高周波用ゲルマニウムNPN成長接合型単結晶の研究
1962 年 31 巻 2 号 p. 106-111
詳細
1962 年 31 巻 2 号 p. 106-111
Doubly doped NPN grown junction crystal for high frequency transistor has been studied. Germanium NPN grown junction transistors have been manufactured by double doped method or rate grown method. Each of these method has inherent limitations of characteristics such as low alpha cut-off frequency and high base spreading resistance.
The low cut-off frequency of germanium NPN grown type crystal is mainly ascribed to the change of resistivity in the base region near the emitter junction by solid diffusion of N type (Sb or As) emitter impurities during the growth of crystal.
The new NPN grown junction crystal has the emitter junction of steep gradient and has a heavily doped emitter region with phosphorus instead of Sb or As.
When phosphorus is used as the emitter dopant, diffusion of the emitter element to the base region decreases remarkably because of the smallness of diffusion constant of phosphorus in germanium. This diffusion constant was smaller than expected; it was found to be 1.3×10-11cm2/sec at 900°C.
The alpha cut-off frequency rose remarkably to about 20mc by the use of bars of about 20μ base width. This base width is comparable to the conventional double doped crystal of 5mc alpha cut-off frequency. Power gains greater than 40db at 455kc were easily obtained. Base spreading, resistance was reduced to a low value in triode type in spite of the high alpha cut-off frequency.
Because the bonded gold wire for base lead spreads over the collector, base and the nearly metallic emitter region, sometimes the breakdown voltage dropped to a low value after bonding.
So it is important to determine the allowable maximum emitter phosphorus concentration to prevent unusual breakdown and also to investigate the behavior of heavily doped PN junction.
The discovery by L. Esaki of the tunnel effect of narrow PN junction was made during these experiments.
