Gate controlled transport in a GaAs:AlGaAs heterojunction
Optical and electron beam lithography has been used to fabricate high mobility GaAs:AlGaAs heterojunction FETs in which the current is controlled by Schottky barrier gates with novel geometries. The two dimensional electron gas (2DEG) at the heterojunction interface had a low temperature mobility of ~250,000 cm2V-1s-1 and a carrier concentration of 4.3x1011cm-2. Narrow channels of the 2DEG were defined by means of a split gate which consisted of two gold pads 15μm long, separated by ~1μm. A negative voltage applied to the gate removes carriers from beneath the gate resulting in a narrow channel in the gap. The channel width can be reduced to zero by further decreasing the gate voltage. At low temperatures (T ≤ 4.2K) the electron phase coherence length, Lφ, is greater than the width, W, and the transport is quasi one dimensional. Analysis of the low temperature magnetoconductance showed that for a channel of width ~450AA the phase coherence length varied as Lφsim 0.16μ m(T/K)^-0.35±0.06. A similar result was obtained from an analysis of the universal conductance fluctuations in channels of width ~ 1800AA. This suggests that the dominant electron scattering mechanism was due to electromagnetic fluctuations in the 2DEG for which Lφ would be expected to vary as T^-1/3. For high magnetic fields (O < B ≤ 8T) the magnetoconductance showed oscillations which were explained in terms of the magnetic depopulation of one dimensional subbands. A number of fine gate FETs were made with gate lengths of ~ 1000AA. The I-V characteristics of a strongly depleted channel were measured at 4.2K and it was found that I ∝V3/2 so that the current flow was dominated by space charge effects. For larger source drain biases I ∝ V and this was explained as being due to velocity saturation. The second voltage differential δ2V/δI2 showed structure at ~ 40meV and ~ 80meV and this was attributed to optic phonon emission by hot electrons.