SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

498 CHAPTER 10. COHERENT TRANSPORT AND MESOSCOPIC DEVICES

1.5

1.0

0.5

0.0

VOLTAGE (V)

300 K

C

URRENT DENSITY

(^10

4 A/cm

2 ) 24 Å 24 Å

} }

1.2 eV

44 Å

0.0 0.5 1.0 1.5 2.0

InGaAs

AlAs

Figure 10.7: Room temperature current–voltage characteristics of an InGaAs/AlAs resonant
tunneling diode.

can be exploited to design digital devices and switches operating at very low power levels. The
general principle of operation is shown in figure 10.9. Electron waves travel from a source to a
drain via two paths. At the output the intensity of the electron wave is (addition is coherent)

I(d)=|ψ 1 (d)+ψ 2 (d)^2 (10.4.1)

If the waves are described by

ψ 1 (x)=Aeik^1 x

ψ 2 (x)=Aeik^2 x (10.4.2)

wherek 1 andk 2 are the wavevectors of the electrons in the two paths. We have

I(d)=2A^2 [1−cos(k 1 −k 2 )d] (10.4.3)

If we can now alter the wavevectors of the electron (i.e., the value of (k 1 −k 2 )) we can
modulate the signal at the drain. This modulation can be done by using an electric bias to alter
the kinetic energy of the electrons in one arm. In figure 10.8b we show a schematic of a split-
gate device in which electrons propagate from the source to the drain either under one gate or
the other. The ungated region is such that it provides a potential barrier for electron transport as
shown by the band profile. Interference effects are then caused by altering the gate bias.
In quantum interference transistors, a gate bias is alters the potential energy seen by the elec-
trons. The electronk-vector at the Fermi energy is given by (Ecis the bandedge i.e the subband