘p’ type semiconductor, such a configuration is known as “forward bias“.
ii. The potential of battery pushes holes and free electrons toward the junction. If applied potential is less than barrier potential, electrons and holes can’t climb the hill of energy across the junction created by the barrier potential.
iii. If applied potential is more than the barrier potential, the electrons and the holes can easily climb the energy hill. The electrons then recombine with the holes. This way the electrons move toward the +ve potential (of battery) and come out of the circuit. Electrons coming out of external circuit enter the crystal at the n-side, and continue to recombine with the holes inside of the crystal, and drift towards positive potential. Electron and hole pairs are created continuously at positive potential and p-side connection.
iv. Large continuous current can flow across the forward biased circuit.
When DC -ve potential is connected to the ‘n’ type semiconductor and +ve potential is connected to the ‘p’ type semiconductor, such a configuration is known as “forward bias“.
When -ve potential terminal is connected to the p-side and +ve potential terminal is connected to the n-side of a pn junction diode, its known as a “reverse bias“ configuration.
i. When -ve potential terminal is connected to the p-side and +ve potential terminal is connected to the n-side of a pn junction diode, its known as a reverse bias configuration.
ii. Holes and (free) electrons flow away from the junction. Depletion layer gets wider.
iii. The barrier potential increases when holes and electrons move away from junction. The depletion layer continues to grow wider.
iv. An equilibrium is reached when potential barrier across junction is equal to the applied potential in the external circuit.
v. Although most of the free charge carriers would recombine, some minority carriers remain in the depletion layer. electron is pushed to right (from p towards n side) by the depletion region potential. (remember electrons move opposite to electric field direction, i.e. from lower to higher potential) Holes in the depletion layer move to left., recombine with electrons created due to thermal energy (at the left end, i.e. p-side). A small current sets up due to these minority carriers. This current is known as “reverse saturation current“. Its called so, because the number of such charge carriers can’t be increased any further by application of potential.