Tunnel Diode Working Principle

The tunnel diode is a PN junction device, that operates in certain regions of V-I characteristics by the tunnelling of electrons across the potential barrier of the junction. This device can be used in high speed switching and logic circuits. Tunnel diodes are useful in many applications such as microwave amplifiers, microwave oscillators etc because of its low cost, light weight, high speed, low power, low noise and high peak current to valley current ratio characteristics.

Principle of Working:

1. Under Equilibrium:

The fermi level is constant throughout the junction. The Fermi level lies below the valence band in the ‘p’ side and above the conduction band in ‘n’ side. Since there are no filled states on one side of the junction which is at the same energy level as the empty state on the other side no flow of charge occurs in either directions and the current is zero. The bands must overlap for the Fermi level to be constant. With a small FB or RB, filled and empty states appear on opposite side separated by width of the depletion region.

2. Under Forward Bias:

When the forward bias is applied across the junction, the potential barrier is decreased by the magnitude of applied voltage. A difference in Fermi levels is created in both sides. While, there are filled states in the conduction band of ‘n’ region at the equal energy level as the authorized empty states in the valance band of p region. Electrons tunnel through the barrier from n region to p region, giving rise to forward tunnelling current from p region to n region. As the forward bias is increased to a max voltage, a maximum number of electrons can tunnel through the barrier producing peak current. If the voltage is increased further, the tunnelling current decreases as there are no empty states available in the p region. When the forward bias voltage is increased further, the current flow increases which is mainly due to minority charge carriers and is known as ‘injection current’.

Tunnel Diode Characteristics:

When the applied forward voltage is between 0 and V_p, the electrons tunnel from n region to p region, thereby increasing the current as the applied forward voltage reaches a value Vp, the current flowing across the junction attains a maximum value called the peak current ‘I_p’. When the applied voltage is increased further a decrease of current occurs this produces a region of negative slope. The maximum value of current achieved in negative resistance region is called Valley Current, I_v at an applied valley voltage V_v. If the voltage is increased beyond the negative resistance region, the current begins to increase due to the flow of minority charge carriers. This characteristic is also known as ‘Voltage controlled negative resistance’ as the current decreases rapidly at a critical voltage.

Tunnel Diode Oscillator:

The value of resistor must be in like a way that it biases the tunnel diode in the middle of negative resistance region. The resistor R₁, is used for setting proper biasing point for the tunnel diode. Resistor R₂, sets proper biasing point for the tank circuit. A parallel combination of resistor Rp, inductor L and capacitor C forms the tank circuit, which resonates at a frequency. Voltage drop in the tunnel diode increases as the applied voltage increases. As a result, the tunnel diode is driven to negative resistance region. In this region, current starts decreasing until the voltage across the diode equals the valley voltage. At this point, further increase of the applied voltage increases the current. This increase in current will raise the voltage drop in the resistor which produces the voltage across the diode.