A pn junction diode is formed when p-type and n-type semiconductor regions are formed in intimate contact with each other. In the pn diode, large concentration gradients exist in the vicinity of the metallurgical junction, giving rise to large electron and hole diffusion currents.
Under zero bias, no current can exist at the diode terminals, and a space charge region forms in the vicinity of the pn junction. The region of space charge results in both a built-in potential and an internal electric field, and the internal electric field produces electron and hole drift currents that exactly cancel the corresponding components of diffusion current.
When a voltage is applied to the diode, the balance in the junction region is disturbed, and the diode conducts a current. The resulting i-v characteristics of the diode are accurately modeled by the diode equation:
iD = IS [ exp (vD / nVT) - 1 ]
where IS
= reverse saturation current of the diode
n
= nonideality factor (typically 1)
VT
= kT/q = thermal voltage (0.025 V at room temperature)
Under reverse bias, the diode current equals -IS, a very small current.
For forward bias, however, large currents are possible, and the diode presents an almost constant voltage drop of 0.6 to 0.7 V.
At room temperature, an order of magnitude change in diode current requires a change of less than 60 mV in the diode voltage. At room temperature, the silicon diode voltage exhibits a temperature coefficient of approximately -1.8 mV/°C.
One must also be aware of the reverse-breakdown phenomenon that is not included in the diode equation. If too large a reverse voltage is applied to the diode, the internal electric field becomes so large that the diode enters the breakdown region, either through Zener breakdown or avalanche breakdown. In the breakdown region, the diode again represents an almost fixed voltage drop, and the current must be limited by the external circuit or the diode can easily be destroyed.
Diodes called Zener diodes are designed to operate in breakdown and can be used in simple voltage regulator circuits. Line regulation and load regulation characterize the change in output voltage of a power supply due to changes in input voltage and output current,respectively.
As the voltage across the diode changes, the charge stored in the vicinity of the space charge region of the diode changes, and a complete diode model must include a capacitance. Under reverse bias, the capacitance varies inversely with the square root of the applied voltage.Under forward bias, the capacitance is proportional to the operating current and the diode transit time. The capacitances prevent the diode from turning on and off instantaneously and cause a storage time delay during turn-off.
Direct use of the nonlinear diode equation in circuit calculations usually requires iterative numeric techniques. Several methods for simplifying the analysis of diode circuits were discussed, including the graphical load-line method and use of the ideal diode and constant voltage drop models.
SPICE circuit analysis programs include a comprehensive built-in model for the diode that accurately reproduces both the ideal and nonideal characteristics of the diode and is useful for exploring the detailed behavior of circuits containing diodes.
Important applications of diodes include half-wave, full-wave, and full-wave bridge rectifier circuits used to convert from ac to dc voltages in power supplies. Simple power supply circuits use capacitive filters, and the design of the filter capacitor determines power supply ripple voltage and diode conduction angle. Diodes used as rectifiers in power supplies must be able to withstand large peak repetitive currents as well as surge currents when the power supplies are first turned on. The reverse-breakdown voltage of rectifier diodes is referred to as the peak-inverse-voltage, or PIV, rating of the diode.
The use of integrated circuit voltage regulators provides an excellent method to produce highly accurate dc output voltages with very low ripple content. IC regulators also provide excellent line and load regulation. These ICs are available with a wide range of fixed output voltages as well as in versions with adjustable output voltages.dc-to-dc converters provide a flexible means of producing the multiple supply voltages required in many electronic systems. High efficiency dc-to-dc converters use an inductor and a periodically operated switch to generate a dc output voltage that is larger than the input voltage in the case of the boost converter, or an output that is smaller than the input in the case of a buck converter.
Diodes are also useful in building circuits that provide clipping, clamping, and generation of piece wise linear voltage transfer characteristics. Finally, the ability of the pn junction device to generate and detect light was discussed, and the basic characteristics of photo diodes, solar cells, and light-emitting diodes were presented.
