Gunn Effect and Gunn Diode ( tranferred electron effect )
Gunn effect :-
From Gunn's observation the carrier drift velocity is linearly increased from zero to a maximum when the electric field is varied from zero to a threshold value.
When the electric field is beyond the threshold value of 3000 Vlcm for the n -type GaAs, the drift velocity is decrease and the diode exhibits negative resistance. This situation is shown in Figure 1.
Figure 1: Drift velocity of electrons in n-type GaAs versus electric field
Gunn also discovered that the threshold electric field Eth varied with the length and type of material.
RIDLEY-WATKINS-HILSUM (RWH) THEORY :-
Many explanations have been offered for the Gunn effect. Gunn's observations were in complete agreement with the RidleyWatkins- Hilsum (RWH) theory
Differential Negative Resistance :-
The fundamental concept of the Ridley-Watkins-Hilsum (RWH) theory is the differential negative resistance developed in a bulk solid-state III-V compound when either a voltage (or electric field) or a current is applied to the terminals of the sample. There are two modes of negative-resistance devices: voltage-controlled and currentcontrolled modes as shown in Fig. 2(a) and (b), respectively .
Figure : Diagram of negative resistance
In the voltage-controlled mode the current density can be multivalued , whereas in the current-controlled mode the voltage can be multivalued. The major effect of the appearance of a differential negative-resistance region in the currentdensity- field curve is to render the sample electrically unstable. As a result, the initially homogeneous sample becomes electrically heterogeneous in an attempt to reach stability. In the voltage-controlled negative- resistance mode high-field domains are formed, separating two low-field regions. The interfaces separating lowand high-field domains lie along equipotentials; thus they are in planes perpendicular to the current direction as shown in Fig. 3(a). In the current-controlled negative-resistance mode splitting the sample results in high-current filaments running along the field direction as shown in Fig. 3(b).
Figure 3: Diagrams of high field domain and high current filament
Expressed mathematically, the negative resistance of the sample at a particular region is
If an electric field Eo (or voltage Vo) is applied to the sample, for example, the current density Jo is generated. As the applied field (or voltage) is increased to E2 (or V2), the current density is decreased to J2. When the field (or voltage) is decreased to E1(or V1), the current density is increased to J1 . These phenomena of the voltagecontrolled negative resistance are shown in Fig.4(a). Similarly, for the currentcontrolled mode, the negative-resistance profile is as shown in Fig. 4(b).