The pn junction acts as a diode to provide rectification or one-way cur- rent flow, similar to the diode vacuum tube. Two pn junctions are used
Construction, symbol, and typical case for a pn junction diode, a pnp and an npn transistor. (A) pn junction diode. (B) pnp transistor. (C) npn transistor.
p n
Junction
Anode Cathode
Type of construction
Anode Cathode
Symbol
Band denotes cathode
Typical case
p n p
Emitter connection
Base connection
Collector
connection Base
Collector
Emitter
Type of construction Symbol Typical case
n p n
Base connection Emitter
connection
Collector connection
Type of construction
Base
Collector
Emitter
Symbol Typical case
A
B
C
F I G U R E 5 . 1 4
in “bipolar” pnp or npn transistors to provide amplification of small input signals to large output signals similar to the triode vacuum tube.
This section concentrates on how the pn junction diode works.
A pn junction is formed when one half of a block of pure semiconduc- tor material is doped with a trivalent impurity and the other half is doped with a pentavalent impurity. The resulting pn junction creates an electro- chemical force due to the excess holes in the p-type material and the excess free electrons in the n-type material. This electrochemical force is responsible for the operation of the pn junction diode and the transistor.
When the pn junction is formed, as shown in Fig. 5.15A, free elec- trons from the n-type material drift across the junction and immedi- ately combine with the numerous holes present in the p-type material.
Thus some atoms in the p-type material gain additional electrons and exhibit a net negative charge. As a group, these atoms, or negative ions, form a negative electric field adjacent to the junction.
The loss of free electrons by the n-type material in the vicinity of the junction leaves some atoms with a net positive charge. These atoms, or positive ions, form a positive electric field next to the junc- tion. Each pair of positive and negative ions are referred to as a dipole (not to be associated with the dipole antenna, covered in Chapter 10).
The formation of negative and positive electric fields at the junc- tion are shown in Fig. 5.15B. This region is referred to as the depletion zone. The depletion zone shown in Fig. 5.15B is not drawn to scale.
Actually, the depletion zone is very thin, virtually a few atoms thick.
The resulting electric field, or barrier potential, varies with the type of semiconductor material being used. For example, the barrier potential at room temperature is approximately 0.3 volt for a germanium pn junction and approximately 0.7 volt for a silicon pn junction. These values of barrier potential are the most useful in describing the action of pn junction diodes and transistors. Note that the p-type material in the pn junction diode is referred to as the anode and the n-type mate- rial is referred to as the cathode. These designators represent a carry- over from the vacuum tube era.
If the temperature of the pn junction is increased, the barrier poten- tial is decreased by a slight amount due to the increased number of electron-hole pairs, or dipoles. Temperature effects in semiconductor materials must be taken into account when designing solid-state elec- tronic circuits.
The pn junction described in the previous section is used as pn junction diodes. Figure 5.16 shows the schematic symbol and several case outlines used to house these diodes. These case outlines repre- sent a general cross section of the numerous types of diodes available on the commercial market. Semiconductor handbooks should be con- sulted for specific technical information on particular diodes. The The pn junction diode. (A) Initial condition when a pn junction diode is formed.
(B) Final condition of a pn junction diode after electric field is stabilized.
Excess holes
p-type semiconductor
pn-junction
n-type semiconductor
Excess electrons
Depletion zone p-type
semiconductor
Excess holes
Electrons entering this area combine with positive ions, producing a net negative charge.
Electrons leaving this area produces a net positive charge.
Anode terminal Cathode terminal
Excess electrons
n-type
semiconductor
B A
F I G U R E 5 . 1 5
diodes in Fig. 5.16B are low-power or sig- nal diodes. These types of diodes are used in RF detector, demodulator, and switch- ing circuits. The miniature rectifier or power diodes shown in Fig. 5.16C are commonly used in low-power applica- tions involving current up to about 1 ampere. Such diodes can be used in ac power supplies involving receivers and low-power transmitters. The high-power rectifier diodes shown in Fig. 5.16D will conduct current levels up to 10 amperes or more with peak inverse voltage ratings of 50 to 600 volts depending on the type.
These high-power rectifiers are used in the power supplies of high-power amateur radio transmitters.
The pn junction diodes are available in many sizes and case types. The basic pn junction diode is used as a rectifier, either in high-current, low-fre- quency rectification circuits or in low-current, high-frequency, and high-speed switching circuits. Power rectifier diodes are generally larger, containing a larger junction area with increased capacitance.
Due to this larger capacitance, these diodes are not suitable for high- frequency or switching circuits. High-frequency signal diodes and switching diodes used in digital circuits are physically smaller and have less internal capacitance. Consequently, their current conducting ability is less than the power diodes.