Patent GB-853457-A (GB)
Title: Very high frequency unipolar transistors
IPC: H03K17/28
Filed: 1958-02-08
Abstract: 853,457. Unipolar transistors. TESZNER, S. Feb. 8, 1958 No. 4185/58. Class 37 A unipolar transistor comprises a bar of semiconductor material with an ohmic electrode at each end and a thinned central portion surrounded by at least two electrodes making rectifying contact therewith. In one embodiment (Fig. 12) a cylindrical body 8 of 10 ohm. cm N type germanium the thinned portion of which has a diameter of 6 Î 10<SP>-</SP><SP>3</SP> cm. is provided with ohmic electrodes 1, 2 and annular rectifying electrodes 14, 15 of length 1À5 Î 10<SP>-2</SP> cm. and 5 Î 10<SP>-4</SP> cm. respectively. In another embodiment (Fig. 13) using a bar of similar dimensions there are three annular rectifying electrodes 19, 20, 21 of length 10<SP>-2</SP> cm., 5 Î 10<SP>-4</SP> cm. and 5 Î 10<SP>-3</SP> cm. respectively. In both devices the rectifying electrodes which may be of the PN junction, metal semi-conductor, or metalinsulator-semi-conductor type are 10<SP>-</SP><SP>3</SP> cm. apart. An upper limit is imposed on the operational frequency of a unipolar transistor using a single gate to which the controlling signal is applied by the RC time constant of the gate. to conductive channel circuit, and by the transit time of charge carriers from one end of the gate electrode to the other. The first is the more stringent limitation and the frequency limit may be raised only to a limited extent by reducing the length of the gate because the edge capacity increases disproportionally when the electrode becomes short relative to the diameter of the channel, and when the carrier velocity exceeds a certain value in the channel the resistance of the latter tends to a constant value despite any further reduction in length. The time constant can of course be reduced if the cross section of the channel is reduced and the resistivity increased since the gate length can then be made much smaller before the edge and carrier velocity effects become appreciable. Constructional difficulties limit the extent of such improvements. In such devices the conductive channel is very much narrower at one end of the gate on account of the potential gradient along the bar so that the change in resistance of the channel produced by a modulating signal occurs almost entirely at the narrow end of the channel. The principal contribution to the gate-channel capacity however comes from the opposite end of the channel where the space charge layer is thin. In the device shown in Fig. 12 the gate electrode is divided into two parts between which the modulating signal is applied. Since the capacity between electrode 14 and the conductive channel is very much larger than that between electrode 15 and the channel substantially the whole of the modulating signal voltage is applied to the latter electrode where it is most effective in changing the channel resistance. For zero bias on the gate the space charge layer has the configuration shown in full lines in Fig. 7 and moves between the limits shown by broken lines in response to the modulating signal. The area of space charge layer affected by the signal and hence the capacity effective in limiting the frequency is much greater than the area of electrode 15 because of the edge effect of electrode 14. A further improvement in this respect is obtained in the device shown in Fig. 13, using two auxiliary electrodes 19 and 21. To ensure maximum effectiveness in such an arrangement it is essential that there should be no region of pinch off beyond the electrode 20 to which the signal is applied so that the latter operates on a region contributing a substantial proportion of the total resistance of the channel. This is ensured by heavily biasing the annular electrodes. The configuration of the space charge region and the extent to which it alters in response to the modulating signal is shown in Fig. 10. Since only the layer immediately beneath electrode 20 changes the effective capacity is much reduced. In addition the resistance R, which in this case comprises resistances 19 to 20 and 20 to 21 inparallel, and is thus less than in the Fig. 12 device, is minimized by making the two resistances equal by using the electrode lengths specified above. The resistance along the conductive channel for a zero value of bias 25 is shown in Fig. 11, the broken lines representing the change in response to signals. The working point i.e. the position of line 23 between 24 and 241 may be controlled by bias 25. Several of the devices,which may be provided with cooling means, may be wired in parallel for high power applications without affecting the upper frequency limits, which are calculated in the Specification to be 600 mc/s for the Fig. 12 device and 5500 mc/s for the Fig. 13 device. Specification 808,734 is referred to.