The Helium Field Effect Transistor (I): Storing Surface State Electrons on Helium Films

Abstract

We present investigation s of surface state electrons on liquid helium film in confine geometry, using a suitable substrate structure microfabricated on a silicon wafer, s imilar to a Field Effect Transistor (FET). The sample 'has a SOUl'ce and drain region , separated by a gate structure, wh ich consists of two go ld electrodes with a narrow gap (channel) through which the tran sport ofthe surface state electrons takes place. The sampie is illuminated to provide a sufficien number of free carriers in the sili con substrate, such that a well-define potential distribution is ac hieved. The eventual goa l ofthese experiments is to study the electron transport through a narrow channel in the variolls states ofthe ph ase diagram ofthe 2D e lectron system. In th e present work we focus on storing the electrons in the source area of the FET, and investigate the spatial distribution of these electrons. lt is shown that under the influ ence of a potential g radi ent in the sili con substrate the e lectron s accllmulate in front of the potential barri er of the gate. The electron di stribution, governed by Coulomb repulsion and by the substrate potentia l, is determin ed ex perim entally. The result is found to be in good agreement w ith a parallel-plate capacitor mode l of the system, developed with the aid of a finit element ca lculation of the surface potential profil of th e device.

The Helium Field Effect Transistor (II): Gated Transport of Surface-State Electrons Through Micro-constrictions

“…[13], and a sample chip with FET geometry as well as examples for gate channels (long, short, and structured) is shown in Fig. 1.…”

“…Since we are using undoped silicon, the substrate is insulating at low temperature. Its conductivity can, however, be increased by external illumination, and under this condition, a voltage applied between the edges of S and D provides a well-defined linear potential gradient for the SSE [ 13]. A typical value for the potential difference between S and D is 1 V, and the gate voltage is also in the range of a few V. The electrons used to charge the source are supplied by a tiny filament, which is periodically heated with short current pulses at a repetition rate of 4 Hz.…”

“…The de pickup current registered for that particular run was 5 pA. Then, the gate was closed at A, /p dropped to zero, and the electrons from the filament filled the source. After a few seconds, a high enough charge density was reached that electrons started to spill over the gate barrier [13], and the filament was switched off at B. Under this condition, the electrons can be stored in S for times much longer than minutes.…”

“…This problem was investigated previously with the same setup in Ref. [13]. There it was shown that due to the externally applied potential gradient along the sample, the electron density is not constant across the source area, but reaches its maximum value no directly at the channel entrance.…”

“…In Ref. [13], a model was developed which allows one to determine no quantitatively from the potentials applied to S, D, and the gate G. The electron densities no studied here were between 10 6 and 10 10 cm-2 , that is in a range where the SSE behave like a classical system. A preliminary report of some of the results presented here has already been given in Ref.…”