Structures of Semiconductor Diodes

All semiconductor diodes are classed into the point-contact type and the junction type.

The arrangement of a point-contact diode is shown in Fig. 3.11. It has a thin pointed wire to which a desired impurity is applied and then is welded by a current pulse (1A/μs) to a wafer of semiconductor.

Fig. 3.11. Structure of a point-contact crystal diode

Junction diodes are mainly fabricated by alloying ( or fusion) and diffusion (Fig. 3.12). The alloying process is a fabrication technique in which a small button of indium is fused at about 500°C into an n -type germanium wafer.

Fig. 3.12. P-N junction diodes manufactured by (a) alloying and (b) diffusion

Zener Diodes

Semiconductor diodes may be operated under conditions of an electric break­down. There are special diodes, sometimes called Zener diodes, oftenused for voltage stabilization.

Figure 3.13 shows the current-voltage characteristic of a typical break­down diode under reverse bias. As is seen, when the diode is operating in the avalanche break­down region (the voltage regulation mode), the output voltage changes very little.

Fig. 3.13. Reverse current-voltage characteristic of a silicon breakdown diode

Figure 3.14 shows a simple circuit using a silicon Zener diode. The load is placed in shunt with the diode. Therefore, in the breakdown region when the voltage across the diode re­mains nearly constant, the same voltage will exist across the load. If the source of voltage E is unstable, any changes in E will almost comple­tely be absorbed by a limiting resistor, R_lim.

Fig. 3.14. Connection of a Zener diode in a circuit

The value of R_lim may be found by the following equation;

R_lim = (E_av - U_B)/(I_V + I_L ),

How well voltage regulation is performed is stated in terms of the voltage regulation ratio (or the stabilization factor), K_stab. For the simple circuit shown in Fig. 3.14 we may write:

K_stab = (∆E/E)/(∆U_B/U_B) = >1.

Varactors

A varactor is a p-n junction semi­conductor diode which utilizes variations in the junction capacitance with reverse bias. Varactors are variable capacitors whose capaci­tance is controlled electrically.

Varactors are mainly used as tuning elements in resonant (tuned) circuits (Fig. 3.15) and some specialized devices, such as parametric amplifiers.

Fig. 3.15. Connection of a varactor in a resonant circuit as a variable capacitor

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