BPH 221 Lecture Series: Lecture 3 (I-V Characteristics of a diode)

To understand how we can make a useful application of a diode, we require the knowledge of its electrical characteristics. The characteristics of diode can be obtained by using a simple circuit like the one in figure 3.1.

Figure 3.1a:  Circuit diagram of diode in forward bias

In the circuit, we have a milli-ammeter and a protective resistor in series with the diode, the voltmeter V is connected to measure the voltage across the diode. Note that the positive pole of the voltage source is connected to the anode of the diode, meaning that it is in a forward bias connection.  Our objective is to study the current flow and voltage across the diode as the Voltage Vs increases.

 

Figure 3.1b:   Virtual implementation of circuit in figure 3.1a

To demonstrate the I-V characteristics of the diode, we implement the circuit in virtual laboratory, figure 3.1b shows the circuit implementation, the potentiometer P1 is used to vary the voltage output of the voltage source V1 across the diode D1. AM1 and VM1 measure the current and voltage across the diode respectively for every stepwise increase in P1 output.

Simulating the circuit we obtain the DC transfer characteristics of the diode, as shown in figure 3.2, bellow;

                                                                          (a)

                                (b)

Figure 3.2:  I-V characteristics curve of forward bias diode.

Figure 3.2(a) is showing the potentiometer output against current AM1 and voltage VM1 on different graphs, while figure 3.2(b) shows the VM1 (V) output against AM1 (I) output from the diode (I-V characteristic curve).

We can observe from the curves that, as the voltage VM1  across the diode increases, initially the corresponding current output is very low, unable to give any meaningful indication on the milli-ammeter, but when VM1 value gets to a certain level, 0.35V in our circuit example, you’ll begin to notice some increase in the level of current and at a point the current increases exponentially with respect to the voltage, VM1.

The explanation is that, current starts to flow in the circuit only when the voltage VM1 across the diode has been able to overcome the built-in voltage barrier of the diode, (see discussion in Lecture 1), the exponential increase in current occur at a voltage level a little above the voltage barrier, this is called the cut-in voltage.

We can also study the I-V characteristics of the diode in reverse bias condition. 

 

Figure 3.3a:  Reverse bias connection of the diode.

Figure 3.3b:   Virtual implementation of circuit in figure 3.3a

Here, we can observe that the polarity of V1 is reversed as against the circuit in figure 3.2, now the positive pole of the supply connects to the cathode and the negative pole to the anode of the diode, i.e. in reversed bias connection (see previous lecture). Simulating the circuit we obtain the DC transfer characteristics of the diode, as shown in figure 3.4, bellow;

(a)   Current AM1 and VM1 as they vary against potentiometer output

(b)   Voltage VM1 against Current AM1, I-V Characteristics curve

Figure 3.4: I-V characteristics of a diode in reverse bias connection.

From Figure 3.4(a) and (b), we can observe only a very small negative current bellow -40nA in AM1, compare to current flow in mA in the case of forward bias connection. The infinitesimal current flow is maintained at this same level even as voltage VM1 increases across its axis. The current is so small and unable to give any meaningful indication on the milli-ammeter so it is assumed as zero current (open circuit) with reverse biased connection of the diode.

We can combine the two characteristic curve i.e. reversed and forward biased in a single graph as shown in figure 3.5, bellow;

Figure 3.5: Combining the Forward and Reverse bias I-V Characteristics curve of a diode.

Types of Diodes:
1.  Light Emitting Diodes (LED):
LEDs are also a type of semiconductor diodes that glows when voltage is applied to it in a forward biased connection. This diode has many applications in electronics. Go to Light Emitting Diodes for discussion on the Construction, Electrical property and Applications of Light emitting diodes (LEDs). 
 2.  Photodiodes:
 Photodiodes are another type of semiconductor diodes, they a designed in such a way which, when exposed to light, generates a potential difference or change its electrical resistance. Go to Photo-diode for a brief discussion on operation and applications of photo-diode. 
3. Varactor Diodes:

A varactor , also known as tuning diode, a variable capacitance diode, a varicap diode or variable reactance diode, is a diode that exploit the principle of operation of semiconductor diode, to act or behave like a variable capacitor i.e. has variable capacitance, which is a function of the external voltage impressed on its terminals. Go to Varactor Diode for a brief discussion on construction and application of varactor diode. 


PREVIOUS LECTURE                                                                       NEXT LECTURE

I am Adeniran Adetunji, contact email: tunji4physics2@gmail.com