Deep learning techniques such as convolutional neural networks (CNNs) and recurrent neural networks (RNNs) have shown strong performance for sentiment analysis of text data. Deep learning models are capable of automatically learning representations of text needed for sentiment classification from large amounts of unlabeled training data through architectures inspired by the human brain.

CNNs have proven effective for sentiment analysis because their sliding window approach allows them to identify sentiment-bearing n-grams in text. CNNs apply consecutive layers of convolutions and pooling operations over word embeddings or character n-grams to extract key features. The final fully connected layers then use these features for sentiment classification. A CNN can learn effective n-gram features in an end-to-end fashion without needing feature engineering.

RNNs, particularly long short-term memory (LSTM) and gated recurrent unit (GRU) networks, are well-suited for sentiment analysis due to their ability to model contextual information and long distance relationships in sequential data like sentences and documents. RNNs read the input text sequentially one token at a time and maintain an internal state to capture dependencies between tokens. This makes them effective at detecting sentiment that arises from longer-range contextual cues. Bidirectional RNNs that process the text in both the forward and backward directions have further improved results.

CNN-RNN hybrid models that combine the strengths of CNNs and RNNs have become very popular for sentiment analysis. In these models, CNNs are applied first to learn n-gram features from the input embeddings or character sequences. RNN layers are then utilized on top of the CNN layers to identify sentiment based on sequential relationships between the extracted n-gram features. Such models have achieved state-of-the-art results on many sentiment analysis benchmarks.

Rule-based techniques such as dictionary-based approaches are also used for sentiment analysis. Dictionary-based techniques identify sentiment words, phrases and expressions in the text by comparing them against predefined sentiment dictionaries or lexicons. Scoring is then performed based on the sentiment orientation and strength of the identified terms. While not as accurate as machine learning methods due to their dependence on the completeness of dictionaries, rule-based techniques still see use for simplicity and interpretability. They can also supplement ML models.

Aspect-based sentiment analysis techniques aim to determine sentiment at a more granular level – towards specific aspects, features or attributes of an entity or topic rather than the overall sentiment. They first identify these aspects from text, map sentiment-bearing expressions to identified aspects, and determine polarity and strength of sentiment for each aspect. Techniques such as rule-based methods, topic modeling, and supervised ML algorithms like SVMs or deep learning have been applied for aspect extraction and sentiment classification.

Unsupervised machine learning techniques can also be utilized to some extent for sentiment analysis when labeled training data is limited. In these techniques, machine learning models are trained without supervision by only utilizing unlabeled sentiment data. Examples include clustering algorithms like k-means clustering to group messages into positive and negative clusters based on word distributions and frequencies. Dimensionality reduction techniques like principal component analysis (PCA) can also be applied as a preprocessing step to project text into lower dimensional spaces better suited for unsupervised learning.

In addition to the above modeling techniques, many advanced natural language processing and deep learning principles have been leveraged to further improve sentiment analysis results. Some examples include:

Word embeddings: Representing words as dense, low-dimensional and real-valued vectors which preserve semantic and syntactic relationships. Popular techniques include Word2vec, GloVe and FastText.

Attention mechanisms: Helping models focus on sentiment-bearing parts of the text by weighting token representations based on relevance to the classification task.

Transfer learning: Using large pretrained language models like BERT, XLNet, RoBERTa that have been trained on massive unlabeled corpora to extract universal features and initialize weights for downstream sentiment analysis tasks.

Data augmentation: Creating additional synthetic training samples through simple techniques like synonym replacement to improve robustness of models.

Multi-task learning: Jointly training models on related NLP tasks like topic modeling, relation extraction, aspect extraction to leverage shared representations and improve sentiment analysis performance.

Ensemble methods: Combining predictions from multiple models like SVM, CNN, RNN through averaging or weighted voting to yield more robust and accurate sentiment predictions than individual models.

While techniques like naïve Bayes and support vector machines formed the basis, latest deep learning and NLP advancements have significantly improved sentiment analysis. Hybrid models leveraging strengths of different techniques tend to work best in practice for analyzing customer feedback at scale in terms of both accuracy and interpretability of results.