Name: Energy Band Gap
Uploaded: 2019-07-19
Duration: 5 min 21 s
Description: We know that certain materials conduct electricity whereas some don’t. Has it got anything to do with their energy bandgap? Let’s find out in this video.

Question

Why does a

p - n

junction diode offer low resistance in forward bias and a high resistance in reverse bias? Explain the characteristic curve of a diode by drawing suitable circuit diagram.

Accepted Answer

A small increase in forward voltage across p-n junction shows large increase in forward current. Hence, the resistance (=voltage /current) of p-n junction is low when forward biased.
A large increase in reverse voltage across p-n junction shows small increase in reverse current. Hence, the resistance of p-n junction is high when reverse biased.

Forward Bias Characteristics:

For the V-I characteristics of a forward biasing P-N junction diode the experimental circuit arrangement is shown in the figure.

Here the applied voltage V on the diode is changed by the potential divider arrangement which can be easily read by the voltmeter connected in parallel to the diode. The current I flowing in the diode related to various values of voltages are noted by a milli- ammeter. In this way, the curve obtained for different values of V and I is shown in the figure, which is also called as the P-N junction diode forward bias characteristic curve.

In forward biasing for every less value of voltage, (for Ge is 0.3 V and for Si is 0.7 V) the forward current is very less. The reason for this is that the value of applied voltage is less than the potential barrier due to which the flow of the current is not uniform. This behaviour of the diode is represented by the OA part of the curve. When the value of applied potential is more increased the current in the diode increases exponentially. This behaviour is shown by part AB. The potential at which the current’s value increase very rapidly is called the diode’s knee voltage or cut in voltage. For germanium diode its value is approximately 0.3 V and for silicon diode its value is 0.7 V.

Reverse Bias Characteristics

For the V-I characteristics of a reverse biasing P-N junction diode the experimental circuit arrangement is shown in the figure. Here with the help of a potential divider arrangement the P and N ends of the diode are connected to the negative and positive ends of the battery. Due to the reverse current being very small a micro-ammeter is used in place of milli-ammeter in this circuit. At different reverse potential the related different reverse current value are noted and a curve is drawn as shown in the figure

As explain in previous part in reverse biasing, the current is very small as it is due to minority- charge-carriers and remains constant till the time the reverse potential reaches the value of breakdown voltage (V_B). The reverse current increases very rapidly with a slight increase in the breakdown voltage.

For a semiconductor diode the complete figure for forward biasing and reverse biasing characteristic curve is shown in the figure. If we consider the curve then in forward biasing more current (~ mA) flows and in reverse biasing (µ or nA) less current flows. As a result P-N junction diode is a unidirectional device. The characteristic curve of the diode also shows that this device does not follow Ohm’s law and hence is a non-linear device.

The dependency of current I flowing in the diode on the biasing V is given by following equation;

$I = I_{s} [e x p \frac{q V}{k T} - 1]$

Where q = 1.6 × 10^-19 C, /e is Boltzmann constant, T is the temperature of the junction and V is the potential difference on the junction. $I_{s}$ is saturation current. In forward biasing when V is positive then;

$e x p \frac{q V}{k T} > > 1 t h e n, I = I_{s} e x p \frac{q V}{k T}$

And current increases exponentially with the voltage. In reverse biasing when V is negative then;

$e x p \frac{q V}{k T} < < 1 t h e n, I ≃ I_{s}$

And in reverse biasing the current is almost constant.

The above equations are similar to the characteristics curve of germanium diode. The diode made of silicon follows this equation only in qualitative form and for both the types of diode in reverse biasing this equation is not true for potential higher than breakdown voltage.

Why does a $p - n$ junction diode offer low resistance in forward bias and a high resistance in reverse bias? Explain the characteristic curve of a diode by drawing suitable circuit diagram.

Important Questions on Electronic Devices

Learn and Prepare for any exam you want !

Why does a p-n junction diode offer low resistance in forward bias and a high resistance in reverse bias? Explain the characteristic curve of a diode by drawing suitable circuit diagram.

Important Questions on Electronic Devices

Learn and Prepare for any exam you want !

Why does a $p - n$ junction diode offer low resistance in forward bias and a high resistance in reverse bias? Explain the characteristic curve of a diode by drawing suitable circuit diagram.