From 314eba0e751be6b96d305648e61a9cc35c4363b3 Mon Sep 17 00:00:00 2001
From: Anton Akhmerov <anton.akhmerov@gmail.com>
Date: Wed, 3 Apr 2019 09:04:26 +0000
Subject: [PATCH] exercise 1

---
 src/14_doping_and_devices.md | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/14_doping_and_devices.md b/src/14_doping_and_devices.md
index d4182a9c..074aae71 100644
--- a/src/14_doping_and_devices.md
+++ b/src/14_doping_and_devices.md
@@ -229,6 +229,13 @@ A $pn$-junction has a **depletion layer** in its middle with the potential in a
 
 ### Exercise 1: Crossover between extrinsic and intrinsic regimes
 
+In the lecture we have identified the intrinsic and extrinsic regimes.
+Let us now work out what happens when the semiconductor is at the border between these two regimes, and the dopant concentration $|N_D - N_A|$ is comparable to the intrinsic one $n_i$.
+
+1. Write down the law of mass action and the charge balance condition for a doped semiconductor.
+2. Solve this system of equations for $n_e$ and $n_h$ without additional assumptions.
+3. Verify that your solution reproduces intrinsic regime when $|N_D - N_A| ≪ n_i$ and the extrinsic regime when $|N_D - N_A| ≫ n_i$
+
 ### Exercise 2: Donor ionization
 
 ### Exercise 3: Performance of a diode
-- 
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