Patent ES-289480-A1 (ES)
Title: Radiation-sensitive semi-conductor device having a substantially linear current-voltage characteristic
IPC: H01L31/00
Filed: 1963-06-28
Abstract: Radiation-sensitive apparatus includes a semi-conductor device having a body of substantially intrinsic or weakly extrinsic material in which upon radiation the conductances of holes and electrons are of the same order, the body being provided with at least two electrodes one capable of injecting mainly only holes and another capable of injecting mainly only electrons. The external circuitry used with such a device applies a bias to the electrodes such that double injection occurs. At least in the centre of the body holes and electrons share a substantially common current path the impedance of which is varied by the radiation. In the simplest embodiment the apparatus includes a PIN or PSN device biased so that the P and N zones and their associated metal contacts act as injectors for holes and electrons respectively and additionally serve as collectors for charge carriers of opposite sign to those they are injecting. To keep down the dark current of the device the distance between the electrode zones measured along the current path in the intermediate zone should be at least five times the average diffusion-recombination length therein. To reduce potential discontinuities which arise when charge carriers are collected at an electrode designed for the injection of carriers of the opposite type, transition regions containing high concentrations of centres causing recombination of collected carriers may be provided adjacent the terminal zones. In operation radiation should preferably be allowed to fall on the terminal zones in addition to the intermediate region thus reducing the lead-in resistance to the common holeelectron current path. With D.C. operation both terminals are continuously injecting carriers, while under A.C. use the advantages of the invention are obtained during the halfperiods corresponding to forward bias. Current amplification is obtained if the potential difference applied is such that the mean transit time in the intermediate region is less than the mean recombination lifetime. Suitable semiconductors are silicon, germanium, and AIIIBV compounds (e.g. indium antimonide and gallium arsenide). The intermediate region of the body should be of high purity, compensated, or weakly extrinsic semi-conductor. The number of predominant charge carriers at the operating temperature and in the absence of radiation should not exceed 1015.cm-3. If the semiconductor used has a small band gap this low number may be achieved by operating the device at a reduced temperature. In the PIN or PSN device above, the electrodes consist of the metal contacts and their associated doped semi-conductor zones in which the extrinsic conductivity is preferably one hundred times greater than that in the intermediate zone. Other electrode structures may be used. In one the terminal zone in association with a metal contact consists of a semi-conductor having a larger band gap than the semi-conductor of the intermediate zone. In another a direct metal to semi-conductor junction acts as an injector or collector. Capacitative connections may be used for A.C. devices though not essential. In further embodiments the incidence of potential discontinuities arising from the use of a single electrode for simultaneous injection and collection is avoided by separating the injecting and collecting functions. In the device shown in Fig. 2 P-type zone 6 and its associated metal contact acts as a hole injector and is paired with an electron collector constituted by N-type zone 23 and its associated contact. At the other end of the body electrons are injected from N-type zone 5 while holes are collected by P-type zone 21. In the arrangement shown, biasing for the hole and electron circuits is provided by batteries 8b and 8a and separate load impedances 9b, 9a are provided one or both of which may be a measuring instrument or relay. The impedances are preferably adjusted to balance the current in the two circuits. If, as shown, the electrode pairs at the ends of the body are identical the device will function equally well with A.C., the injecting and collecting functions of the electrodes being interchanged each time the applied polarity is reversed. During operation the holes and electrons share a common path in the intermediate region, the paths only diverging in the parts near the electrodes. To reduce losses radiation is directed on to these parts as well as on to the centre. To obtain adequate separation of their respective functions the injecting and collecting zones of each pair are mutually separated by no more than five diffusion lengths. In general, to obtain further separation of the injection and collection functions a bias is applied between the two electrodes of each pair with the positive side of the bias source connected to the P-type zone. Separation of the functions gives the device a low dark current but even so the body length should preferably be made longer than three diffusion lengths. Although as shown the collector is especially designed for collecting, it is sufficient that it should be better at collecting than the adjacent injector. For example, an ohmic electrode may be used as the hole collecting electrode associated with an electron injector having an N-type region. The injecting and/or collecting electrodes may be subdivided each to two or more electrodes, electrodes having the same function being connected in parallel. Suitable semi-conductors and alternative electrode structures are the same as those mentioned for the simplest embodiment. Devices need not have the shapes shown, for example an at least partly annular body can be used. Electrodes need not be disposed opposite each other. Auxiliary resistors (for circuitry) may be incorporated in the electrode zones. In the arrangement of Fig. 2 there are two external circuits, one effectively deals with the injection and collection of holes and the other effectively deals with the injection and collection of electrodes. The circuits are coupled by the body. In the arrangements described with reference to Figs. 3, 4 and 5 (not shown) a device similar to that shown in Fig. 2 is used (its zone positions are slightly altered), the differences between these arrangements lying in the external circuitry used. These circuits are arranged to provide a bias between each injector and its associated collector of opposite type carriers. A practical device is illustrated in Fig. 6 (not shown). Its 20 ohm cm. P-type germanium body is about 15 mm. long, 1 mm. wide, and 0À2 mm. thick and has Á #p # 1800 cm.2.v-1.sec-1, Á #N # 3600 cm.2.v-1.sec-1, #E # 0À72 eV, T ~ 10-4 sec., and 1d ~ 1 mm. Two 250 Á spheres of lead/gallium alloy (Ga = 0À5 wt. per cent) are heated in contact with the opposite ends of the upper surface of the body for 5 minutes in a hydrogen atmosphere at a temperature of 700 C. to produce 4 Á thick P regions (6, 21) with [Ga] ~ 1020 atoms cm.-3. Similarly sized spheres of lead/antimony alloy (Sb = 2 wt. per cent) are similarly alloyed to the lower surface to produce N regions (5, 23) with [Sb] ~ 1019 atoms cm.-3. As described, the device is connected in several arrangements, some according to the invention and some included for comparison. During tests the device is subjected to constant illumination over its entire upper surface and current-voltage measurements made. Results are given in Figs. 7 and 8 (not shown) for the device used in the arrangement described with reference to Fig. 2, used in an arrangement in which two electrodes of each pair are shorted together, and used in various arrangements in which one electrode from each end of the body is selected, the other two electrodes being open-circuited. Those arrangements in which double injection occurs show a substantially linear relationship between current and voltage at a constant level of illumination.