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The Tunnel and Inversed Diodes

Electrons in Solids | Intrinsic Conduction | Extrinsic Conduction | Diffusion Currents in Semiconductors | The Capacitances of a Semiconductor Diode | Semiconductor Diodes as Rectifiers | Physical Processes in a Transistor | The Basic Circuit Configurations of Transistors | Models of Transistors | Bias Supply and Temperature Compensation for Transistors |


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  4. Microwave Semiconductor Diodes
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  6. Storage Tunnel
  7. Structures of Semiconductor Diodes

A tunnel p-n junction diode that is made from germanium or gallium arsenide with an extremely high level of impurity concentration (or doping) on each side of the junction (of the order of 1019-1020

cm-3).

In a tunnel diode, as is the case with ordinary semiconductor diodes, carriers diffuse across the p-n junction and drift in the reverse direction under the influence of the field. Both processes, however, are of minor importance as compared with what has come to be known as the tunnel effect or tunnelling.

 

Fig. 3.16. Current-voltage characteristic of a tunnel diode

 

The current-voltage characteristic of a tunnel diode show in Fig. 3.16. At U = 0, current is zero. An increase in the forward voltage to 0.1 V leads to a rise in the forward tunnel current to a maximum (point A). The further increase in forward voltage to 0.2 V is accompanied by a decrease in the tunnel cur­rent. Therefore, the current at point В is a minimum, and the characteristic has a down-sloping portion, AB, corresponding to a ne­gative resistance to alternating current:

. (3.7)

By connecting a tunnel diode in a circuit in a suitable manner, it is possible to use its negative resistance so as to balance the positive resistanceand to obtain an amplifier or an oscillator. A oscillator circuit using a tunnel diode VD is shown in Fig. 3.17.

Fig. 3.17. Connection of a tunnel diode as an oscillator

 

Electrons tunnel through the potential bar­rier during an extremely short time: 10-3-10-5 ns. Therefore, tun­nel diodes do their job especially well at mic­rowave frequencies as high as tens or even hundreds of gigahertz.

Fig. 3.18. Current-voltage characteristic and graphical (circuit) symbol of an inversed diode

 

If the material used to fabricate a tunnel diode has an impurity concentration of about 1018 cm-3, there will be practically no tunnel current flowing under forward bias, and the current-voltage characteristic will not have a negative-resistance (down-sloping) portion (Fig. 3.18). Such devices have come to be known, quite appropriately, as inversed diodes; they can effectively operate as detectors at far higher frequencies than the usual diodes.

 

 

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