Diodes and their applications

I. Basic knowledge of semiconductors

1. Semiconductors and their characteristics

(1) Definition: A class of substances whose conductivity is between conductors and insulators is called

Semiconductor

.Commonly used semiconductor materials are silicon and germanium.

(2) Sensitive characteristics: The conductivity of semiconductors is very sensitive to temperature, light, magnetic field, electric field, etc.

(3) Conductivity characteristics:

①The number of free electrons and holes in a pure semiconductor is equal and small, and the conductivity is extremely poor.(So ​​use the "doping" feature)

②Free electrons and holes participate in conduction at the same time, which is an important conduction characteristic that distinguishes semiconductors from conductors.

Note:

N-type and P-type semiconductors are still electrically neutral.

2.PN junction and its unidirectional conductivity

Formation: In a pure semiconductor, different doping processes are used to simultaneously form N-type and P-type semiconductors.A special thin layer formed at the junction of the two is called

PN junction

.

Second, diodes

1. Structure and symbols

Extract one electrode from each of the P and N regions of the PN junction, and then encapsulate it in the package.That constitutes

diode

.

The one drawn from the P area is the positive electrode, and the one drawn from the N area is the negative electrode.

2. Working principle

The diode is turned on when forward biased and turned off when reverse biased.

3.①Forward characteristics

is divided into

forward dead zone

and

forward guidePass

area two parts.

(a) Forward dead zone: When the forward voltage is less than the dead zone voltage, the diode is cut off.The forward current is approximately zero, and the tube exhibits high resistance to the outside.(Dead zone voltage:

silicon tube is about 0.5V, germanium tube is about 0.2V

).

(b) Forward conduction region: When the forward voltage is greater than the dead zone voltage, the diode is turned on.The forward current increases sharply, and the voltage across the tube is basically stable, which is equivalent to closing the switch.(

tube voltage drop: silicon tube is about 0.7V, germanium tube is about 0.3V

).

②Reverse characteristics

is divided into

reverse cut-off area

and

reverse breakdownArea

in two parts.

(a) Reverse cut-off area: the reverse resistance is less than the breakdown voltage, the diode is cut off, the reverse current is about zero, and does not change with the reverse voltage.It presents a high resistance characteristic to the outside, which is equivalent to the switch off.

(b) Reverse breakdown region: When the reverse voltage is greater than the breakdown voltage, the diode reverses breakdown, the reverse current increases sharply, and the unidirectional conductivity is lost.

③The diode has unidirectional conductivity; the diode is a nonlinear element.

4. Main parameters of the diode

①Maximum rectified current

: The maximum forward average current allowed by the diode for long-term operation.When selecting, the actual working current is not allowed to exceed the maximum rectified current.

②Maximum reverse working voltage

: The peak value of reverse voltage allowed across the diode.When selected, the reverse voltage across the tube cannot exceed the maximum reverse working voltage.

Note:

The maximum rectified current and the maximum reverse working voltage are the main parameters for choosing the diode

.