An analysis has been carried out for a silicon diffused junction having a back surface field, with particular reference to the generation of photocurrent. The minority carrier distribution, and, photocurrent, as a function of width of the base region of this device, has been calculated. It has been observed that for a back surface field silicon diffused junction, the photocurrent reaches its maximum value at a much shorter width of the base layer. The dependence of photocurrent on the base layer impurity concentration has been studied after taking into account the dependence of diffusion length and diffusion constant on the impurity concentration in the base layer. The photocurrent for the same structure having a drift field in the base layer and a low high junction at the back has also been calculated, and the results indicate that the effect of a high-low junction on photocurrent in this case is less pronounced. A qualitative description of the photoeffect in pp + junction has also been included and the importance of including photovoltaic effect at the low-high junction to explain the overall performance of device has been stressed.

Effect of back surface field on photocurrent in a semiconductor junction

A method is described that selects, for each transistor in a circuit, the model of least complexity that will give acceptable accuracy. The capability to assess model adequacy derives from a self-consistency test in which the values of currents and voltages computed in a simulation of the circuit behavior are compared with onset parameters, to determine whether these computed values are consistent with the approximations underlying the device models used in the simulation. The onset parameters for a model are the terminal currents and voltages above or below which the model fails to give a satisfactory representation of device behavior. The authors set forth the onset parameters for the Ebers-Moll model and discuss their determination by terminal measurement and by calculation based on the transistor makeup. The paper limits consideration to the static behavior of transistors operating in the forward-active mode.

Assessing model adequacy and selecting model complexity in integrated-circuit simulation

Various expressions for the emitter-collector transit time of a heterojunction bipolar transistor (HBT) can be found in the literature. It is not obvious whether the product of the emitter contact resistance and the base-emitter capacitance should be included in the emitter-collector transit time expression. This paper theoretically demonstrates that the above product does not appear in the transit time expression. Additional physical insights observed from the derived transit time expression and comparison to a similar expression for homojunction bipolar transistors will be discussed.

Derivation of the emitter-collector transit time of heterojunction bipolar transistors

A simple analytical model for a base with an exponential doping profile is presented. The proposed approach improves previous work in that it accounts for all significant effects, namely the dependence of the mobility on doping concentration and electric field, heavy-doping effects, and finite carrier velocity at the collector edge. The model also applies to HBTs with a linearly graded composition. The model results show that the dependence of the mobility on the electric field has a significant impact on device performance, strongly limiting the beneficial effect of the built-in field on the minority-carrier current and transit time.

Analytical relations for the base transit time and collector current in BJTs and HBTs

The effect of mobility degradation at high impurity concentrations on the cut-off frequency of a diffused base transistor with exponential impurity profile in the base has been considered analytically. It has been shown that the cut-off frequency exhibits a maximum with increase in emitter and concentration. The results are compared with those of complementary error function and Gaussian base impurity profiles.

N.K.D. Choudhury

Papers

On the variation of cut-off frequency with emitter end concentration of a diffused base transistor

Cut-off frequency of a drift transistor

On the variation of cut-off frequency at high injection level with emitter end concentration of a diffused base transistor