PN Junction Diodes: Voltage Characteristics
Without watching old sit-coms, you miss an opportunity to enjoy humor that modern writers seem to ignore. During the 1960s, Petticoat Junction featured the three Bradley sisters named Billie Jo, Bobbi Jo, and Betty Jo who helped their mother and her uncle manage the Shady Rest Hotel. The hotel sat next to a large water tower that also served as a swimming pool for the sisters and as a water stop for an 1891 Rogers 4-6-0 steam locomotive. Each episode covered some mini-crisis or get-rich-quick scheme and always involved simple humor.
In electronics, old and simple still has a role. While none of us will ever discover a get-rich-quick scheme connected to PN junction diodes, those devices continue to have a place and continue to provide value.
Flux Capacitors Require Atomic Power
The Hooterville Cannonball that steamed along an isolated railway between Hooterville and Pixley in Petticoat Junction also appeared as a time machine in the Back to the Future 3 movie. Achieving the 1,210,000,000 watts of power needed for the time travel-enabling flux capacitor requires some knowledge of atomic theory.
Every atom contains protons and neutrons in the nucleus and electrons that orbit the nucleus. The electrons travel in orbital paths—called shells--with the outermost path designated as the valence shell.
While valence shells can contain a maximum of eight electrons, the number of electrons in the valence shell determines the conductivity of the atom. We call an atom with one electron in the valence shell a perfect conductor. An atom with eight electrons in the valence shell is an insulator. Conductivity decreases with an increase in the number of valence electrons.
Semiconductor materials such as silicon, germanium, and carbon have four valence electrons and—as you know—do not provide good conductance or good insulation.
Each orbital shell has a specific energy level and the electrons traveling within the shell have the same amount of energy. Because the valence shell locates the farthest distance from the nucleus, it has the highest energy level.
Electrons can jump from one orbital shell to another if the electron can absorb enough energy to jump from one shell to the next. This amount of absorbed energy equals the difference between the initial energy level and the energy level of the target shell. Another band called the conductance band exists outside of the valence shell of an atom. An electron jumping from the valence band to the conductance band must absorb an amount of energy equal to the difference of the conductance band energy level and the valence band energy level.
An electron that absorbs enough energy to make the valence-to-conductance band jump enters its excited state. An excited electron gives up the absorbed energy in the form of light or heat as it returns to its original energy level. Covalent bonding occurs when atoms complete their valence shells by sharing valence electrons with other atoms. While this type of bonding is different than the bond shared by Petticoat Junction’s Bradley sisters, it forms a solid substance that keeps the atoms electrically stable. Remember, though, that completing the valence shells causes a semiconductor material to turn to the dark side and become an insulator.
About the PN Junction Materials
The Hooterville Cannonball crew included a fireman who also served as the conductor. Achieving conductivity in the reality of covalent bonding occurs through a process called doping that adds impure electrons to the intrinsic or pure silicon or germanium. This process adds extra electrons to the covalent bond. Trivalent doping occurs when the impure element contains three electrons and produces a p-type material. Pentavalent doping elements contain five electrons and produce an n-type material.
Knowing the types of diodes at work in your circuit allows for better circuit design.
The p-type material has an excess of holes and few free electrons while the n-type material has an excess of electrons and few valence band holes. When considering the p-type material, the holes are majority carriers while the electrons serve as minority carriers. The opposite is true for n-type materials. Within the two types of materials, the valence bands and convection bands have different energy levels.
Utilizing PN Junction Material Effects in Electrical Designs
A standalone n-type or p-type material does the same amount of work as Uncle Joe performs at the Shady Rest hotel.
Joining the p-type and n-type materials together allows the conduction and valence bands of the materials to overlap as the pn junction forms. Free electrons from the n-type material diffuse—or travel—to the p-type material. This action causes the free electron to become trapped in one of the valence bond holes of the p-type material. As a result, one net positive charge exists in the n-type material and one net negative charge exists in the p-type material.
The only drama that existed within the Petticoat Junction series occurred when a stranger stayed at the hotel or when Homer Bedloe attempted to shut down the Hooterville Cannonball. Large-scale drama hits when a pn junction forms.
Each electron that diffuses across the junction leaves one positively charged bond in the n-type material and causes one bond in the p-type material to have a negative charge. Because the n-type material has lost a conduction band electron and the p-type material has lost a valence band hole, both bands become depleted of charge carriers and a depletion layer exists on both sides of the junction. The overall charge of the layer is positive on the n-side of the junction and negative on the p-side. As the negative charges build up on the p-side of the junction and the positive charges build up on the n-side, a difference of potential—called the barrier potential—exists.
Rather than sit asleep like Uncle Joe, a PN junction offers useful work. Controlling the width of the depletion layer allows to control the resistance of the junction and the amount of current that can pass through the semiconductor device. The potential used to control the width of the junction is the bias. A forward bias occurs when the applied potential causes the n-type material to become more negative than the p-type material. Current passes through the junction with little opposition. Reverse bias happens when the applied potential causes the n-type material to become more positive than the p-type material. The junction widens and the current reduces to almost zero.
Lotsa Curves, You Bet. Even More, When You Get to the Junction
Figure two shows the schematic symbol for a pn-junction diode. The n-type material becomes the cathode for the diode while the p-type material serves as the anode. Referring to the simple circuit that uses a resistor to limit the amount of current flowing through the diode, the diode conducts when n-type cathode is more negative than the p-type anode. Increasing the forward bias voltage causes the forward current and the voltage across the diode to increase.
Referring to the current-voltage characteristic curve for the diode, the forward current has a slight increase until the pn junction reaches 0.7 volts at the knee of the curve. Then, the forward current has a rapid increase.
Applying a reverse bias voltage across the diode allows only a small portion of reverse current to flow through the pn junction. Increasing the reverse bias voltage produces a very small reverse current and an increase in voltage across the diode. Continuing to increase the reverse bias voltage causes the voltage across the diode to reach a breakdown value. Then, the reverse current increases rapidly and causes overheating and damage to the diode.
While PN Junction diodes may seem like old pieces of history, they still serve an important lesson for designers looking to work with diodes in their PCB designs. PSpice offers models and parameters for thousands of diodes in their vast model library. It is also equipped with waveform generators, current measurements, and tolerance tools to ensure that your circuit is capable of handling the voltage necessary for your design.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.