A P-N junction diode is a semiconductor diode constructed by joining p-type and n-type doped semiconductor materials to form a P-N junction. A P-N junction diode has two basic connections, or terminals:
- Cathode - terminal is located on the n-type material
- Anode - terminal is located on the p-type material
Within the P-N junction diode, forward current (on-state current) flows into the anode terminal and flows out of the cathode terminal.
Figure 1. P-N junction diode and circuit symbol
For an ideal diode, the forward direction is the only direction current is able to flow. P-N junction diodes, however, and in fact all real diodes, do allow reverse current, but only very little for moderate values of reverse voltage. Since P-N junction diodes only allow significant flow of current in the forward direction, one of their most common uses are as rectifiers, which convert AC current to DC current.
Figure 2. I-V curve for a P-N junction diode
In figure 1, the middle region of the P-N diode consists of n-type material. However, the type of doping within the middle section of the P-N diode (n-doping or p-doping) does not influence the direction (forward or reverse) of current flow.
Figure 3. P-N junction diodes with different types of doping in middle region
The forward flow characteristics of a P-N junction diode depend on the narrowness of the middle zone between anode and the cathode. A narrower middle section will allow more forward current flow through the P-N junction diode. The amount of doping in the middle region does not affect the forward characteristics of the P-N junction diode but is important in determing its reverse, or blocking characteristics.
Heavily doped outer sections (n+, p+) are essential for allowing good flow of forward current through the P-N junction diode.
Doping and Field Strength in a P-N Junction Diode
The n- zone (or p- zone) in the middle of a P-N junction diode serves to accommodate the depletion zone in the case of reverse voltage. In order to cope with high reverse voltages, the n- zone must be thick and not heavily doped.
Figure 4. Doping and field strength in a P-N junction diode
The highest electric field strength is found at the P-N junction. The lower the silicon doping, the flatter the field strength curve. Electric field strengths for which reverse current is blocked are limited by the avalanche field strength, above which reverse current increases sharply due to charge carrier multiplication.
Figure 5. Electric field strength in the depletion zone of a P-N junction diode
The area under the field strength (the green area) in Figure 5 is equal to the reverse voltage accross the P-N junction diode.
Non Punch Through (NPT) Design of P-N Junction Diodes
Figure 6. Non Punch Through (NPT) P-N junction diode design
For a non punch through design, doping and thickness of the n- zone is chosen in such a way that the depletion zone does not protrude into the n+ doping area. This results in a triangular field shape.
Punch Through (PT) Design of P-N Junction Diodes
Figure 7. Punch through (PT) P-N junction diode design
For a punch through design, the n- doping is reduced so that the depletion zone extends into the n+ zone. For this reason, the field strength curve is trapezoidal or almost rectangular.
Due to the shape of this field strength, punch through P-N junction diodes can cope with almost double the reverse voltage as a non punch through P-N junction diode with the same thickness since the area of a rectangular or trapezoidal area is double, or almost double, the area of a triangle. Switching characteristics of both P-N junction diode designs further duistinguish NPTs and PTs.
For more infromation, please read:
Basic Principles of Electricity and Physics of Semiconductors
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