Semiconductor Devices Lecture 05 Prepared by: Eng.Maira Naz Date:06-06-11

Lecture Contents: PN junction(Diode) Forward biased PN junction. Hole current in forward biased PN junction. Electron current in forward biased PN junction. Reverse biased PN junction. Peak Inverse voltage. Reverse saturation current. Hole current in reverse bias PN junction.

PN Junction (Diode) We begin our study of semiconductor devices with pn junction for three reasons. The device finds application in many electronic devices, e.g. in adaptors that charge the batteries of cell phones The pn junction is among the simplest semiconductor devices, thus provide a good entry point into the study of the operation of such complex structures as transistors. The pn junction also serves as part of transistors. We also use the term “diode” to refer to pn junction.

PN Junction (Diode) A pn junction is formed when the acceptor impurities are introduced into one side and donor impurities are introduced into the other side of a single crystal structure of a semiconductor. Single crystal means that the continuity of structure is maintained. When a single crystal of semiconductor contains both the impurities, the new material we get is called as PN-junction.

PN Junction (Diode) The dotted cross sectional area in the middle of junction is called a metallurgical junction where the effective doping densities changes fro acceptor to donor. PN-junction is not created by joining together P-type and N-type semiconductors because such a creation cannot result into a single crystal structure. PN-junctions are formed by carefully controlled manufacturing process.

PN Junction (Diode)

PN Junction (Diode) Only one dimensional behavior will be considered, meaning the charge carrier density is a function of x only. The doping levels of P and N-regions are uniform, i.e. NA=ND. The transition from NA to ND is abrupt near the junction meaning semiconductor changes its characteristics from P-type to N-type at the boundary where the concentration of either P-type or N-type is zero. This is a called as step-graded junction.

PN Junction (Diode) Fig shows the virgin PN-junction (no external voltage is applied) just after PN-junction is manufactured. Holes are the majority charge carriers in P-region while the electrons are minority charge carriers in P-region. Electrons are the majority charge carriers in N-region while the holes are minority charge carriers in N-region.

PN Junction (Diode)

PN Junction (Diode) So holes (or electrons) having a tendency to move from the region of higher concentration towards the region of lower concentration (due to diffusion). So, the diffusion current will occur immediately just after the formation of PN-junction. The total diffusion current will bee the sum of two.

PN Junction (Diode) Once, the holes diffuse across the junction from P to N-region they are surrounded by a lot of electrons and are quickly neutralized and will disappear from the scene. In identical manner electrons diffuse into P-region from N-region. They immediately find a block of holes in P-region, they fall into them and vanish from arena.

PN Junction (Diode) This means N-region will be deprived of electrons near the junction. Similarly P-side will loose an equal number of holes near the junction. This will eventually produces a thin layer of equal but opposite charges near the junction. The thin charge region across the junction is termed as depletion charge layer. It is also called as space charge layer, dipole layer or transition zone.

PN Junction (Diode)

PN Junction (Diode) Consequently, a potential barrier, Vo, is created across the junction. Because of this Vo, electric field will develop ant its direction will be from N to P-region. The development of this electric field will give a birth to another phenomena i.e. drift current. The direction of electric field support the flow of minority carriers. So, the movement of minority current is due to drift current.

PN Junction (Diode)

PN Junction (Diode)

PN Junction (Diode) Since, no external dc battery is connected to PN-junction, So, the net current is zero. This means the diffusion and drift current in an unbiased PN-junction are equal and opposite in direction I diffusion = -I drift This means both the currents are in opposite direction.

PN Junction (Diode)

PN Junction (Diode) Equilibrium condition: If diffusion current is going to increase, its means holes cross junction from P to N-region. This will cause an expansion in space charge layer. As a result, Vo and associated electric field also increase.

PN Junction (Diode) So, a sort of equilibrium condition will always prevail and there will be no net current flow in an un biased PN-junction. Potential barrier across PN-junction is typically 0.6 to 0.7V for silicon and 0.2 to 0.3V for germanium.

Forward Biased PN junction When the positive terminal of battery is connected to P-side and negative terminal to the N-side of diode, it is said to be forward biased. In this condition, diode is capable of carrying a substantial amount of current and is said to be in on state.

Forward Biased PN junction

Forward Biased PN junction If the magnitude of battery is more than the internal built in potential barrier, Vo, then the negative side of battery will repel the free electrons of N-region towards he junction. Similarly, positive side of battery will repel the holes of P-region towards the junction. This correspond to the majority carrier current.

Forward Biased PN junction So, holes from P-region are compelled to move into depletion layer, they will neutralize some electrons. Similarly, the electrons from N-region are compelled to move into depletion layer, they will neutralize some holes. Consequently depletion layer will reduce.

Forward Biased PN junction

Forward Biased PN junction Hole current:

Forward Biased PN junction Electron current:

Reversed Biased PN junction When positive terminal of battery “VBB” is connected to N-region and the negative terminal of battery is connected to the P-region, then the diode is said to be reverse biased. In, an reverse biased diode majority current does not flow. Instead of that minority current can flow.

Reversed Biased PN junction The free electrons in N-region are attracted by the positive terminal of battery and they move away from the junction. In a similar manner, free hoes in P-region are attracted by negative terminal of battery and move away from the junction. So, they leave behind immobile ions. They are negative ions in P-region and positive ions in N-region.

Reversed Biased PN junction This corresponds to the widening of space charge layer across the junction. So, the potential barrier becomes large compared to unbiased PN-junction.

Reversed Biased PN junction

Reversed Biased PN junction Punch through: Whatever the magnitude of battery, it will simply appear across the space charge layer as reverse bias. At a sufficiently large reverse bias the space charge layer can expand so much that it touches the boundaries of PN-junction. This phenomena is called as punch through and should be avoided.

Reversed Biased PN junction

Reversed Biased PN junction Peak reverse voltage: Various diodes having the capability of withstanding various amount of reverse bias. They are rated by device manufacturer. Typical values of reverse bias rating of diodes may range from a tens of volts to thousands of volt. Diodes are selected for use in practical circuits keeping in view their reverse bias rating.

Reversed Biased PN junction Peak reverse voltage (Cont..): Low voltage circuits such as used in radio receivers and tape recorders will require diodes with low reverse bias rating. High voltage circuits used in television receivers and oscilloscope will deploy diodes with high reverse bias voltage. Reverse bias rating of diode is termed as Peak reverse voltage(PRV) Or Peak inverse voltage(PIR).

Reversed Biased PN junction Reverse saturation current: The increased space charge layer will also increase the intensity of associated electric field. The direction of electric field aids the flow of minority charge carriers across the junction. Minority charge carriers i.e. holes in N-region and electrons in P-region cross the space charge layer and go to the other side. At the same time, the increased space charge layer and its increase electric field will completely stop the majority carriers to migrate across the junction.

Reversed Biased PN junction Reverse saturation current (Cont..): Therefore, the majority electrons in N-region will face an enhanced opposition from the electric field intensity of reverse bias diode. These electrons will not be able to overcome this opposition. Hence, they will never cross the junction and migrate to P-region. Same situation is valid for holes in P-region.

Reversed Biased PN junction Reverse saturation current (Cont..): Reverse bias diode does not support the majority carrier current. It will allow the minority carrier current to flow across the junction. This current is called as reverse current or reverse saturation current or reverse leakage current. Its magnitude is smaller than the forward majority current of forward biased diode.

Reversed Biased PN junction Why the silicon semiconductors are used frequently in the manufacturing of diodes and transistors? The generation of minority charge carriers in semiconductors is highly temperature dependent and increased at elevated temperature. Therefore, the minority current of reverse bias diode is also temperature dependent. The reverse current in small purpose silicon diodes is typically in the range of nano-amperes and can be ignored.

Reversed Biased PN junction It may be of the order of tens of micro-amperes for medium sized silicon diodes. It is not as frivolous even in small purpose germanium diode. It may be in the range of tens of micro-amperes to hundred of micro-amperes. Because of this low reverse current, silicon semiconductors are replacing germanium diode in the manufacture of diodes and transistors.

Reversed Biased PN junction Hole current: