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Electronics I Laboratory Manual

Diode Biasing

A semiconductor diode is a non-linear device whose most outstanding feature is the fact that current is allowed to flow – basically – in one direction only.  The diode is built by joining together two semiconductor materials: an N-Type material and a P-Type material.  The area of contact is called thejunction.  This is the reason why sometimes we refer to the diode as a PN Junction.

When an applied voltage forces the diode to conduct we say that the diode is operating in the forward bias condition; when the supply voltage is connected such that the current in the diode is minimal (practically zero) the diode is in the reverse bias condition.  When in the forward bias mode1 the diode behaves much like a closed switch; on the other hand, in reverse bias2 the diode behaves like an open switch.  To understand the operation of a diode we must understand the diode I-V characteristics.

Diode Characteristics

Figure 5.1 shows the typical I-V characteristics of a silicon diode3.  The two operating regions are clearly labeled.  Notice the different scales used to indicate the current in each case.  For the forward bias case the current in the particular I-V curve is expressed in milli-amps (mA), whereas in the reverse bias region the current is expressed in micro-amps (µA).  The main features of these two operating conditions are explained below.

Forward Bias Region

As you can see, at low levels of diode voltage the current is very small.  After reaching a particular voltage – called the threshold voltage, or Vthr – the current increases abruptly.  The threshold voltage is dependent on the type of material the diode is built with:  For a Silicon diode the Vthr = 0.7 V, and for a Germanium diode Vthr = 0.3 V.


In the forward bias region we can distinguish two important areas in relation to the amount of current in the diode.
 

Diode Voltage isVthrFor any diode voltage (VD) between zero and Vthr the current is very small.  In general, as an approximation, we may consider this current to be zero.  This means that in this range the diode behaves like an open circuit, or like a device with a very high resistance.


Diode VoltageVthr In this region the current increases very fast.  As you can see in Fig. 5.1 the voltage across the diode (VD) remains more or less constant and equal to Vthtr.  In this area the diode voltage is independent of the source voltage.  Sometimes we assume that the diode voltage is equal to zero, so the diode behaves like a closed switch, or like a device with a very small resistance.


1And the junction voltage is bigger than the threshold voltage – approximately 0.7 V for Silicon diodes and 0.3 V for Germanium diodes.

2And when the junction voltage is less than the breakdown voltage.

3See Experiment No. 2 for general considerations of characteristics of non-linear devices.

Reverse Bias Region


In reverse bias the current through the diode is very small (practically zero) when the junction voltage is between zero and the so-called breakdown voltage, VBD.  This is the voltage that produces an abrupt increase in current.  The breakdown voltage is not a constant value like the threshold voltage in forward bias.  VBD is different for each diode.  This value is a specification parameter given by the manufacturer.

Like in the last case, in reverse bias we can distinguish two important areas. 

Diode Voltage isVBD In this area the current is very small.  We call this current the leakage current.  In practical applications you may consider it to be zero.  Thus, in this area the diode behaves like an open switch, or like a device with a very large resistance.

Diode Voltage isVBD In the breakdown region the current increases very fast as a function of the diode voltage.  The diode behaves like a closed switch, or like a device with a very small resistance.  Notice that the diode voltage in this case is very close to VBD for practical applications, for any source voltage.

 

Current

Resistance

Ideal Behavior


 
Forward Bias
(VD    Vthr)

 

 

  0

 

very large ()

 

 

open switch

 


 
Forward Bias
(VD   Vthr)

 

 

large

 

very small

 

 

closed switch

 

 
Reverse Bias
(VD    VBD)

 

  0

 

very large ()

 

 

open switch

 


 
Reverse Bias
(VD    VBD)

 

 

large

 

very small

 

 

closed switch

 

Table 5.1:  Diode Conditions
 

Table 5.1 is a summary of the diode operating conditions.  The last column of the table indicates the behavior of an ideal diode.  When an ideal diode is forward biased it would behave like a closed switch with a resistance equal to zero ( 0 Ω).  In reverse bias the ideal diode is similar to an open switch with current equal to zero and infinite () resistance. 
                                                                                                                                                   
  

Note:
 

Of course ideal diodes do not exist, so the forward resistance of the diode is not 0 Ω but a low resistance, and the reverse bias resistance is notbut a high resistance.
                                                                                                                                                   

Diode Identification
The schematic symbol for the diode is typically a filled arrow with a short line across the tip.  The cathode is the N-type material and is represented by the tip of the arrow.  The anode is the P-type material and is indicated by the base of the arrow. 

The manufacturers of diodes used diverse methods to indicate the anode and the cathode.  In the most common method the cathode (N-type material) is usually identified with a colored band.  Thus, the end of the diode closest to this band is the cathode.  The other end is the anode (P-type material.)


Connections


Figure 5.2:  Diode Biasing:  Forward and Reverse           

When a diode is used in a completed circuit, if the positive potential (highest) is connected to the P material and the negative potential to the N material, then the diode is in forward bias.  If, on the other hand, the highest potential is connected to the N material and the lowest potential to the P material (or, in other words, if the N material is at higher potential than the P material) the diode is reverse biased. Figure 5.2 shows the forward and reverse biasing of a diode (ideal, in the case) connected to a circuit.  The student should pay special attention to the connections needed to achieve a particular biasing.

Ohmmeter Measurement

To find the condition of a diode we can measure its resistance with an ohmmeter.  The actual resistance can be measured, but the student should be aware that this process is often a source of some confusion. 

The equivalent circuit of an ohmmeter is shown in Figure 5.3.  The meter movement may be represented as some resistance which could be lumped with the ohmmeter resistance R, as shown in the figure.  If we connect the test lead of the meter as shown, the circuit is really a simple forward biased circuit as shown in Figure 5.3.  What causes the confusion is that the equivalent voltage varies from one meter to another and even changes whenever the ohmmeter’s range is changed.  This varies the amount of forward biasing applied to the diode, and as a consequence varies the resistance measurement, so the same diode might measure a forward resistance, say, of 20 on one range and 300 Ω on another range or on another meter.  This is quite normal and, once understood, will not bother the student.  

The important things that must be known when taking the resistance measurements concern the ohmmeter’s test lead polarity, and the ratio of forward and reverse diode resistance.  The student should test the ohmmeter with a voltmeter to be sure of the test lead polarity, or else refer to the operating manual for the instrument. The actual measured forward or reverse resistance of a diode is not particularly important.  What is important is that the resistance should be much higher in one direction than in the other, typically 1 kΩ or less in the forward bias direction and 1 MΩ in the reverse direction.  The measurement of any semiconductor resistance will always depend upon the ohmmeter’s internal voltage.

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