P-N Junction Diode

A p–n junction is a boundary or interface between two types of semiconductor material, p-type and n-type, inside a single crystal of semiconductor. It is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy (growing a layer of crystal doped with one type of dopant on top of a layer of crystal doped with another type of dopant). If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that severely inhibits its utility by scattering the electrons and holes.

When a P-type semiconductor is joined to a N-type semiconductor such that the crystal structure remains continuous at the boundary, a PN junction is formed. A PN junction forms a very useful device and can be called a semiconductor diode, PN junction diode, or simply a crystal diode. Simply joining two pieces together cannot form a PN junction, because it would produce a discontinuous crystal structure. Special techniques are required to assemble a PN junction. Normally P-type carriers are to the left of the junction and N-type carriers to the right.

Since the junction diode is a two terminal device, the application of voltage across its terminals leaves three possibilities:

1. No bias
2. Forward bias
3.  Reverse bias

No bias 

This occurs when there is no external voltage applied. The holes from the P-region diffuse into the N-region. They then combine with the free electrons in the N-region. The free electrons from the N-region diffuse into the P-region. These electrons combine with the holes.

Forward bias 

When a battery is connected to the PN junction diode such that the positive terminal is connected to the P-side and negative terminal to the N-side, forward bias is created. When the PN junction is forward biased, the holes are repelled from the positive terminal and are forced to move towards the junction.  Similarly, the electrons are repelled from the negative terminal of the battery and drift towards the junction. The current within the PN junction is the sum of electron current and hole current.

Reverse bias 

A voltage source is connected with the positive terminal attached to the N-region and negative to the P-region. The holes in the P-region are attracted towards the negative terminal of the applied voltage, and the electrons in the N-region are attracted to the positive terminal. Thus the majority carriers are drawn away from the junction. This increases the barrier potential, which makes it more difficult for the carriers to diffuse across the junction.

P-N Junction Diode characteristics

The PN junction region of a Junction Diode has the following important characteristics:

1). Semiconductors contain two types of mobile charge carriers, Holes and Electrons.

2). The holes are positively charged while the electrons negatively charged.

3). A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so that it contains mobile charges which are primarily electrons.

4). A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so that it contains mobile charges which are mainly holes.

5). The junction region itself has no charge carriers and is known as the depletion region.

6). The junction (depletion) region has a physical thickness that varies with the applied voltage.

7).When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer which is approximately 0.5 to 0.7v for silicon diodes and approximately 0.3 of a volt for germanium diodes.

8). When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short circuit allowing full current to flow.

9). When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow, (only a very small leakage current).