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Revision 2.15

18th October 2011

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What the parameters mean.

Depletion layer p.d.

The p.d. or potential difference (voltage) set up in the depletion layer by the combining holes and electrons.

This p.d. must be overcome for the diode to conduct. For a silicon junction the p.d is about 0.6V and for germanium about 0.2V.

Reverse leakage current.

When a PN junction is reverse biased in a transistor or a diode, a very small leakage current will flow due mainly to thermal activity within the semiconductor material "shaking loose" free electrons, which then form the leakage current. Typically a germanium device may have a reverse leakage current of several micro-Amperes but in silicon devices it is much smaller; only a few nano-Amperes

Forward current.

The maximum current a diode may pass when forward biased (anode + cathode -) without the device being damaged.

Max. reverse voltage.

The maximum voltage that may be applied to a diode when it is reverse biased (anode - cathode +) without damage to the device. (also called the Reverse breakdown voltage)

Current Gain.

In a transistor, current gain is the number of times the value of collector current is greater than the base current. (I_CE / I_BE)

Note: When reading parameters such as these in technical information, they are sometimes also written as I_ce / I_be. The difference between using capitals or lower case for the subscript letters denotes that the parameter refers to large signal or DC operatiing conditions (Capitals) or small signal AC operation (lower case).

Power output.

The total amount of Power in Watts (P=V x I) that the device can supply to a load. The greater the power output the more heat the device must dissipate.

Silicon Diodes.

Fig. 2.1 Siicon Junction Rectifier Diode.

Silicon rectifier diodes, like that shown in Figure. 2.1 are used in such applications as power supplies, where they rectify the incoming mains voltage. They must pass all of the current required by whatever circuit they are supplying; this may be several tens of Amperes or more.

Carrying such currents requires a large junction area so that the forward resistance of the diode is kept as low as possible. Even so the diode is likely to get quite warm. The black resin case helps dissipate the heat.

The resistance to current in the reverse direction (when the diode is "off") must be high, and the insulation offered by the depletion layer between the P and N layers extremely good to avoid the possibility of "reverse breakdown", where the insulation of the diode fails due to the high reverse voltage across the junction.

Silicon diodes are made in many different forms with widely differing parameters. They vary in current carrying ability from milli-amps to tens of amps, some will have reverse breakdown voltages of thousands of volts; others use their junction capacitance to act as tuning devices in radio and TV tuners. Look in suppliers lists to see the many types available.

The advantages and disadvantages of silicon diodes are summarised in Table 1 below:

Table 2.1 Diodes

Parameter	Germanium	Silicon	Comments
Depletion layer p.d.	0.15V	0.6V	Germanium can be useful for low voltage applications.
Forward current	A few milli-amperes	Tens of amperes	Silicon much better for high current applications.
Reverse leakage current	A few micro-amperes	A few nano-amperes	Germanium 1000 times more leaky than silicon.
Max. reverse voltage	Volts	Hundreds of volts	Silicon the only real choice for high voltage applications.
Temperature stability	Poor	Good	Germanium more sensitive to temperature. Can be a problem or can be useful.
Junction capacitance	Very low (point contact)	Comparatively high	This is a useful feature for high frequency use. Note: low capacitance silicon diodes are also available but still higher capacitance than point contact types.

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