PN Junction Diodes Current Flow Diodes are two terminal devices, like a resistor. Unlike resistors, they only allow electricity to flow through them in one direction. This makes them a useful and valuable component, like a plumber’s check-valve. Current Flow

The first junction diodes were made by mounting a piece of naturally occurring crystal into a holder, which was one terminal of the diode. A springy sharp wire called a “catwhisker” was used to probe for “hot spots”, or places where pockets of some impurities existed. The catwhisker became the other terminal of the diode. Early radio receivers used these crystal diodes to extract voice signals from a radio wave in a process known as detection or demodulation. Modern diodes are made by processing semiconductor materials, altering their properties with impurity “dopants”, and packaging them in a variety of forms.

Some Diodes Small signal silicon diodes Power Rectifiers Surface mount diodes The largest packages, in the upper left, are about the size of a hockey puck. These diodes are used in powerful circuits for motor control, welding, etc. Smaller diodes come in a variety of packages that can often be found in consumer electronic devices. Some Diodes can’t be found at all any more, like the guys in the lower right. Miscellaneous diodes from the junk box A Canadian band from the 1980’s

Doping of Semiconductors The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors. Pentavalent To understand the diode, it is helpful to understand how impurities can be added to silicon (valence 4) to change its properties. Consider some candidate impurities: Antimony, arsenic, phosphorous…all pentavalent, Boron, aluminum, gallium, all trivalent Trivalent

P and N Type Semiconductors If we were to insert one of those pentavalent atoms into the crystal lattice of silicon, that atom would fit pretty well, but there would be one more electron in the neighborhood than if the silicon atom where in place. Silicon which has had a small number of crystal locations where pentavalent atoms have been substituted for silicon exhibit an excess of free electrons. Silicon which has been doped this way is also called N type. If we were to make the same substitution, but with trivalent atoms, our chunk of crystal would be deficient of electrons. We would say that it has an excess of holes. Silicon which has been doped this way is also called P type. P-type N- type

Formation of Depletion Region Now what would happen if we pushed a piece of N type silicon and a piece of P type silicon into contact with one another? The excess electrons on the N side would find comfortable places to go to on the P side, where electrons are in short supply. The result is the creation of a neutral zone around the junction, where there is neither an excess nor a deficiency of electrons. The P region of the diode is conductive, as there are carriers available there. Holes can move, and participate in current flow. The N region is also conductive, as there are plenty of free electrons there. The neutral zone in the middle, called a depletion region, as it has been depleted of active charge , is an insulator. There are no carriers (no free electrons, no available holes) that can participate in electrical conduction. anode cathode

Reverse Bias No appreciable current flow Depletion region widens What would happen if we connected the diode with a battery as shown, N type to positive terminal, P type to negative terminal? The electric field from the battery would pull electrons to the right, and holes to the left. The depletion region would only widen, and no appreciable current would flow through the diode. The energy diagram beneath our diode suggests a substantial energy barrier that electrons must climb if they are to cross to the P side. We call this condition reverse bias.

Forward Bias Depletion region narrows Substantial current flows If we reverse the battery polarity, something very interesting happens. The energy barrier preventing the movement of electrons is reduce. Electrons are pushed into the depletion region, and so are holes from the P side. The depletion region narrows or even vanishes, and a substantial current flow develops through the diode. This condition is called forward bias.

Current-versus-Voltage (IV) Characteristic Curve If we were to plot the magnitude of the current flowing through the diode versus the voltage across it, we would get something like this. The forward bias region is in the first quadrant. Typically, for silicon diodes, there is a sharp and exponential uplift in current as the voltage begins to exceed 0.6 volts. The reverse bias behavior, in the third quadrant, shows a nearly negligible current flow over a wide range of voltage. What little current that does flow is the result of thermally excited electrons getting swept up in the reverse bias electric field. When that reverse bias field gets strong enough, the diode can no longer stand off the applied voltage, and a breakdown or avalanche current flows.

Avalanche Breakdown Thermally excited electrons cause a leakage current to flow in the reverse bias state. These electrons accelerate in the bias field, picking up energy, and collide with the lattice, releasing more electrons. At sufficiently high field strength the process achieves a runaway condition called avalanche. Avalanche is a little like a nuclear reaction going critical. At a critical level of electric field strength, the thermally excited electrons crash into the crystal lattice and release more electrons, which also become accelerated in the field.

Zener Diodes Zener diodes have well-controlled and specified avalanche voltages. The avalanche or zener voltage is often used to establish stable reference potentials in circuits. Usually avalanche breakdown is to be avoided by selecting diodes whose breakdown voltage is much larger than the voltages we expect to find in our circuit. Zener diodes, however, exploit the avalanche effect by engineering it to be a very sharp transition that takes place at a specified and controlled voltage. Zener diodes then become very useful, as they can be used in the avalanche mode to establish precise reference voltages or limit voltages.

Schottky Diodes Schottky diodes are formed with metal/semiconductor junctions Schottky diodes are not PN junctions at all, but are made by joining a semiconductor and a metal. They will enter forward bias and conduct strongly with a potential of only about 0.3 volts, versus 0.6 for silicon diodes. This makes them attractive in high power applications where diode heating could be a problem. They exhibit lower forward voltage drops and higher switching speeds between forward and reverse states. Reverse leakage currents tend to be higher than with PN junction diodes.

Varicap Tuning Diodes In reverse bias, a diode exhibits capacitance between its terminals. As the reverse bias potential increases, the depletion region grows wider and capacitance decreases. As the reverse bias potential decreases, the depletion region narrows and the capacitance increases. Diodes that are designed specifically to be voltage-variable capacitors are called varicaps. Virtually all modern radio and television receivers, as well as cell-phones, handy-talkies, etc. are tuned by varicap diodes. The depletion region of a diode can be exploited to produce another interesting property. By applying differing amounts of reverse bias, the capacitance of the (non-conducting) diode can be made to vary over a range.

Next Steps Explore diode properties with curve tracer in lab. Investigate use of diode as half-wave rectifier.

In the lab, we will explore the current versus voltage characteristics of several diodes using a circuit like this. Basically, we will plot the voltage across the diode on the horizontal axis of an oscilloscope, while plotting the current through the diode as the vertical axis signal. An AC power supply will allow this graph to be generated for us 60 times per second.

Half Wave Rectifier We will also explore how AC is converted to DC by experimenting with this half wave rectifier circuit. Finally, we will look inside one of the “wall-wart” power bricks that power many of the consumer electronic devices we own, and see how it works.