Diode Characteristics

Semiconductor diodes have properties that enable them to perform many different electronic functions. To do their jobs, engineers and technicians must be supplied with data on these different types of diodes. The information presented for this purpose is called diode characteristics. These characteristics are supplied by manufacturers either in their manuals or on specification sheets (data sheets). Because of the scores of manufacturers and numerous diode types, it is not practical to put before you a specification sheet and call it typical. Aside from the difference between manufacturers, a single manufacturer may even supply specification sheets that differ both in format and content. Despite these differences, certain performance and design information is normally required. We will discuss this information in the next few paragraphs.

A standard specification sheet usually has a brief description of the diode. Included in this description is the type of diode, the major area of application, and any special features. Of particular interest is the specific application for which the diode is suited. The manufacturer also provides a drawing of the diode which gives dimension, weight, and, if appropriate, any identification marks. In addition to the above data, the following information is also provided. A static operating table of electrical characteristics (giving spot values of parameters under fixed conditions), sometimes a characteristic curve similar to the one in this figure (showing how parameters vary over the full operating range), and diode ratings (which are the limiting values of operating conditions outside which could cause diode damage).

Electrical characteristics

Manufacturers specify various diode characteristics and ratings with "letter symbols" in accordance with fixed definitions. The following is a list, by letter symbol, of the major electrical characteristics for the rectifier and signal diodes.

Rectifier Diodes

Average Forward Voltage [V_F(AV)] - the average forward voltage drop across the rectifier given at a specified forward current and temperature.

Forward Voltage [V_F] - the instantaneous forward voltage drop across the rectifier given at specified conditions.

Average Reverse Current [I_R(AV)] - the average reverse current at a specified temperature, usually at 60 Hz.

Reverse Current [I_R] - the instantaneous (or DC) value of reverse current at specified conditions, usually at rated DC blocking voltage.

Signal Diodes

Forward Voltage [V_F] - the instantaneous forward voltage drop across the diode given at specified conditions.

Reverse Current [I_R] - the instantaneous (or DC) value of reverse current at specified conditions.

Reverse Recovery Time [t_rr] - the maximum time taken for the forward-bias diode to recover its reverse bias.

Maximum ratings

The maximum ratings of a diode (as stated earlier) are the limiting values of operating conditions, which if exceeded could cause damage to a diode by either voltage breakdown or overheating. The following is a list of the major ratings for the rectifier and signal diodes.

Peak Repetitive Reverse Voltage (V_RRM, PRV, or PIV) - the maximum allowable instantaneous value of reverse voltage that may be applied across the diode repetitively under the specified conditions.

DC Blocking Voltage [V_R] - the maximum reverse DC voltage which the diode may block on a continuous basis.

Average Rectified Forward Current [I_F(AV)] - the maximum amount of average rectified forward current at a specified temperature, usually at 60 Hz with a resistive load.

Peak Surge Current [I_FSM] - the maximum current permitted to flow in the forward direction in the form of nonrecurring pulses, specified for a given number of cycles or portion of a cycle.

The following is a list of some other ratings for the signal diodes.

Repetitive Peak Forward Current [I_FRM] - the maximum peak current that can be permitted to flow in the forward direction in the form of recurring pulses.

Forward Continuous Current [I_F] - the maximum DC current that can be permitted to flow in the forward direction.

All of the above ratings are subject to change with temperature variations. If, for example, the operating temperature is above that stated for the ratings, then the ratings must be decreased.