Patent GB-1138029-A (GB)
Title: Semiconductor devices
IPC: H01L23/58
Filed: 1966-03-04
Abstract: 1,138,029. Semi-conductor devices. RADIO CORPORATION OF AMERICA. 4 March, 1966 [19 March, 1965], No. 9709/66. Heading H1K. In a transistor having an electrode contact bonding pad extending over a dielectric layer, the capacitance between the pad and the collector region is reduced by forming a low impedance screen region in the body below the location of the pad and connecting this region to a source of electrical potential. As shown, Fig. 1, the surface of an N-type silicon substrate 10 is masked and boron is diffused-in from the vapour phase to produce P-type regions 12 and 14 which constitute the base region and the screen region respectively. Phosphorous is diffused into region 12 to form N-type emitter region 18. The surface of the wafer is covered with a layer 20 of silicon oxide, windows are formed to expose parts of the emitter, base and screen regions, and aluminium is evaporated-on to form the electrodes. The emitter electrode comprises a portion 22 which contacts region 18 and a portion 21 which forms the bonding pad and overlies arm 15 of screen region 14 from which it is insulated by oxide layer 20. The base electrode is similarly constructed and arranged over arm 16 of region 14. Bonding pad 26 provides a contact to screen region 14 and may be earthed in use. The device is enclosed in a four-lead encapsulation. In a second embodiment, Figs. 2, 2a, 2b and 2c (not shown), the starting wafer comprises an epitaxial layer grown on a more highly doped substrate and the device has a plurality of emitter regions (47), each having an electrode (48) connecting it to a common emitter bonding pad (46), alternately arranged with base contact regions (33), each having an electrode (44) connecting it to a common base bonding pad (43). The transistor is produced by growing the epitaxial layer, diffusing-in heavily doped regions to form the base contacts (33) and the screen region (35) and then diffusing-in base region (41) and emitter region (47), covering the surface with silicon oxide, forming windows and depositing the contact regions. In further embodiments: (i) the screen region surrounds the base region and is electrically connected to the emitter region by means of a common bonding pad (76), Figs. 4, 4a and 4b (not shown); (ii) as in (i) but with the screen electrically connected to the base region instead of the emitter region, Fig. 5 (not shown); (iii) two separate screen regions are provided for the base and emitter bonding pads, the base pad screen region being connected to the emitter and the emitter pad screen region being left floating, Figs. 6 and 6a (not shown); and (iv) as in (iii) but with emitter pad screen region connected to the base and the base pad screen region being left floating, Fig. 7 (not shown). A plurality of transistors may be formed in a single wafer of semi-conductor material which is then divided into separate devices. The semiconductor material may be silicon, germanium, silicon-germanium alloy, gallium arsenide, or indium phosphide. The following impurities are referred to in connection with silicon devices, boron aluminium, gallium, indium, phosphorus, arsenic and antimony. Selenium and tellurium are mentioned as donors for gallium arsenide and indium phosphide. The electrodes may be of aluminium, gold or chromium and may be applied by evaporation, electroplating or electroless plating. If the semi-conductor material is gallium arsenide the insulating layer may be produced by forcing the pyrolytic decomposition products of a vaporized siloxane compound through a jet on to the wafer. An encapsulation, Fig. 3 (not shown), is described and comprises a header (50) with four lead-out wires one of which (57) is shorted to the header (50) and is also connected to the screen region. The body of the wafer (the collector) is mounted on a platform (59) connected to a lead-out wire (55<SP>11</SP>), and the emitter and base electrodes are connected to two other lead-out wires (55, 55<SP>1</SP>) respectively.