Double diffused gallium arsenide transistors

Abstract

Double diffused gallium arsenide transistors utilising a basewidth of 0·3 μ to overcome low minority carrier lifetime have been made. The npn mesa structures use zinc as a base-dopant and tin for the emitter. The impurity concentration of the starting material is limited to about 1 × 10 16 cm −3 because of the tendency to thermal conversion. The highest emitter doping achieved is about 2 × 10 19 cm −3. These restrictions require a base diffusant with surface concentrations of the order 10 17 cm −3. Among the available acceptor impurities zinc was chosen over manganese (incomplete ionization at room temperatures), and cadmium (surface reactions). A three-step diffusion cycle was developed in order to reduce the zinc surface concentration several orders of magnitude from ordinary vapor phase diffusions to the desired level. To achieve complex emitter patterns, methods parallel to those used for silicon were developed. Resulting devices have operated at junction temperatures from liquid nitrogen to 350°C. The d.c. current gain is typically 6 at a collector current of 80 mA, with values as high as 15 observed. The variation of current gain with temperature is typically ±15 per cent from −196°C to +350°C. An a.c. power output of 1 W was measured at 50 Mc with 60 per cent conversion efficiency ( I C = 80 mA, V CE = 20 V). The units show a current gain-bandwidth frequency f t = 300 Mc with the best transistors having a f t as high as 600 Mc.