1 Transistor Negative Resistance 

Negative Resistance in NPN Transistors

When a positive voltage is applied to the emitter and a negative voltage is applied to the collector of an NPN transistor with the base left disconnected, the transistor junctions break down and the transistor starts to conduct. This is known as the avalanche breakdown region. As the voltage is increased, the current flowing through the transistor actually drops. This is because the avalanche breakdown process generates more holes than electrons, which reduces the net charge carrier density in the transistor. This negative resistance effect can be used to create a variety of low-cost circuits, such as relaxation oscillators and voltage regulators.

One way to understand the negative resistance effect is to consider the energy band diagram of an NPN transistor. In the avalanche breakdown region, the electric field across the transistor junctions is high enough to cause electrons to gain enough energy to jump from the valence band to the conduction band. This process generates more holes than electrons, which reduces the net charge carrier density in the transistor. As a result, the transistor becomes less conductive, and the current flowing through the transistor decreases as the voltage is increased.

The negative resistance effect in NPN transistors is typically observed at voltages between 6 and 15 volts. At higher voltages, the transistor eventually reverts to the positive resistance region. This is as the avalanche breakdown process becomes severe.

Negative resistance devices are often used in electronic circuits to generate oscillations, amplify signals, and regulate voltages. Some examples of applications for negative resistance devices include:

• Relaxation oscillators: Relaxation oscillators are circuits that produce a periodic waveform without the need for an external clock signal. Negative resistance devices can be used to create a variety of relaxation oscillators, such as the astable multivibrator and the monostable multivibrator.

• Amplifiers: Negative resistance devices can also be used to amplify signals. For example, a negative resistance device can be used to create tunnel diode type amplifiers.

Overall, the negative resistance effect in NPN transistors is a fascinating and useful phenomenon. It can be used to create a variety of low-cost electronic circuits with a wide range of applications.