Như đã nêu trong hình. , phát (E) là rất nhiều pha tạp (n +) và có thể được truy cập trực tiếp từ trên. Các cơ sở (B) là nhẹ nhàng pha tạp, liên quan đến nước thải, nâng cao hiệu quả tiêm điện tử. Một hiệu quả phun cao có nghĩa là phát hiện tại bao gồm chủ yếu là của các điện tử được tiêm vào khu vực cơ sở chứ | BIPOLAR JUNCTION TRANSISTOR BJT 353 fi VtHe- This resistance is in the k 2 range and thus much lower than the input resistance of an FET. The speed of a BJT often is quantified by its unity current-gain frequency . transition frequency fr and its maximum frequency of oscillation max- From Eq. we find that gm JIc dVvE Id T and with Cbe tF IdVr Cje and Cbc Cjc we can derive cf. Eq. fT -L. 8m . - 1 . 2tĩ Cbe Cbc 2ir TF Cje Cjc VT Ic We see that fa is determined by the carrier transit time TF the junction capacitances Cje and Cjc and the collector current Ic. An important difference to the FET is that the carrier transit time through the base is controlled by diffusion rather than drift in an electrical field. The transit time T r is proportional to xị Dn where XB is the base thickness and D is the diffusion constant. Thus the transit time is bias independent to a first A more detailed analysis reveals that if the current density Id Ae exceeds a critical value the transit time increases rapidly because of an extension of the base region into the collector region known as base pushout or Kirk effect. The same effect also causes a reduction of the current gain. Despite these differences the speed of FETs as well as BJTs are improved by high-mobility semiconductor materials because the diffusion constant and the mobility are linked by the Einstein relationship Dn Vp. The higher electron mobility compared with the hole mobility also is the reason why n-p-n devices are faster than p-n-p devices. The maximum frequency of oscillation max depends strongly on the intrinsic base resistance Rb cf. Eq. and thus can be controlled by the base doping and the layout style a narrow emitter-stripe layout with a sufficient number of base contacts leads to a low Rb- For additional information about fr and max see Section . In switching and limiting amplifier applications the BJT may enter a regime where the base-emitter and the .
