The model function for the exponential model is defined as $$ f(d,\theta)=E_0+E_1(exp(d/\delta)-1)$$
exponential(dose, e0, e1, delta)Dose variable
Placebo effect
E1 parameter
Delta parameter, controlling the convexity of the model.
Response value
This model is intended to capture a possible sub-linear or a convex dose-response relationship.
Pinheiro, J. C., Bretz, F. and Branson, M. (2006). Analysis of dose-response studies - modeling approaches, in N. Ting (ed.). Dose Finding in Drug Development, Springer, New York, pp. 146--171